Fisheries science JSFS , tập 78, số 1, 2012 1

Four bag-and-size-limit scenarios regulating catch were compared on these trips: 1 the 2006 state-specified reg-ulations control scenario that had a bag limit of eight fish with a minimu

O R I G I N A L A R T I C L E Fisheries

Recall bias in recreational summer flounder party boat trips

and angler preferences to new approaches to bag and size limits

Eleanor A Bochenek•Eric N Powell•

John DePersenaire

Received: 13 December 2010 / Accepted: 12 July 2011 / Published online: 27 October 2011

Ó The Japanese Society of Fisheries Science 2011

Abstract Three innovative approaches to bag and size

limits were evaluated in the recreational summer flounder

Paralichthys dentatus fishery Each approach was designed

to reduce discard mortality while increasing angler

satis-faction, yet still limiting recreational take within

manage-ment goals Each was compared to the 2006 legal bag and

size limits on party boat trips from New Jersey and New

York Angler-specific catch data were collected during the

trips, and anglers completed a questionnaire while sailing

back to port Comparison of questionnaires to observer

records revealed that anglers could not accurately recall the

number of fish kept or released Anglers overestimated both

kept and discarded fish by a factor of about two Neither

fishing scenario, age, sex, nor years fished significantly

influenced the accuracy of survey reports of kept fish

Anglers on three of five boats over-reported landings

Reported landings were nearly accurate on two boats

Survey accuracy for reported discards was influenced by

bag-and-size-limit scenario and differed among boats,

sexes, and fishing experience, but no predictable pattern

was evident In particular, bias in reporting was unrelated to

angler sex, age, experience, and performance on observed

trips or any other criterion measured in this study Anglers

preferred the slot limit most and the 2006 legal bag and sizelimit least High grading and transfer of fish among anglerswere rare occurrences Our study demonstrated that thesummer flounder fishery is a consumptive fishery

Keywords Angler recall Bag and size limits  For-hire Summer flounder

IntroductionSummer flounder Paralichthys dentatus supports importantcommercial and recreational fisheries along the northeastcoast of the United States This species is readily accessible torecreational anglers In 2006, summer flounder ranked fourth

in total number of recreationally caught fish in the Atlanticand Gulf region [1] Of the top recreational species encoun-tered in 2006, summer flounder ranked third in number of fishreleased but was not one of the top five harvested species [1];thus the discard-to-landings ratio was extremely high

By the late 1980s, summer flounder was severelyoverfished [2,3] Consequently a stock-rebuilding programwas initiated in the early 1990s Spawning stock biomassreturned to near historically high levels by 2004 [3] Therecreational fishery is managed through an annual harvestlimit computed as landings plus 10% of discards based onthe estimated discard mortality rate [2,3] As the summerflounder stock rebuilt, more older, heavier fish becameavailable to anglers Responsive harvest controls includedshortened fishing seasons, increased minimum legal sizes,and reduced bag limits These efforts to constrain landingsduring successful stock rebuilding resulted in landings oflarger, predominately female fish and increased regulatorydiscards because of the greater availability of fish smallerthan the legal size limit in the rebuilding stock with a

Haskin Shellfish Research Laboratory, Institute of Marine and

Coastal Sciences, Rutgers, The State University of New Jersey,

6959 Miller Ave., Port Norris, NJ 08349-3167, USA

e-mail: bochenek@hsrl.rutgers.edu

E N Powell

e-mail: eric@hsrl.rutgers.edu

J DePersenaire

Recreational Fishing Alliance,

PO Box 3080, New Gretna, NJ 08224, USA

e-mail: jdepersenaire@joinrfa.org

DOI 10.1007/s12562-011-0413-0

corresponding increase in discard mortality The steady rise

in discards coupled with the increasing minimum size

limits effectively reduced the realized number of fish

anglers could keep because total allowable catch includes

both landings and dead discards

Three alternative approaches to bag and size limits in

the recreational summer flounder party boat fishery were

evaluated in 2006 These were designed to reduce discard

mortality while increasing angler satisfaction within

con-straints that would retain catch within management goals

Bochenek et al [4] described the impact of each alternate

bag-and-size-limit scenario on discards, landings, and the

discard-to-catch ratio Powell et al [5] evaluated the health

of discarded fish relative to the presumed 10% mortality

rate [3] Inaccuracy in angler recall and nonresponses to

angler surveys can bias recreational catch and effort data

[6 11] Chase and Harada [12] noted that reducing the time

between the event and the reporting of the event could

possibly reduce the impact of recall bias Little information

on recall bias is available for the summer flounder fishery

The objectives of this study were to (1) evaluate angler

recall for the number of kept and discarded summerflounder on directed summer flounder party boat trips and(2) conduct an angler survey to determine their preferencesfor bag-and-size limit alternatives, reasons for discardingsummer flounder, and whether they high grade

Materials and methodsExperimental designFive party boats fishing for summer flounder during the

2006 fishing season were selected for this study Theseboats encompassed a range of vessel sizes and areas fishedalong the coasts of New York (southwest coast) and NewJersey, with three vessels with homeports in New Jersey(NJ) and two vessels with homeports in New York (NY).Vessels ranged in size from 50 to 90 ft; angler capacityranged from 50 to 131 anglers Locations fished includednear-coast state waters, offshore federal waters, and baysand estuaries (Fig.1) Detailed descriptions of the party

boats, areas fished, and number of trips are provided in

Bochenek et al [4]

Four bag-and-size-limit scenarios regulating catch were

compared on these trips: (1) the 2006 state-specified

reg-ulations (control scenario) that had a bag limit of eight fish

with a minimum length of 16.500(41.91 cm) for New Jersey

and four fish with a legal size limit of C1800(45.72 cm) for

New York; (2) a reduced-minimum-size limit with a 1400

(35.56 cm) minimum length and the 2006 state-specified

bag limit (NJ eight fish, NY four fish; (3) a slot limit in

which anglers were allowed to keep two fish between 1400

and the state-specified minimum size limit with the

remaining kept fish being greater than or equal to the

state-specified minimum size limit, with the state-state-specified bag

limit enforced; and (4) a cumulative size limit with kept

fish C1400set by conflating the state-specified size and bag

limit to produce a cumulative number of inches that could

be harvested, determined for, e.g., New Jersey, by

con-flating the 2006 legal bag limit and minimum size as 8

fish 9 16.500= 13200

Field sampling occurred only on weekdays (Monday–

Thursday) during June through September with the fishing

season split into an early (June to mid-July), mid (end of

July to mid-August), and late (end of August to

mid-Sep-tember) season Each boat was sampled once over a

con-secutive 4-day period in each season The

bag-and-size-limit scenario was randomly selected without replacement

for each 4-day period such that one scenario was sampled

per day and such that each scenario was fished once by

each boat in each season Most anglers were unaware of the

change in regulation until they boarded the boat, although a

few may have gleaned information from the boat owner/

operator prior to the trip Each angler was given a

num-bered tag and a one-page flier describing the current day’s

fishing scenario One or two observers were present on

each trip and collected angler-specific data on kept and

discarded summer flounder as the fish were caught [4] At

the end of the trip, observers gave each angler C14 years of

age a one-page questionnaire and asked them to complete

and return the survey to a marked box prior to leaving the

boat Some younger anglers also completed the survey with

assistance from their parents Fishermen remained

anony-mous, except for a numbered tag given to each angler at the

inception of the trip used to relate observer data to the

respective questionnaire Anglers were requested to record

the unique tag number; number of summer flounder kept

and discarded on the current trip; their gender, age, and

fishing experience (years fished); and information

pertain-ing to the angler’s preference for bag and size limit fished,

reasons for releasing fish, and whether they high graded,

that is whether they released otherwise legal fish that were

deemed to be of lower quality

Statistical analysisThe difference in summer flounder kept or discardedbetween that observed during the trip (observed kept/dis-carded) and that reported on the angler survey (reportedkept/discarded) was calculated as:

D kept¼ observed kept  reported keptand

D discard¼ observed discard  reported discard

To standardize the degree of over- or under-reporting ofkept or discarded summer flounder, the reporting accuracy(RA) was compared to the observed value For the fish kept

FDiff¼ abs(RA  1Þ:

Reporting accuracy (RA) was not normally distributed asindicated by significant Kolmogarov-Smirnov one-sampletests (a B 0.05) Accordingly, nonparametric statisticswere used to assess the degree of recall bias in anglerreports of fish kept or discarded Wilcoxon signed-ranktests were used to test this expectation for kept and dis-carded fish across the entire study and, as well, by boat,bag-and-size-limit scenario, angler sex, angler age, andangler experience measured as years fished Angler agewas assigned to a series of age groups for analysis identi-fied subsequently as 8 = anglers aged C70 years, 7 =anglers aged 60–69, 6 = anglers aged 50–59, 5 = anglersaged 40–49, 4 = anglers aged 30–39, 3 = anglers aged20–29, 2 = anglers aged 10–19, and 1 = anglers aged

\10 Angler experience was binned into a series of fishingexperience groups identified subsequently as 12y = yearsfished C50, 11y = years fished C45 to \50, 10y = yearsfished C40 to \45, 9y = years fished C35 to \40,8y = years fished C30 to \35, 7y = years fished C25 to

\30, 6y = years fished C20 to\25, 5y = years fished C15

to \20, 4y = years fished C10 to \15, 3y = years fishedC5 to \10, 2y = years fished C2 to \5, and 1y = yearsfished \2

The tendency for anglers to over-report or under-reportwas assessed using a sign test This test merely comparesthe number of times anglers over-reported or under-reported fish regardless of the degree of error Nonstan-dardized (D kept, D discarded) and standardized [RA(kept),

RA(discarded)] reporting accuracies were compared

between boats, fishing scenarios, angler age groups, fishing

experience levels, and angler sexes using Kruskal-Wallis

tests

Correspondence analysis [13] was used to visualize the

interrelationship of boat, bag-and-size-limit scenario,

angler age group, fishing experience group, and angler sex

with angler survey responses Correspondence analysis is a

data-reduction technique that permits evaluation of

rela-tionships within categorical datasets [14, 15] and is

anal-ogous to principal components analysis for continuous or

meristic data [16] Inputs to the correspondence analysis

included the following: survey responses that pertain to the

angler’s preference for keeping and releasing legal summer

flounder, survey responses that pertained to why anglers

released some summer flounder that they could have kept

legally, and survey responses that pertain to their

prefer-ence for the three experimental bag and size limits and the

control Additional variables included were angler age

group, fishing experience group, and sex Supplementary

variables positioned on the axes were boat and

bag-and-size-limit scenario Factor loads for each variable for the

first 10 dimensions resolved by correspondence analysis

were used as variables describing each angler category and

survey response in cluster analysis Response and angler

category variables were clustered using an unweighted

pair-group algorithm with Euclidean distance as the

simi-larity index [17]

To investigate possible reasons for angler bias in the

reporting of fish kept and discarded, we added to the list of

supplementary variables in correspondence analysis a

ser-ies of variables describing the degree of angler bias These

included whether the angler over-reported or

under-repor-ted landings and/or discards and whether the bias in

reporting fell in the upper or lower 25% of all respondents

in each regard We also distinguished anglers that landed

fish from those that did not catch any legal-size fish Angler

bias measured as the raw difference between observed and

reported landings and discards and standardized to total

angler catch was included

Results

Survey statistics

In 2006, 76 summer flounder fishing trips (full day,

morning half-day, afternoon half-day) were sampled on

five party boats A total of 1,860 anglers fished on these

boats and 1,090 anglers completed the survey (58.6%) Of

these, 49 were discarded because of inaccurate trip or

angler designations Thus, our study relies on 1,051 angler

surveys

The number of completed angler surveys was unevenlydistributed among the three bag-and-size-limit scenariosand the control (Table1) The fewest number of surveyswas completed for control trips (19.5%) and the greatestnumber for reduced-minimum-size trips (30.6%) (Table1).More surveys were completed on morning trips (N = 508)than on afternoon (N = 367) and full-day trips (N = 215)

To a large extent, these differentials reflected the ential in the number of anglers participating in morning andafternoon trips and the tendency for observed vessels tocarry out half-day rather than full-day trips [4] Anglersthat completed surveys were predominately males (83.6%)and angler mean age was 47.6 years and ranged from 6 to

differ-85 We had asked that participants C14 years of agecomplete the survey However, 4.7% of respondents wereyounger than 14 and some parents assisted these youngeranglers in completing the survey Other young anglers fil-led out their own questionnaire These data were included

in the analysis Anglers had fished on average 27.7 yearsand ranged from new anglers with 0 years of experience toanglers with 75 years of experience encompassing a broadrange of angler experience from novices to the veryexperienced (Table1)

A total of 156 anglers reported that they did not catch asummer flounder Anglers reported keeping 2,108 summerflounder, an average of 2.0 fish per angler (Table2) anddiscarding 3,676 summer flounder, an average of 3.5 fishdiscarded per angler Fishermen also disclosed that theydiscarded 3,297 fish below the minimum size, a per-angleraverage of 3.2 fish In extremum, one angler reported 50discarded fish, all below the legal minimum size, and thatall discards were dead Observers on this boat did not seeone dead fish discarded nor any single angler catching 50summer flounder

When we asked anglers if they ever high graded, 169anglers (15.5%) answered affirmatively, but only 3.4%reported high grading during the previous year (the 2005fishing season) Fishermen were also questioned as towhether they gave fish away on the boat instead of highgrading: 146 fishermen answered affirmatively, but only2.4% did so during the previous year (the 2005 fishingseason) (Table2) Anglers were asked how many fishingtrips they made on party boats, private boats, and from theshore, bank, or jetty in the previous fishing year (2005).Fishermen participating in this study averaged approxi-mately eight trips on party boats, four trips on privateboats, and three outings from the shore, bank, or jetty(Table1)

Angler preferences/descriptive statisticsAnglers were asked to identify the bag-and-size-limit sce-nario under which they fished for that trip Only nine

anglers (0.8%) answered incorrectly Anglers were also

asked to rate their satisfaction with the bag-and-size-limit

scenario under which they fished that day on a five-point

scale Most anglers rated the slot-limit scenario as highly

preferred (56.7%), and 4.6% of the anglers ranked this

scenario as not preferred (Table3) For the cumulative-size

scenario, 46.5% of the anglers ranked this scenario as

highly preferred, and only 5.1% did not prefer this

sce-nario The rankings for the reduced-minimum-size scenario

were similar to the cumulative-size scenario In contrast,

only 28.9% of the anglers highly preferred the control

scenario, and 22.9% of the anglers ranked this fishing

scenario ‘‘not preferred’’ (Table3) The mean rank for the

control, slot-limit, reduced-minimum-size, and

cumulative-size scenarios was 3.2, 4.2, 4.1, and 4.0, respectively, with

a 5 indicating ‘‘highly preferred.’’

Fishermen were queried as to their relative preferences

for the three alternative bag-and-size-limit scenarios and

the control (Table4) In this case, anglers were asked to

respond concerning the desirability of each alternative as a

future management scenario, even though they had

par-ticipated in only one of the four alternatives For the 2006

state-specified bag and size limits (control scenario), about

32% of the anglers were satisfied and 32% not satisfied

with this scenario In contrast, approximately 43% of the

anglers favored the reduced-minimum-size scenario and

about 47% of the anglers favored the slot-limit scenario

Approximately, 33% of the anglers preferred the

cumulative-size scenario, but more anglers indicated satisfaction with this scenario (*26%) than for the slot-limit and reduced-minimum-size scenarios (Table4).Increased dissatisfaction by anglers for the cumulative-sizescenario may come from the greater difficulty for anglers tokeep track of their total inches caught More anglersexpressed dissatisfaction with the control scenario than any

dis-of the alternatives

Fishermen were asked about their predilections forkeeping and releasing summer flounder that they catch Sixhundred and fifteen anglers preferred to keep all legallyallowed summer flounder, 246 anglers preferred to keepmost of the summer flounder legally allowed, 49 anglerspreferred to release most of the legally allowed summerflounder, and 17 anglers would release all legally allowedsummer flounder These preferences support the generallyheld view that the summer flounder fishery is predomi-nately a consumptive fishery, since a majority (66.3%) ofthe anglers would rather keep all legal summer flounderwhereas only 1.8% of the anglers practice true conservation

by releasing all summer flounder that could have beenlegally kept Forty-two percent responded that the questiondoes not apply to them

Anglers were queried as to why they released some ofthe summer flounder that could have been kept legally(Table5) Only 16.9% (N = 126) responded that theyrelease legally allowed summer flounder because they donot consume them, whereas 63.6% (N = 473) of anglers

Preference was ranked from 1 to 5 with 1 equal to ‘‘do not prefer’’ and 5 equal to ‘‘highly prefer’’ No explicit alternative was provided

prefer

agree

Somewhat disagree

know

fish, NJ 8 fish (reduced-minimum-size scenario)

current legal size limit and bag limit: NY 4 fish, NJ 8 fish (slot-limit scenario)

Fish for summer flounder under a cumulative total size limit (e.g., NJ: total size limit of

(cumulative-size scenario)

Does not include no-response answers

disagreed with this reason About 68% of the anglers

responded that they release these fish because some of the

summer flounder were too small to keep and about 55%

released fish because they caught what they wanted to eat

Approximately 46% of the anglers responded that their

conservation ethic was the reason for release of some fish

However, in response to the direct question concerning

their desire to carry out a catch-and-release fishing

expe-rience, only about 20% agreed or somewhat agreed and

about 39% disagreed These responses also support the

dominance of consumptive fishing as the prime motivating

force for angler participation in this fishery

Observed versus reported summer flounder catch

The mean difference between summer flounder observed

kept and reported kept (D kept) was -0.370, indicating that

anglers reported landing more fish than they actually

lan-ded per trip The mean standardized accuracy of reports

[RA(kept)] was -0.800 indicating that the reported number

of kept fish exceeded the observed kept by a factor of

1.800 For discarded summer flounder, the mean difference

(D discard) was -1.600 indicating that anglers also

over-estimated their discards The mean standardized accuracy

was -1.423 Anglers tended to overestimate their summer

flounder discards by about a factor of 2.423

The accuracy of angler reports for kept and discarded

fish differed significantly among boats, but only among

bag-and-size-limit scenario for kept fish (Table6,

Kruskal-Wallis test) Angler age did not influence reporting

accu-racy overall, and angler sex influenced only the accuaccu-racy of

reported discards with males being more biased with a

mean D discard = -1.81, whereas for females the mean D

discard = -0.56 Fishing experience influenced only the

accuracy of reported landings (Table6)

Significant results do not imply inaccurate reporting,

however; they merely indicate that reporting accuracy

varied between main-effect categories To further

investi-gate the degree of accuracy within main-effect groups, we

evaluated each category separately with tests designed to

identify bias The nonstandardized differences in observed

and reported discards were significantly different from zerofor all bag-and-size-limit scenarios, boats, angler agegroups except for the very youngest (age 1), sexes, andfishing experience groups (Table7) That is, in no case wasreporting accuracy unbiased relative to the true (observed)value For the three alternative bag-and-size-limit scenar-ios, discards were over-reported with the degree of over-estimate ranging from a factor of 1.41 fish for thecumulative-size scenario to 1.61 fish for the reduced-min-imum-size scenario In contrast, discards were overesti-mated by 2.04 fish for the control scenario Male anglersoverestimated discards by a factor of 1.81 fish; femaleanglers were somewhat more accurate, overestimatingdiscards by 0.56 fish (Table8)

Reporting was consistently better for kept fish than fordiscarded fish For kept fish, the difference betweenobserved and reported fish was significantly different forthree of five boats, all bag-and-size-limit scenarios but theslot limit, both sexes, five of eight angler age groups, and 6

of 12 fishing experience groups (Table7) Less enced anglers tended to over-report landings more thananglers with greater experience Younger fishermen tended

experi-to report more accurately than older fishermen

In order to standardize the level of bias in the reporting

of kept and discarded summer flounder, we computed thestandardized reporting accuracy (RA) (Table8) Thehighest over-reporting of kept fish occurred for the controlscenario at 3.922 Lesser reporting bias occurred for kept

agree

Somewhat disagree

Does not include no-response answers

the observed and angler-reported kept and discarded summer flounder

by boat, bag-and-size-limit scenario, age group, sex, and angler fishing experience

fish for the alternative scenarios that ranged from the

cumulative-size scenario for which anglers reported 1.403

more kept fish than observed to the reduced-minimum-size

scenario for which anglers reported 1.45 more kept fishthen observed Under-reporting occurred for some anglers,but only in two cases (fishing experience) was the meanRA(kept) value positive Discards were over-reported byanglers fishing under the reduced-minimum-size scenario

by a factor of 3.090 In contrast, least bias occurred in thecumulative-size scenario in which anglers over-reporteddiscards by a factor of 1.801 The value for the control(2006 legal) scenario was 2.318 Under-reporting occurredfor some anglers, but in no case was the mean RA(discard)

signed-rank tests [RA(kept), RA(discard)] to evaluate the differences

in the observed and reported kept and discarded summer flounder by

boat, bag-and-size-limit scenario, age, sex, and fishing experience

NS Not significant at a = 0.05 Abbreviations for angler age and years

Paren-theses indicate negative values

cate-gories in which the significance of reporting accuracy was evaluated

be obtained as |(RA - 1)|

value positive For boats, RA values for kept fish ranged

from 1.011 to 3.082 for boats A and B, respectively For

discards, RA ranged from 1.516 (boat E) to 2.941 (boat B)

Female anglers tended to over-report kept and discarded

fish less than male anglers (Table8) RA fell between

about 1 and 2 for kept fish for all angler age groups; the

range for discards was 1.524–2.945 Regardless of the level

of fishing experience, all anglers tended to over-report kept

and discarded fish except for kept fish for anglers that had

fished C20 to \25 years (6y) and C35 to \40 years (9y)

who under-reported their catch RA for kept fish ranged

from about 1 to 2 excluding the anglers that under-reported

their kept fish; the range for discarded fish was about

1.5–2.8 (Table8)

Correspondence/cluster analysis

Angler characteristics and survey responses were used to

define axes in correspondence analysis and the degree to

which any vessel or bag-and-size-limit scenario fell

pref-erentially on one of the assessed characteristics Survey

responses were little influenced by vessel or

bag-and-size-limit scenario overall nor did biases exist in the distribution

of anglers by age, fishing experience, or sex between

vessels and trips None of the vessels or bag-and-size-limit

scenarios received a factor loading score [ |0.22| on any of

the first 10 dimensions of the correspondence analysis

Thus, most angler reports and angler descriptors (e.g., age,

years fished) were relatively randomly distributed among

boats and bag-and-size-limit scenarios

To examine the interaction of survey responses and angler

descriptive variables, the various responses and descriptive

variables were clustered using the factor loading scores for

the first 10 dimensions of the correspondence analysis to

define between-variable and/or between-response similarity

This analysis revealed the following interactions (Table9)

(1) Young anglers \20 years old and anglers of limited

fishing experience (2–15 years) provided indecisive answers

as to the reason for discarding summer flounder Most

‘‘somewhat disagreed’’ with the range of options provided

for why they release some summer flounder from the survey

(Table9) Separately clustered were anglers with

15–20 years of experience These anglers were associated

with ambivalent feelings about the bag-and-size-limit

alternatives, recording the opinion ‘‘somewhat disagree’’

with the control, reduced-minimum-size, and

cumulative-size scenarios (Table5) In total, anglers with relatively little

fishing experience tended to be more indecisive about

management scenarios and reasons for discarding than older

and more experienced anglers (2) Females clustered with

anglers 20–30 years of age and anglers with less than 2 years

of experience The latter two criteria described most female

anglers These anglers typically provided no response to the

question concerning their preference for keeping or releasinglegal-size fish, although they responded to other questions onthe survey (3) A series of anglers responded that they did notknow why they discarded fish (Table 4) These anglers werenot dominantly male or female, nor were they dominantly inone age or fishing experience group (4) A series of anglersindicated that they did not have an opinion as to their pref-erence for the four studied bag-and-size-limit scenarios(Table5) These anglers also, interestingly, had no opinionwhen asked about their preference for landing or releasinglegal-size fish Such anglers might be expected to express noopinion as to bag-and-size-limit scenario These anglerswere not dominantly male or female, nor were they domi-nantly in one age or fishing experience group (5) A series ofanglers did not respond to survey questions pertaining to whythey released some summer flounder that they could havekept and to the management measures they prefer for sum-mer flounder (Tables4,5) Failure to respond fell into twodistinctive clusters differentiating the two questions, indi-cating that such responses came from two different groups ofanglers These anglers also were not dominantly male orfemale, nor were they dominantly in one age or fishingexperience group (6) Two clusters defined anglers that eitherfished primarily for consumption or did not The first clusterdisagreed with the following reasons as to why they releasesome summer flounder that they could have kept legally:some summer flounder were too small to keep, they hadcaught what they wanted to eat, and their conservation ethic(Table4) The second cluster of anglers agreed with thecatch-and-release philosophy, agreeing that they do notconsume summer flounder and like to catch and release(Table4) This latter group also expressed a preference forreleasing most or all summer flounder caught Separationinto two clusters is indicated by some anglers choosing toexpress the catch-and-release approach in two distinctiveways on the survey form Interestingly, in neither case werethese anglers associated with a preference for any bag-and-size-limit scenario, nor were they associated dominantlywith either sex, any angler age group, or any angler fishingexperience group (7) The oldest anglers, with[50 years offishing experience and [60 years old, were not associatedwith any discrete set of survey responses Such anglersresponded in a multitude of ways to the survey questionnaire.(8) Anglers with 40–50 years of fishing experience indicateddiscomfort with the three alternative bag-and-size-limitscenarios by checking ‘‘disagree’’ (Table5) This anglergroup was unique in this respect and, significantly, was notassociated with any predominant response to preference forthe control scenario (Table5) They were simply more likely

to accept the status quo without stipulating a clear preferencefor it (9) An important group of anglers expressed a pref-erence to keep most, but not all, legal-size fish These anglerswere ambivalent as to why they discarded fish (Table4) and

Table 9 Angler responses to survey questions 16–18aand the descriptive variables of sex, age group, and fishing experience group eterized as factor loads using the first 10 dimensions in correspondence analysis fell into 13 clusters

variables

(b), caught what want to eat (c), conservation ethic (d), and catch and release (e)

Angler ages

\20 years, 2–15 years of fishing experience

for keeping vs releasing

summer flounder that

were caught

Angler ages 20–30 years,

\2 years of fishing experience, females

fish too small (b), caught what want to eat (c), conservation ethic (d), and catch and release (e)

preferences for keeping

scenario, reduced-minimum-size scenario, slot-limit scenario, and cumulative-size scenario

what want to eat (c), conservation ethic (d)

release most of what is

legally allowed to be

kept

Agrees with don’t eat (a) and fish too small (b)

small (b), caught what want to eat (c), conservation ethic (d), and catch and release (e)

fishing experience, angler age [60 years

scenario, slot-limit scenario, and cumulative-size scenario

45–50 years of fishing experience

scenario, reduced-minimum-size scenario, and cumulative-size scenario

15–20 years of fishing experience

most of

summer

flounder legally caught

Somewhat agree with don’t eat (a), fish too small (b), wants to eat (c), conservation ethic (d), and catch and release (e); somewhat disagree with catch and release (e)

Somewhat agree with control scenario, reduced-minimum-size scenario, cumulative-size scenario, and slot-limit scenario; somewhat disagree with the slot-limit scenario

legally allowed to keep

Agree with fish too small (b), wants to eat (c), and conservation ethic (d); disagree with don’t eat (a) and catch and release (e)

Agree with control scenario, minimum-size scenario, slot-limit scenario, and cumulative-size scenario;

reduced-disagree with control scenario

Angler ages 30–60 years, 20–45 years of fishing experience, male

as to their preference for bag-and-size-limit scenario

(Table5) This group of anglers checked options ‘‘somewhat

agree’’ or ‘‘somewhat disagree’’ in answer to most

alterna-tives in these two questions The tendency to choose both

pairs of responses (a) ‘‘do not eat summer flounder’’ and

‘‘like to catch and release summer flounder’’ and (b) ‘‘some

summer flounder were too small to eat’’ and ‘‘conservation

ethic’’ (Table4) equivalently, alternatives that are

inher-ently distinctive and exclusionary of the alternative,

char-acterizes this ambivalence These anglers were not

dominantly male or female, nor were they dominantly in one

age or fishing experience group (10) One set of anglers was

characterized by definitive opinions on questions pertaining

to why they release some summer flounder that they could

have kept legally and questions pertaining to the

manage-ment of summer flounder and preference for managemanage-ment

scenarios (Tables4, 5) These anglers typically checked

options ‘‘agree’’ or ‘‘disagree.’’ Anglers in this category

preferred to keep their catch and not release them and thus

preferred consumptive fishing They routinely indicated a

desire to keep all, not most, legal-size fish They strongly

favored each of the alternative bag-and-size-limit options

but indicated a distinctly lesser preference for the control

scenario These anglers were predominately male,

30–60 years old, and with 20–45 years of fishing experience

Overall, the survey showed that younger anglers were

more indecisive as to their desires on the fishing trip They

expressed ambivalent reasons for discarding and

ambiva-lent opinions on bag-and-size-limit scenarios These

anglers were dominantly young males or females, as most

females fell into this category At the other extreme were

the oldest anglers, who had a range of opinions or indicated

a preference for the present management approach,

depending on age category In between were anglers,

mostly male, with mostly intermediate fishing experience,

who were strongly associated with a desire to keep all fish

and with a predilection for alternative bag-and-size-limit

scenarios Several other groups were not as age-, fishing

experience-, or sex-dependent These included a subset that

preferred catch and release and subsets that tended, for one

reason or another, to express no opinion when queried

about bag-and-size-limit preference or reasons for

dis-carding A range of ambivalences existed among these

anglers, some having no opinion while others expressed

ambivalence in checking options defined as ‘‘somewhat’’

agree or disagree In either case, these ambivalent anglers,

with the exception of the younger and less experienced

individuals discussed previously, were not characterized by

age, fishing experience, or sex

Finally, we examined the relationship of reporting bias

to angler survey responses by including measures of

reporting bias in the correspondence analysis No measure

of reporting bias achieved a factor loading score [ |0.23|

on any of the first 10 dimensions of the correspondenceanalysis This included standardized (RA) and nonstan-dardized (e.g., D kept) measures of bias Equivalent resultsoccurred if the upper or lower 25% of anglers, ranked byreporting bias, were selectively evaluated or if all anglerswere evaluated together Reporting bias was a relativelyrandom variable, not obviously associated with angler sex,age, or experience or survey respondents categorized byboat or fishing scenario

DiscussionMany researchers have reported on recall and other surveybiases [6 8,18] This study had a 58.6% survey completionrate over 76 trips Whether nonrespondents would haveprovided different answers to our questionnaire than thoseanglers that responded is unknown Answers from childrenyounger than 14 years may have had an effect on theoutcome of the answers from the survey because many ofthe children’s answers may reflect the opinions of theirparents or guardians In addition, we used years fished as

an indication of an angler’s fishing ability by assuming thatangler’s fishing ability increases with more years fished.This could be another possible source of error because wereally did not know the anglers’ true fishing abilities.Origin of recall bias

In our study, anglers could not accurately recall the mer flounder caught, either kept or released, just minutesafter completing their targeted summer flounder fishingtrips Anglers overestimated their trip landings and discardsalmost twofold Landings were reported with less bias thandiscards In fact, anglers on two of the five boats did notoverestimate their landings, whereas anglers on five boatsoverestimated discards The Marine Recreational FisheriesStatistics Survey (MRFSS), the primary database main-tained by the US National Marine Fisheries Service(NMFS) for the recreational harvest, relies on angler recall

sum-to obtain information on discards, some kept fish that arenot verified at the dock, and effort If our observations aretypical of responding MRFSS anglers, recreational summerflounder discards for the party boat sector may likely beoverestimated in this database

Overall, the reporting accuracy observed in this study islower than that observed in many other studies Historicalsummer flounder landings in the United States were esti-mated from national marine angler surveys that precededthe MRFSS and were based in part on angler catch recallfrom up to 1 year earlier [2] These estimates are thought tohave overestimated landings by 100% [2, 11] This esti-mated recall accuracy is higher than observed in our 2006

study that relied on near-instant recall for landings

Con-nelly and Brown [8] investigated recall biases associated

with a diary and 12-month recall from mail questionnaires

for anglers that fished in Lake Ontario and found

signifi-cant differences between estimates reported in diaries and

the mail questionnaire for both fishing effort (days fished),

which was overestimated by about 45% in the 12-month

recall mail questionnaire, and fish consumption, which was

under-reported in diaries No significant differences were

found for catch rates and fishing expenditures [8] Sullivan

[19] investigated whether walleye anglers fishing in

Alberta, Canada, exaggerated their catch of walleye

Stiz-ostedion vitreum He calculated an exaggeration factor as

the ratio of reported catch to estimated catch and found that

exaggeration in catch was not constant, but increased

exponentially with decreasing catch rate These anglers

reported catching 2.2 times more protected-length walleyes

per legal-length walleyes than during test angling fishing

Another study examined the potential biases in interview

responses using onboard independent observers and creel

survey interviews in the Georgia Strait recreational fishery

on the Pacific Coast [6] The number of kept fish reported

in the creel survey and by onboard interviews was not

significantly different, but a significant difference existed

for released fish The estimated releases for two salmon

species were approximately 40% underestimated in the

creel survey [6], a result opposite that observed in this

study

The inaccuracy of discard estimates from responding

anglers, even after so short a time as towards the end of the

fishing trip and even with observers onboard, suggests that

measures of discarding obtained without direct observation

may be biased, as was observed in this study We found no

consistent differential influence of bag-and-size-limit

sce-nario, age, sex, or fishing experience on the accuracy of

discard reporting and only one on the accuracy of landings

reporting The latter was a tendency for anglers with more

fishing experience to more accurately provide landings

information Anglers on these boats over-reported their fish

landed, whereas those on boats A and D reported landings

with greater accuracy The reason why anglers reported

more accurately on two of five boats studied cannot be

determined Discards, in comparison, were over-reported

for the three experimental bag-and-size-limit scenarios, and

for the control Each bias in discard reporting distinctly

exceeded bias in landings reporting, except for the control

Overall, over-reporting of discards by anglers was a

gen-eral characteristic of the anglers and was influenced only

moderately by bag-and-size-limit alternative; the tendency

to exaggerate landings, while lower than with discards and

much more frequently not significant, was well established

throughout the data set, regardless of angler sex, age, or

fishing experience

Angler characteristics and preferencesAngler mean age was 47.6 years and ranged from 6 to 85.The majority of the anglers were male These results aresimilar to other studies [20] and re-emphasize that the U.S.angling population is aging and still male-dominated.Males and females responded differently to survey ques-tions, indicating a different view of the fishing experience.Females tended to be less experienced and younger and, asseems to be true for most younger and inexperiencedanglers, they were ambivalent concerning their preferencefor bag-and-size-limit alternatives and less sure about theirreasons for discarding The older anglers tended to be moreconservative, favoring the present-day legal scenario butwere not associated with any specific discarding behavior.The mass of anglers intermediate in age and experience,mostly males in this case, were the most definitive of allangler groups in their responses These anglers fished forsummer flounder for the table, preferred to take all legal-size fish home, and preferred any alternative to the controlscenario A study of preferences for harvest regulations byred drum Sciaenops ocellatus anglers showed that anglerswith more fishing experience were less interested inrelaxing current management regulations, and casualanglers demonstrated a strong preference for catching morered drum by relaxing regulations [21] Schroeder et al [22]compared male and female anglers’ motivations and ethicsfor recreational fishing through a survey of licensed Min-nesota fishermen He noted that men reported higherinvolvement in fishing than women and agreed more withthe ethics of catch and release fishing as well as lower baglimits than women Females often reported keeping alllegal fish that they catch and keeping larger fish oversmaller fish whereas males tended to release all the fish thatthey caught and kept smaller fish over larger fish Our studyalso found differences in gender related to the fishingexperience and preferences, but young females wereindifferent to management preferences and males withintermediate fishing experience desired alternative bag-and-size-limit scenarios and keeping all their fish In con-trast, in our study older anglers were much more supportive

of the control (2006-legal) bag-and-size-limit scenario,consistent with results reported by Oh and Ditton [21].Approximately 16% of the anglers reported that theyhigh grade on summer flounder fishing trips The majority

of anglers also responded that they do not give summerflounder away However, these same anglers reported that

in 2005, they gave away summer flounder on only 2.4% ofthe summer flounder fishing trips and that they high graded

on only 3.4% of their summer flounder fishing trips Weobserved few instances of either activity Both high gradingand the giving of summer flounder to other anglers are rareoccurrences on party boats and this behavior tends to be

more exaggerated by memory recall than likely actually

occurs based on our trip experience However, we cannot

exclude the possibility that angler behavior may have been

modified because of the observers on the boat

When anglers were asked to rate the various

bag-and-size-limit scenarios fished under during that trip, 56.7% of

the anglers ranked the slot-limit scenario as highly

pre-ferred, 46.5% of the anglers ranked the cumulative-size

scenario as highly preferred, the reduced minimum-size

scenario was ranked about the same as the cumulative-size

scenario, and 28.9% of the anglers ranked the control

scenario as highly preferred The least preferred scenario

was the control, probably because the fewest fish can be

kept and more fish are discarded under this scenario

Overall, little angler preference discriminated the

bag-and-size-limit alternatives, but the control (2006 legal option)

was clearly less preferred Stoll and Ditton [23] studied the

winter bluefin tuna Thunnus thynnus recreational fishery in

North Carolina and noted that these anglers were fairly

harvest oriented as in our study of summer flounder anglers

and were less willing to pay for the most restrictive catch

management scenario (total catch and release) and most

willing to pay for the least restrictive management scenario

(catch and release fishery with one bluefin per person kept)

In our study, the control scenario was the most restrictive

management scenario and the least preferred

Anglers were queried to rate their reasons for keeping and

releasing summer flounder that they catch About 66% of the

anglers preferred to keep all legal summer flounder; only

about 2% of the anglers practice true conservation by

releasing all summer flounder that could have been legally

harvested Anglers practicing catch and release were not

biased by sex, age, or fishing experience, nor did they

iden-tify a preferred bag-and-size-limit alternative Preference for

bag-and-size-limit alternatives was expressed by anglers

seeking to take fish home for food This demonstrates that the

summer flounder fishery is a consumptive fishery As these

anglers are the preponderant majority and include a core

group of relatively old, experienced anglers, they likely

represent the important constituency for the summer

floun-der party-boat industry These anglers were definitive in their

survey responses in comparison to all other participants

One interesting trend is the indecisiveness of several

groups of anglers to survey questions Some of these

anglers expressed ambivalent answers or were without

opinion as to bag-and-size-limit scenario or reason for

discarding, but these anglers were distributed across age

groups, fishing experience groups, and sexes The survey

tool did not permit further evaluation of this group of

anglers, but the results suggest that some fraction of party

boat anglers are rather indifferent to the fishing experience

The second block is strongly represented by younger

anglers and those least experienced These anglers were

slightly more definitive in their answers, but still answeredmost queries by using the adjective ‘‘somewhat’’ beforedisagree or agree The survey responses suggest thatdefinitive opinions on the angling experience take someyears to mature or that a demographic shift is occurring inthe intensity of angler investment in the angling experi-ence In either case, the future of the recreational fishery islikely to be vested in the capture of these ambivalentanglers into the older angler group with more definitiveopinions and, we infer, more serious commitment to rec-reational fishing This result and the recognition of thedegree of bias in angler reporting of landings and discardsacross all age groups, fishing experience groups, sexes, andbag-and-size-limit scenarios are the two most importantoutcomes of this study However, our study targeted asmall component (party boat sector) of the summer floun-der recreational fishery Future studies need to be con-ducted to determine if angler recall bias occurs in otherrecreational fishing sectors in this fishery and while tar-geting other fish species

OverallGiven the variety of age-dependent, sex-dependent, andexperience-dependent responses by anglers in our surveyquestionnaire, we anticipated identification of certainangler characteristics leading to increased bias in the recallreporting of landings and discards No such trend could bediscerned, however The degree of reporting bias was notobviously influenced by sex, years of fishing experience,angler age, or any subset of angler characteristics resolved

by correspondence analysis of survey questionnaireresponses Angler bias in reporting of landings and discardsseems to be a wholly random outcome Of course, someunmeasured characteristics of anglers may yet resolve thereasons for some anglers to be more biased than others inreporting landings and discards Our study suggests, shouldthose characteristics exist, that they are more complex thanthe simple sex, age, and years of experience categoriesupon which our description of anglers participating in thisstudy were based

References

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2006 Fisheries Statistics Division, NMFS-F/STI 4 Silver Spring, MD

2 Terceiro M (2002) The summer flounder chronicles: science, politics, and litigation, 1975–2000 Rev Fish Biol Fisher 11:125–168

3 Terceiro M (2006) Stock assessment of summer flounder in 2006 NEFSC Ref Doc 06-17 Northeast Fisheries Science Center, Gloucester, MA

4 Bochenek EA, Powell EN, DePersenaire J, King SE (2010)

Evaluating catch, effort, and bag limits on summer flounder

directed trips in the recreational party boat fishery Mar Coast

Fish Dyn Manag Ecosyst Sci J 2:412–423

5 Powell EN, Bochenek EA, DePersenaire J, King SE (2011) Injury

frequency for discarded summer flounder in the recreational

fishery of the Mid-Atlantic Bight: influence of landing size

reg-ulations In: Beard Jr TD, Arlnghaus R, Sutton SG (eds) The

angler in the environment: social, economic, biological, and

ethical dimensions Proceedings of the 5th World Recreational

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Bethesda, MD, pp 171–187

6 Diewert RE, Nagtegaal DA, Hein K (2005) A comparison of the

results of the 1988 Georgia Strait creel survey with an

indepen-dent observer program Can Manuscr Rep Fish Aquat Sci

2716:1–39

7 Connelly NA, Brown TL, Knuth BA (2000) Assessing the

rela-tive importance of recall bias and nonresponse bias and adjusting

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biases associated with 12-month recall on mail questionnaires.

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estimates of statewide fishing activity N Am J Fish Manag

10:111–113

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(1997) Catch rate estimation for roving and access point surveys.

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ability to estimate catch and recall catch and effort over time.

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intro-14 Green RH (1993) Relating two sets of variables in environmental studies In: Patil GP, Rao CR (eds) Multivariate environmental statistics Elsevier Science, New York, pp 149–163

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21 Oh C-O, Ditton RB (2006) Specialization differences in anglers’ preferences for red drum (Sciaenops ocellatus) harvest regula- tions Proc Gulf Caribb Fish Inst 57:869–880

22 Schroeder SA, Fulton DC, Currie L, Goeman T (2006) He said, she said: gender and angling specialization, motivations, ethics, and behaviors Hum Dimens Wildl 11:301–315

23 Stoll JR, Ditton RB (2006) Understanding anglers’ willingness to pay under alternative management regimes Hum Dimens Wildl 11:27–42

O R I G I N A L A R T I C L E Fisheries

Association of early juvenile yellowfin tuna Thunnus albacares

with a network of payaos in the Philippines

Yasushi Mitsunaga•Chikayuki Endo •

Kazuhiko Anraku•Cornelio M Selorio Jr.•

Ricardo P Babaran

Received: 24 December 2010 / Accepted: 30 August 2011 / Published online: 10 November 2011

Ó The Japanese Society of Fisheries Science 2011

Abstract To understand how early juvenile yellowfin

tuna use the habitat and environment provided by fish

aggregating devices (FADs), fish (19–31 cm FL) implanted

with ultrasonic transmitters into their abdominal cavities

were released in a network of payaos in Panay Gulf, the

Philippines Self-recording receivers were attached to the

anchor ropes of the payaos to detect the presence of the fish

Some aspects of the behavior of juveniles were similar to

those reported in adults One juvenile showed a diurnal

vertical swimming pattern, swam within a limited shallow

range during the nighttime, and dived to deeper waters

during the daytime Two juveniles performed deep dives

over 100 m during payao-to-payao excursion Three

juve-niles showed a diurnal horizontal swimming pattern that

was synchronized In contrast, juveniles stayed \6 days in

the network, shorter than adults No juveniles returned to

the same payao after an interruption of over 24 h It is

suggested that juveniles in this area are just starting to

migrate and are temporarily staying around a payao for a

few days to forage before continuing their migration

Keywords FAD
Juvenile
Payao
Telemetry

Yellowfin tuna

IntroductionPhilippine waters are important regions for yellowfin tunaThunnus albacares stocks because they include spawninggrounds and nurseries from where juveniles start tomigrate when they reach about 30 cm fork length (FL)[1] A fish aggregating device (FAD) called a payao istraditionally used to catch pelagic species includingjuvenile yellowfin tuna in the Philippines A payao is ananchored FAD composed of a bamboo raft, an anchoringrope, a cement anchor, and suspended palm fronds [2].Most telemetry studies on tuna behavior around FADsinvolved adult and large juveniles [40 cm in FL usingtelemetry techniques [3 15] However, no information isavailable on the behavior of early juveniles \40 cm in FL.Moreover, no experiment had been conducted in thePhilippines, except for the work of Babaran et al [16],who studied early juvenile yellowfin tuna around a singlepayao in the Philippines They determined the feasibility

of undertaking a telemetry experiment on early juvenilesand found some behaviors of juveniles that were verysimilar to those of adults Juvenile yellowfin tuna swamwithin a limited shallow range during the nighttime, dived

to deeper waters during the daytime, and then movedaway from the payao at midnight The results also sug-gested that juveniles associate with several payao unitswithin a network, because two juveniles were recapturedsimultaneously at another payao over 3 km away from thereleasing payao

In this study, we present the results of more hensive telemetry studies in the Philippines to record howearly juvenile yellowfin tuna use the habitat and environ-ment provided by a network of payaos in a region whereinformation is inadequate for management of Philippinetuna, which are part of the Pacific stocks

Faculty of Agriculture, Kinki University,

204-3327 Nakamachi, Nara 631-8505, Japan

e-mail: mittsu@nara.kindai.ac.jp

K Anraku

Faculty of Fisheries, Kagoshima University,

4-50-20 Shimoarata, Kagoshima 890-0056, Japan

College of Fisheries and Ocean Sciences,

University of the Philippines Visayas,

Miagao, 5023 Iloilo, Philippines

DOI 10.1007/s12562-011-0431-y

Materials and methods

Installation of receivers

In Panay Gulf, several payaos were installed in the same

network within a few kilometers of each other The

experiments were conducted around nine payaos (P1–9) in

the same network deployed approximately 10 km off the

coast of Miagao in Panay Island, Philippines (Fig.1) The

depth of the water was approximately 500 m Before the

experiment, we searched all payaos in the network and

marked their positions using global positioning system

(GPS) According to the rich experience gained by local

professional fishermen, 6 payaos with abundant palm

fronds were preselected for attachment of set receivers

(VR2-DEL; Vemco Ltd., Canada) A set receiver was

installed on the payao anchor line at depth of

approxi-mately 20 m by scuba diving The receiver decodes the ID

numbers and swimming depths of fish implanted withtransmitters within the detection zone and records theinformation and time stamp in flash memory A previousstudy determined the detection zone to be at least 500 m inradius [16] In case of poor fishing, receivers were rein-stalled at other payaos Receivers were installed at P1–6 on

30 August, but those attached to P1–3 were reinstalled atP7–9 on 5 September 2006 because of poor fishing.Fishing and tagging

A professional fisherman captured experimental fisharound the payaos at depth of approximately 30 m by handline Sixteen yellowfin tuna (YT01–16, 19–31 cm FL) werecaptured from 1 to 20 September 2006 Juvenile yellowfintuna in the Western and Central Pacific Ocean grow up toabout 50 cm FL per annum and mature at about 120 cm FL[17,18] Tagged juveniles are estimated to be age 0? andimmature The details of each fish are given in Table1.The fish were implanted with coded ultrasonic transmitters(V9P-2H-S256 or V7-2L-R256; Vemco Ltd.) The V9Ptransmitter, which has a pressure sensor, weighs 2.9 g inwater and measures 9 mm in diameter and 46 mm inlength This transmitter emits a train of eight pings atoutput power of 147 dB every 40 ± 20 s for identificationand depth measurement [19] The accuracy of the pressuresensor is ±10 m The battery life is 64 days The V7transmitter, which does not have a pressure sensor, weighs0.9 g in water, has diameter of 7 mm, and measures 20 mmlong This transmitter emits a train of six pings at outputpower of 136 dB every 80 ± 40 s only for identification.The battery life is 48 days Because our previous studyindicated high fishing pressure [16], 3 fish tagged with V7transmitters were also implanted with data loggers (DSTmicro; Star-Oddi Ltd., Iceland) to record time-series data

of swimming depth and body temperature in expectation ofrecapture The data logger weighs 1.9 g in water andmeasures 8.3 mm in diameter and 25.4 mm in length Theaccuracy of the temperature and pressure sensor is ±0.2°Cand ±1.5 m, respectively The sampling interval was set at

1 min Tag implantation was conducted according to theprocedure described by Babaran et al [16] The surgicaloperations were performed just after catching the fish andtook \90 s for each fish The fish were released immedi-ately after the operation To facilitate retrieval of recap-tured fish, notices of monetary rewards were distributed innearby fishing communities and fish markets

Measurement of ambient water temperatures

To understand the horizontal distribution of water perature, data loggers (DST milli; Star-Oddi Ltd.) wereattached to the receivers at P1–3 or P6–8 and the ambient

tem-122.0

10.0

11.0

122.5 121.5

20km

NegrosIsland

1km

(b)

lines are isobaths per 1000 and 500 m, respectively The black star

indicates the recapture point of YT10 b Formation of payaos in the

network Dotted circles indicate the approximate detection distance of

receivers attached to payaos, 500 m in radius

water temperatures were recorded every 10 min To

understand the vertical profiles of water temperature, a data

logger (DST milli) was submerged from the surface to

300 m depth at P3, recording water temperature every

10 m, at the beginning of the experiment on 31 August

2006 The accuracy of the temperature and pressure sensor

is ±0.1°C and ±1.2 m, respectively

Data analysis

Detection rate as defined by Ohta and Kakuma [13] was

calculated to relate the swimming patterns of juvenile

yellowfin tuna with known patterns of adult yellowfin tuna

They suggested five patterns of adult yellowfin tuna

asso-ciated with FADs Pattern A was characterized by a higher

detection rate during nighttime than daytime, while

pat-tern B was characterized by a higher detection rate during

daytime than nighttime Meanwhile, pattern C was

char-acterized by a few hours absence at around sunset, and

pattern D was characterized by several hours absence

beginning around noon Pattern E was assigned to

indi-viduals that showed no clear pattern Hourly detection

numbers were divided by the maximum number of

trans-missions determined by the average transmission interval

of each transmitter Continuous residence time (CRT) and

intermittent residence time (IRT) were also calculated

CRT indicates the duration for which a tagged fish was

continuously monitored at a single payao without absence

over 24 h, whereas IRT indicates the duration for which a

tagged fish stayed within the network of payaos To

examine diurnal swimming patterns, daytime was defined

from sunrise to sunset at 10.5°N, 122.0°E When more thanone receiver detected a tagged juvenile fish, its horizontalmoving speed was roughly estimated using the middlerecording time of each receiver and the distance betweeneach receiver Vertical moving speed during both ascentand descent was also roughly estimated using differences

of time and depth between successive time-series datapoints Statistical analyses were computed using Statcel2statistical software (OMS, Japan) Values are presented asmean ± standard error (SE)

ResultsPayao-associated behavior of juvenileP4 was destroyed, possibly due to bad weather, but theattached receiver was salvaged by a fisherman on 7 Sep-tember P5 disappeared because of the appearance of a 15thtyphoon in 2006 (Xangsane) on 25 September, and norecords were obtained The receivers at P6–9 remaineduntil 7 October

YT01–05 were released between 09:35 and 09:55 on 2September at P9 When these fish were released, noreceiver was installed at P9 yet When a receiver wasreinstalled at 13:10 on 5 September, YT01–04 were notdetected and remained missing until the end of the exper-iment CRTs and IRTs were unknown YT05 was released

at 09:55 on 2 September at P9 and detected by the stalled receiver at P9 from 13:10 on 5 September soon after

rein-it was reinstalled until 13:46 on 7 September The fish was

(days)

IRT (days)

monitored continuously without any interruption over 24 h

until the end of recording CRT or IRT was \6 days

because the behavior until 5 September was unknown and

then the fish was not detected by any other payao

YT06–10 were released at 06:24–07:06 on 13

Septem-ber at P5 Unfortunately, P5 disappeared because of a

typhoon on 25 September and no records were obtained

YT06 and YT07 were missing until the end of the

exper-iment CRTs and IRTs were unknown YT08 was released

at 06:42 on 13 September at P5 and detected at P8 from

00:43 to 01:18, at P9 from 01:38 to 01:46, and at P6 from

02:08 to 02:12 on 14 September CRT was \1 day IRT

was also \1 day because the fish was detected by the

payaos within 24 h Horizontal moving speed was

esti-mated as 44 cm/s (1.8 FL/s) between P8 and P9 and as

83 cm/s (3.3 FL/s) between P9 and P6 Figure2shows the

time-series data of the swimming depths of YT08 during

the excursion YT08 was swimming close to the surface

until 5 min before leaving P8 Suddenly, the fish dived to

deeper waters over 90 m with maximum vertical moving

speed of 76 cm/s (3.0 FL/s) then swam out of the detection

zone of P8 When the fish was detected by P9 and P6 again,

it continued to swim in relatively deep waters of about

60 m YT09 was detected by P7 from 15:30 to 15:47 on 13

September At that time, the fish was swimming in deeper

waters ranging from 195 to 207 m CRT and IRT were

\1 day, the same as YT08 YT10 was also missing but was

recaptured by a ring netter on 24 September at another

southern payao over 60 km away CRT and IRT were

unknown

YT11–14 were released in rapid succession between

07:38 and 07:51 on 13 September at P7 YT11 was released

at 07:38 on 13 September at P7 and continued to swim in

the detection zone of P7 from time of release until 10:05 on

15 September The fish was monitored continuously

without interruption over 24 h during the recording period.CRT was 3 days IRT was also 3 days because the fish wasnot detected by any other payao There was no difference

in the hourly detection rate during daytime and nighttime(Mann–Whitney test, P [ 0.05) Figure3 shows the time-series data of the swimming depth of YT11, whichexhibited a diurnal vertical swimming pattern The fishfrequently stayed in a relatively shallow and narrow layerbetween 5 and 10 m during the nighttime; in daytime, thefish swam at a wider depth range between 10 and 40 m(Fig.4) The fish swam in significantly deeper waters(24.8 ± 0.2 m, n = 1833) during daytime than nighttime(6.7 ± 0.1 m, n = 1738) (Mann–Whitney test, P \ 0.01).The fish swam vertically at significantly higher speed(12.1 ± 0.2 cm/s, n = 1832) during daytime than night-time (2.3 ± 0.1 cm/s, n = 1737) (Mann–Whitney test,

P\ 0.01) There was no difference in the vertical movingspeed between ascent and descent during either daytime ornighttime (Mann–Whitney test, P[ 0.05) YT12–14showed a diurnal horizontal swimming pattern YT12 wasreleased at 07:44 on 13 September at P7 and stayed in thedetection zone of P7 from release to dusk on 13 September.Then, the hourly detection rate declined at dusk andremained low until dawn (05:09) on 14 September Thehorizontal moving pattern continued for 2 days Finally,the fish left the payao at 08:08 on 16 September CRT was

4 days because the interruptions were \24 h IRT was also

4 days, because the fish was not detected by any otherpayao YT13 was released at 07:48 on 13 September at P7and also showed the diurnal pattern for 4 days and left thepayao at 16:47 on 17 September CRT and IRT were

5 days YT14 was released at 07:51 on 13 September at P7and showed the diurnal pattern for 3 days, leaving thepayao at 08:53 on 16 September CRT and IRT were

4 days There were significant differences in the hourly

payao-to-payao excursion Horizontal double-headed arrows indicate the

recording duration by each payao

juvenile swam within a limited shallow range during nighttime (indicated by a horizontal black bar) and dived to deeper waters during daytime (white bar)

detection rates of YT12–14 during daytime and nighttime

(Mann–Whitney test, P \ 0.01)

YT15 was released at 08:17 on 20 September at P9 The

fish was not detected from right after its release until the

end of the experiment, probably because the transmitter

was broken CRT and IRT were unknown

YT16 was released at 08:52 on 20 September at P6

The fish also showed a diurnal horizontal swimming

pattern The fish stayed in the detection zone of the

payao from release to dusk on 20 September The next

morning, the fish returned to P6 and was recaptured by a

ring netter CRT and IRT were [1 day because the fish

might have stayed around the payao without the

recapture

Payao-associated pattern of juveniles

Figure5shows the sufficiently long time-series data of the

hourly detection rate of fish that were detected by some

receivers YT05 and YT11 were monitored continuously

without interruption during the recording period, being

assigned to pattern E, i.e., no clear pattern YT12–14

showed a diurnal horizontal swimming pattern that ched pattern B, characterized by a higher detection rateduring daytime than nighttime YT16 also showed a patternsimilar to that of YT12, and YT14 matched pattern B.Ambient water temperature

mat-Water temperatures recorded by data loggers attached tothe receivers were stable in the range from 28°C to 29°C.There was no remarkable difference in temperature at thelocation of payaos during the recording period The ver-tical profile of water temperature was stratified as shown

in Fig.4 From the surface to 120 m depth, water perature gradually decreased from 28°C to 24°C Athermocline existed from 120 to 160 m, where the watertemperature declined from 24°C to 17°C From 160 to

tem-300 m, the water temperature decreased moderately from17°C to 13°C

DiscussionHorizontal movement around a payao after releaseYT05 and YT11 were assigned to pattern E Babaran

et al [16] also assigned an individual to pattern E andmentioned the possibility that such observations of juve-nile yellowfin tuna may be just one of several associationpatterns YT12–14 showed a diurnal horizontal swimmingpattern that matched pattern B Holland et al [3] alsoreported a diurnal pattern of adult yellowfin tuna andindicated feeding excursions In addition, as all the taggedfish were captured by hand line, the presence of prey was

an important factor in the association of juvenile yellowfintuna with a payao Small fish usually make a school toavoid predators and to find prey effectively [20] There-fore, as they were caught in rapid succession around thesame payao, they were probably swimming together in thesame school Babaran et al [16] suggested that juvenileswere swimming around a payao in the same school.Klimley and Holloway [8] reported the school fidelity andhoming synchronicity of adult yellowfin tuna WhileYT12 and YT14 left P7 in the morning on 16 September,YT13 only remained until the evening on 17 September.More observations are needed to reveal the schoolingbehavior

All tagged fish left the payao where they were released

in daytime Babaran et al [16] reported an individual thatleft the payao in the middle of the night Departure fromthe tagging site at nighttime has also been reported in adultyellowfin tuna [3] These observations of juvenile yellow-fin tuna may be just some among the alternative payao-associated patterns

YT11 and vertical profile of ambient water temperature The juvenile

was swimming in significantly deeper waters during the daytime A

thermocline existed from 120 to 160 m where the water temperature

declined from 24°C to 17°C

Vertical movement around a payao

YT08 and YT09 were released at P5 Unfortunately, P5

dis-appeared because of a typhoon, so we do not know the time of

their departure from P5 due to the lack of the receiver

However, YT08 and YT09 were detected by other payaos

YT08 was detected near 3 other payaos at nighttime,

swim-ming in deep waters over 90 m YT09 was also detected by

another payao and reached deeper swimming depths of over

200 m Holland et al [3] also reported deep diving of adult

yellowfin tuna during FAD-to-FAD excursion

Adult yellowfin tuna is a deep diver Dagorn et al [14]

reported an adult individual that dived over 1000 m,

experiencing a minimum temperature of 5.8°C,

represent-ing a difference of over 20°C in ambient water

tempera-ture From the measurements of ambient water

temperature, there was no remarkable difference

horizon-tally Vertically, a thermocline existed at depth ranging

from 120 to 150 m In the case of YT09, the coldest

temperature was 14°C and the difference of ambient water

temperature was 14°C Dickson [21] revealed that 20.7 cm

FL tunas can elevate their body temperatures significantly

above ambient water temperature by using vascular

coun-tercurrent heat exchangers YT09 was the largest fish in

this experiment and 31 cm FL was larger than 20.7 cm to

maintain body temperatures elevated above ambient water

temperature

YT11 stayed around the releasing payao and did notshow such deep dives The fish demonstrated the diurnalvertical swimming pattern in the surface mixed layer Josse

et al [6] indicated the important role of the sound tering layer (SSL), assimilated as food, in vertical andhorizontal tuna movements, during daytime and nighttime.Babaran et al [16] also suggested the possibility of coin-cidence of swimming depth of juveniles and prey In thisstudy, YT11 demonstrated a diurnal vertical swimmingpattern, as if following the diurnal migration patterns ofprey organisms in the SSL All tagged fish were capturedduring the daytime by hand line at depth of approximately

scat-30 m, which coincides with the swimming depth of YT11during daytime As mentioned above regarding horizontalmovements, presence of prey is an important factor in theassociation of juvenile yellowfin tuna with a payao,because they are at a life stage in which they need to growquickly

Moving speedThe estimated horizontal moving speed of YT08 in thenetwork of payaos was between 1.8 and 3.3 FL/s Holland

et al [3] tracked four adult yellowfin tuna, averaging2.1 FL/s Meanwhile, Yuen [22] observed that yellowfintuna averaging 51.9 cm long swam at 0.5 to 14.4 BL/s.Dewar and Graham [23] calculated the optimal swimming

100 0

100 0 0:00

hourly detection rates of each

juveniles a YT05 at P9, b YT11

at P7, c YT12 at P7, d YT13 at

P7, e YT14 at P7, and f YT16 at

P6 ‘‘R’’ indicates recapture

velocity of 51 cm yellowfin tuna as 2.0 FL/s Dewar and

Graham [24] calculated the maximum swimming velocity

of 40 cm yellowfin tuna at 25°C as 27 FL/s The horizontal

moving speed of YT08 was within these ranges The

maximum vertical moving speed at 76 cm/s (3.0 FL/s)

demonstrated by YT08 was also within these ranges

YT08 moved horizontally at nighttime YT12–14 and

YT16 also showed a diurnal horizontal swimming pattern,

assuming that they stayed near a payao during daytime and

away from the payao at nighttime, while YT11 stayed in

the detection zone all day and moved vertically at

signifi-cantly higher speed during daytime than nighttime High

horizontal moving speed at nighttime and high vertical

moving speed during daytime might be related to presence

of prey as mentioned above

Associative behavior of juvenile with a network

of payaos

No tagged juveniles returned to the same payao after an

interruption over 24 h YT08 and YT09 moved to other

payaos but within 24 h and did not stay around any payao in

the network So, we could not distinguish between CRT and

IRT clearly Ohta and Kakuma [13] reported that adult

yellowfin tuna stayed around a single payao for a maximum

of 55 days, while Dagorn et al [14] reported a maximum

duration of 151 days with a network of FADs In those two

studies, some individuals returned to the same FAD after an

interruption over 24 h In the current study, not only CRTs

but also IRTs were \6 days for all juveniles These results

suggest that the juveniles were probably starting to migrate

In this study, we revealed some aspects of the behavior

of juvenile yellowfin tuna in a network of FADs in the

Philippines Growing juveniles might stay around a payao

for a few days, forage in a school, and then continue with

their migration while associating with other payao

net-works The distribution of horizontal water temperature is

stable and does not seem to limit the horizontal movement

of juveniles However, the vertical distribution of water

temperature may limit the vertical movement of juveniles

in the surface mixed layer during association with a payao

except during excursion between payaos Unlike other

regulated regions such as Okinawa and Hawaii waters, the

actual numbers and positions of payaos and networks were

not clarified in this region Additional comprehensive

studies are needed to reveal the behavior of juveniles

swimming among numerous networks of payaos for the

management of Pacific yellowfin tuna stocks A juvenile

with data logger attached was not recaptured in this study,

but a relatively high recapture rate (12.5%, two of sixteen)

resulting from high fishing pressure will provide an

opportunity to clarify the behavior of juvenile yellowfin

tuna in greater detail for a longer period in a larger area

Mr Michael Morit and Pepe Severino Jr., for all help in the ments around the payaos This study was conducted as part of a 10-year collaboration between the Japan Society for the Promotion of Science (JSPS) and the Department of Science and Technology (DOST) of the Philippines This study was also partly supported by the 21st Century COE program and Global COE program of the Ministry of Education, Culture, Sport, Science, and Technology, Japan.

experi-References

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G, Itano DG, Aumeeruddy R, Girard C, Million J, Fonteneau A

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O R I G I N A L A R T I C L E Fisheries

Performance of a conical jellyfish exclusion

device installed in a trawl net

Chang-Doo Park•Kyounghoon Lee •

Seong-Hun Kim• Yasuzumi Fujimori

Received: 19 October 2010 / Accepted: 12 September 2011 / Published online: 7 December 2011

Ó The Japanese Society of Fisheries Science 2011

Abstract Recently, the increasing population of giant

jellyfish Nemopilema nomurai has negatively affected

coastal fisheries in Korea As a result, the fishing industry has

begun developing devices to prevent jellyfish capture In this

study, we assessed the performance of a conical jellyfish

exclusion device in the coastal areas of Yokji Island in

southern Korea during 2009 After hauling, we measured the

length, diameter, and weight of the jellyfish and fish that were

captured by the cod end and cover net We found that the

captured species included N nomurai, silver croakers

Pennahia argentata, yellow croakers Larimichthys

polyac-tics, shotted halibut Eopsetta grigorjewi, largehead hairtails

Trichiurus lepturus, and melon seeds Psenopsis anomala

The catch ratios of the giant jellyfish that entered the cod end

in terms of weight ranged from 0.005 to 0.027 In contrast,

the catch ratios of total fish in terms of weight and number

were 0.793 and 0.835, respectively On selectivity analysis

of a conical separator for individual fish species, their

exclusion ratios were independent of their length, and were

similar to their observed exclusion ratios in terms of number

These results indicated that the conical jellyfish exclusion

device performs well; however, some improvements are

needed to minimize the escape of fish from the net

Keywords Jellyfish Nemopilema nomurai  Jellyfish

exclusion device Selectivity  Separator  Trawl

IntroductionThe recent increase in the population of jellyfish, especiallygiant jellyfish Nemopilema nomurai and moon jellyfishAurelia aurita, in the coastal waters of Korea and Japan hasnegatively affected the fishing industry Jellyfish areundesirable catches because they have poison-filled ne-matocysts on their tentacles, which they use to stingpotential predators, and they are relatively heavy since theyare approximately 95% water [1,2]

In general, jellyfish move both horizontally and cally within the water column, and they tend to moveupward during the day and downward during night; forexample, giant jellyfish Nemopilema nomurai can movefrom the water surface to depths up to 176 m [3, 4].However, this species is most often found at depth of 40 m

verti-in a warm, low-salverti-inity water column [5] The averageswimming speed of giant jellyfish is 0.11 m s-1, and most

of them grow until their bell diameter and weight are 2 mand 200 kg, respectively [6,7] Due to these characteristics

of jellyfish, their unwanted capture causes many negativeeffects, including damage to fishing nets due to theirweight, increased water resistance and exclusion of desiredfish due to their large size, reduction of the commercialvalue of captured fish due to their toxin, and increasedwork for and health and safety risks to fishery laborers whohave contact with jellyfish to sort them from other fish [8,

9] Furthermore, jellyfish damage affects almost all fishingequipment, including trawls, Danish seines, set nets, pairtrawls, stow nets, shrimp beam trawls, and gillnets [10].Because of the high cost and risk associated with jel-lyfish damage, the operations of many fisheries in Koreaare limited by jellyfish conditions To overcome thisproblem, the fisheries industry has begun developingdevices that effectively prevent jellyfish capture by

Fisheries System Engineering Division, Fundamental Research

Department, National Fisheries Research and Development

Institute, Busan 619-705, Korea

e-mail: cdpark1@nfrdi.go.kr

Y Fujimori

Graduate School of Fisheries Sciences, Hokkaido University,

3-1-1 Minato, Hakodate, Hokkaido 041-8611, Japan

DOI 10.1007/s12562-011-0416-x

separating and releasing them from fishing nets Several

recent studies have recommended using a combination of

an exclusion device that is attached to the trawl nets and an

interception net at the entrance of the set net [10–13]

Jellyfish exclusion devices use a sloping panel, which

consists of square mesh netting, diamond mesh netting, or a

metal grid panel, in front of the cod end to separate jellyfish

from trawl nets The efficacy of these devices to exclude

jellyfish and limit fish escape depends on their shape and

material composition Although jellyfish exclusion devices

have a higher exclusion rate and a lower fish escape rate

than many other exclusion devices, further improvement is

needed to increase their efficacy and ease of use in

com-mercial fisheries

Recently, fishermen who use stow net fishery (FAO

Home: http://www.fao.org/fishery/geartype/227/en

acces-sed 04 May 2011) on the western coast of Korea have

developed and adopted a jellyfish exclusion device that

uses a conical separator, which is formed from an

isos-celes triangular or trapezoidal piece of netting This

cone-shaped separator works similarly to sieve nets that

are inserted in shrimp trawls to direct unwanted bycatch

to an escape hole under the body of the trawl net [14,

15] Although the stow net is one of the fixed gears

using tidal current, it appears to have almost the same

characteristics as the trawl net hydrodynamically In this

study, we assessed the performance of a jellyfish

exclusion device that uses a conical separator in a trawl

net

Materials and methods

Experimental net

Our jellyfish exclusion device (0.86 m depth 9 1.42 m

width 9 8.50 m length), including extension and conical

separator with the outlet, was attached between the body

and cod end of the experimental trawl net, which was

43.12 m long (Fig.1) The cone-shaped separator was

connected to the inside of the cylindrical extension

(Fig.1) When the jellyfish is in contact with the net

panel, it is more likely for jellyfish to be cut due to its

weight and momentum as the twine diameter becomes

thinner or the mesh size becomes larger To decrease the

entering of jellyfish fragments cut by the netting of the

separator to the cod end, we used thick twine with

diameter of 8 or 5.5 mm for the netting of the separator

and designed the length of the separator to be as long as

possible to minimize the slope angle of the upper line

In addition, the 2-m-long outlet was placed parallel to thewater flow on the bottom panel of the extension, similarly to in

a conventional stow net (Fig.2), so that jellyfish would urally move toward the outlet due to water resistance Thedistance between the outlet and the sea bed was designed to beapproximately 0.7 m For the experiment, we attached a covernet [14,15], which has a 50 mm mesh opening as the samemesh opening as the cod end has, to the outside of the outlet tocollect any jellyfish or fish that escaped through the outlet(Fig.2) Several sinkers were also attached to the bottom side

nat-of the cover net to prevent the masking effect caused byoverlap of the cover net with the outlet during towing [16–18].Sea trials and measurements

We investigated the performance of this conical jellyfishexclusion device on the R/V Tamgu 3rd (GT369) nearYokji Island in Korea for 7 days in September 2009 Wetrawled for approximately 30 min at speed of 4 knots anddepth of approximately 40 m to maximize exposure of thedevice to giant jellyfish

Lacing line PP rope dia 24 mm, Total length 8.5 m, Outlet length 2 m

(72 mesh) (46 mesh)

Separator : PE net

that was inserted between the cod end and aft body of the experimental trawl net

After hauling, we measured the length and weight of the

fish as well as the bell diameter and weight of the jellyfish

that were caught by the cod end and cover net with a

tapeline and a spring type balance However, we only

measured the weight of the fragments of jellyfish that

entered the cod end

Catch ratio

We calculated the catch ratio (Eq.1) of jellyfish as the ratio

of the catch weight in the cod end to the total catch weight

of the cod end and cover net combined

Rc¼ catch in cod end= catch in cover netð

In addition, we calculated the catch ratio for fish in

terms of both catch weight and number of fish

Selectivity analysis

In general, meshed fishing gear allows selective capture of

a specific species or a certain range of fish sizes When the

target fish encounter the separator net, some of them pass

through the separator net into the cod end, while the rest

escape through the outlet, which plays the role of an

exclusion vent In this study, we defined the selectivity of

the separator net (Eq.2) as the exclusion ratio of fish with

body length l through the outlet [14]

Re¼ 1  Rc

¼ catch in cover net=ðcatch in cover net

We modeled selectivity as a logistic function (Eq.3),

such that the exclusion ratio increases with fish length l, or

a constant function (Eq.4), which is independent of fish

length l:

Logistic function : Re(lÞ ¼ expða þ blÞ=½1 þ expða þ blÞ;

ð3Þ

where a and b are logistic function parameters, and c is a

constant

The parameters were estimated by using the likelihood method [16,19–21], and Akaike’s informationcriterion (AIC) values (Eq.5) were used to choose the bestfit model [22–24]:

where MLL is the maximum log-likelihood and q is thenumber of parameters Smaller AIC value indicates betterfit of the model

Since giant jellyfish in the cod end were mostly mented, the bell diameter of the original jellyfish could not

frag-be measured, so we could not quantify the selectivity of theseparator for jellyfish in this study

Statistical analysis

We used the Kolmogorov–Smirnov test to determine tistically significant differences in distributions of thelengths of fish that were captured in the cod end with thosethat were caught in the cover net The level of significance(a) was defined to be 0.05

sta-ResultsCatch composition and ratios

As shown in Tables1 and 2, we caught giant jellyfishand various fish species by using our experimental fish-ing gear with the conical jellyfish exclusion device Asexpected, giant jellyfish were the predominant species ofjellyfish Poorly marketable fish species such as Kammalthryssa Thryssa kammalensis are included in ‘‘others’’ inTable2

The catch ratios of the fish and giant jellyfish thatwere caught in the cod end and cover net are summa-rized in Tables1 and 2 The catch ratios of giant jel-lyfish that entered into the cod end in terms of weightranged from 0.005 to 0.027, whereas the value for thetotal was 0.010 As a result, the jellyfish exclusiondevice separated and excluded 99% of the giant jellyfishthat entered the trawl net However, we found somedifferences in the catch ratio of the individual fish spe-cies that were caught in the cod end; the catch ratiosbased on the number or weight of fish were 0.0–1.0,whereas the catch ratios based on the total number orweight were 0.835 and 0.793, respectively

Size distributionThe bell diameter of the giant jellyfish that were caught inthe cover net and cod end ranged from 25 to 95 cm, and themaximum wet weight was 31 kg (Figs.3,4) The weight of

giant jellyfish whose bell diameter was measured

corre-sponded to 42.8% of their total weight caught in the

experimental net When giant jellyfish grows to 2-m bell

diameter, the calculation is done by using the regressionequation given in Fig 4to give its weight, being approx-imately 230 kg

ratios of giant jellyfish caught in

the experimental fishing gear

with the conical jellyfish

exclusion device

catch weight in the cod end to

the total catch

ratio

(kg)

(kg)

Japanese Spanish

mackerel

Scomberomorusrus niphonius

yokohamae

The length distributions of fish species that had more

than 50 individuals in the cod end are shown in Fig.5 A

slight difference was observed between the length ranges

of fish that were caught in the cod end and cover net

(Fig.5) However, it should be noted that we could not

compare the length distributions of yellow croakers in the

cod end and cover net because we did not catch any yellow

croakers in the cover net The Kolmogorov–Smirnov test

was used to compare the catch length distributions for each

fish species in the cod end and cover net (Kolmogorov–

Smirnov test, a = 0.05) [24] When the yellow croaker

data were excluded, the differences in the length

distribu-tions of the fish that were caught in the cod end and cover

net were not statistically significant (Table3)

Relationship between length and girth

The maximum total lengths (TLs) of silver croaker and

yellow croaker were 37.0 and 25.0 cm, respectively By

using regression analysis [25] of the maximum length and

girth (G) of fish, we determined that the girths applicable to

the maximum TLs of these fish were 24.4 and 13.9 cm,respectively Similarly, the maximum anal length (AL) oflargehead hairtail and the maximum fork length (FL) ofmelon seed were 28.0 and 21.0 cm, respectively, corre-sponding to girths of 11.4 and 20.1 cm, respectively [25].From the other fishing results carried out in the sameground and season using trawl net, their regression equationsand coefficients of determination for redwing searobin,marbled sole, and shotted halibut were G (cm) =0.3982TL - 4.533 (R2= 0.8025), Bd (cm) = 0.3837TL

- 0.184 (R2= 0.9171), and Bd (cm) = 0.435TL - 1.064(R2= 0.9978), respectively, where Bd is the body depth offlat fish For these fish, their maximum TLs were 35.0, 39.0,and 23.0 cm, respectively, corresponding to girth of 18.5 cmand body depths of 15.1 and 8.9 cm, respectively

The standardized relative girth (G/P), which is theratio of body girth (G) to the mesh perimeter (twice themesh opening, P), is a primary factor of contact selec-tion [21, 25, 27] When the fish encountered the400-mm-mesh front panel (mesh opening, 360 mm) ofthe separator, the relative girths applicable to the maxi-mum lengths of silver croaker, yellow croaker, redwingsearobin, largehead hairtail, and melon seed were 0.340,0.193, 0.257, 0.159, and 0.280, respectively For flat fishsuch as marbled sole and shotted halibut, the corre-sponding ratios of body depth to mesh opening, which issimilar to the relative girth, were 0.421 and 0.248,respectively When the fish encountered the rear panel(mesh opening, 188 mm) of the separator, the relativegirths applicable to the maximum lengths of fish wereslightly less than twice the corresponding values for thefront panel, because the mesh opening of the rear panelwas larger than half of that of the front panel

Parameter estimates and model selectionThe parameter estimates of the models for the fish speciesthat were relatively commonly caught in the cover net(silver croaker, largehead hairtail, melon seed, redwingsearobin, and Japanese Spanish mackerel) are shown inTable4 Length classes with zero catch were eliminatedfrom the calculation of selection curves In addition, theselection curve, which was calculated from both theobserved values and the estimated parameters, is shown inFig.6 Likelihood ratio statistic values, which are twice thelog of the likelihood ratio between the full and currentmodels, were calculated to test the goodness of fit for eachmodel (Table4) [20,24] There was no evidence of a lack

of fit in either model The AIC value of the constantselectivity model tended to be slightly less than that of thelogistic selectivity model Therefore, we chose the constantfunction as the best-fit model of the selectivity of theseparator net for each fish species In this model, the

net of the experimental trawl net

selectivity of the separator for silver croaker, largehead

hairtail, melon seed, redwing searobin, and Japanese

Spanish mackerel was 0.185, 0.058, 0.292, 0.247, and

0.238, respectively (Table4) These values were similar tothe observed exclusion ratios of individual numbers for fishspecies (Table2)

0 100 200 300 400 500

Total length (cm)

Codend Cover net

Pennahia argentata

0 20 40 60 80 100 120

Anal length (cm)

Codend Cover net

Trichiurus lepturus

Total length (cm)

Codend Cover net

Lepidotrigla microptera

0 10 20 30 40 50

Total length (cm)

Codend Cover net

Scomberomorusrus niphonius

0 10 20 30 40 50 60 70

Fork length (cm)

Codend Cover net

Psenopsis anomala

0 8 16 24 32

Total length (cm)

Codend Cover net

Eopsetta grigorjewi

0 8 16 24 32

0

8 4

12 16 20

Larimichthys polyactis

the various fish species caught

in the experimental trawl net

Our results showed that the exclusion ratio of our conical

jellyfish exclusion device for giant jellyfish in terms of

weight (0.99) is higher than that of any other previously

reported device For example, Matsushita et al [11]

reported that a grid jellyfish excluder device (grid spacing,

0.18 m; fragment size of jellyfish C28 9 26 9 5 cm3),

which was designed for towed fishing gear, had an

exclu-sion ratio of 0.89 for giant jellyfish in terms of weight

Similarly, Okino et al [12] showed that the exclusion ratio

for a jellyfish excluder device with an intercepting net

(mesh size 400 mm; jellyfish bell diameter 70–90 cm)

ranged from 0.40 to 0.74 in terms of weight These results

suggest that our conical jellyfish exclusion device

effec-tively excludes giant jellyfish

The results of Kim et al [10] for the same ground in

July 2004 showed that, on increasing the tilt angle of the

square mesh separator panel by 10°, 15°, or 20°, the escape

ratios of fish in terms of weight were 0.49, 0.51, and 0.56,

respectively, whereas the jellyfish exclusion ratios in terms

of weight were 0.66, 0.41, and 0.44, respectively Thus, a

smaller tilt angle tends to increase the jellyfish exclusion

ratio but decreases the exclusion ratio of fish As a result,

we used a relatively small slope angle in this study (5.8°)

We believe that this small slope angle, which was smallerthan any angle previously reported for a separating net orgrid panel, also contributed to the high performance of ourconical jellyfish exclusion device

In the present study, the exclusion ratios based onnumber for Japanese jack mackerel, melon seeds, yellowcroakers, largehead hairtails, and shotted halibut were0.043, 0.282, 0.000, 0.052, and 0.047, respectively Thesevalues tended to be less than those reported by Kim et al.[10], which were 0.94, 0.91, 0.89, 0.74, and 0.00, respec-tively In addition, Okino et al [12] showed that theexclusion ratio of shotted halibut in terms of number was0.123, which is higher than the value that we determined inour study However, these differences might have been due

to differences in the position of the outlet in the jellyfishexclusion device in our study compared with those in otherstudies; the outlet of our conical jellyfish exclusion devicewas located on the bottom panel of the net, while the outlet

of other exclusion devices was located on the top panel[10–12] The different effects of the position of the outlet

on the exclusion ratio may have been influenced by the fish

microptera

Scomberomorusrus niphonius

Logistic Re(l) = exp(a ? bl)/[1 ? exp(a ? bl)]

Constant Re(l) = constant

swimming behavior in different parts of the net [26],

because fish swimming near an outlet may escape from it

more easily than from a more distant outlet A comparison

of our exclusion ratios with those of Kim et al (2008)

suggested that Japanese jack mackerel, melon seeds,

yel-low croakers, and largehead hairtails swim near the top

panel of the net On the other hand, our exclusion ratios of

ocellate spot skate and olive flounder suggested that these

species usually swim near the seabed

Although a 100% catch ratio of fish in the trawl net

would be ideal, it is not possible with the exclusion device

because both fish and jellyfish can escape from the

exclu-sion device Initially, we thought that the fish that escaped

from the outlet were larger than the fish that were caught in

the cod end net, because the fish had to pass through the

separator, which may limit the size of the fish that are

captured in the cod end However, the absence of any

statistically significant differences in the distribution oflengths of fish between the cod end and cover net suggestedthat the separator used in the experiment did not limit thesize of the fish that were captured in the cod end or escapedfrom the exclusion device

Mesh size selectivity for fishing gear is usually sed as a function of fish girth [20,21,25,27] The selec-tivity of fishing gear starts at the range where the relativegirth, the ratio of girth to mesh perimeter, is greater than0.5 [20, 25,27] In the case of fish with cylindrical bodysuch as conger or hagfish, the selection probability occursnear one in the relative girth [20] Except for JapaneseSpanish mackerel without girth data, the relative girthsapplicable to the maximum lengths of most fish in thisstudy were less than 0.5, when the fish encountered the400-mm-mesh front panel of the separator In the case ofthe 200-mm-mesh rear panel of the separator, the relative

expres-0 0.1 0.2 0.3 0.4 0.5

Total length (cm)

Observed Constant

Pennahia argentata

0 0.1 0.2 0.3 0.4

Anal length (cm)

Observed Constant

Trichiurus lepturus

0 0.1 0.2 0.3 0.4 0.5

Fork length (cm)

Observed Constant

Psenopsis anomala

0 0.2 0.4 0.6 0.8

Total length (cm)

Observed Constant

Lepidotrigla microptera

0.2 0.4 0.6 0.8

Scomberomorusrus niphonius

separator net for each fish

species

girths applicable to the maximum lengths of silver croaker

and melon seed were slightly greater than 0.5 Also, the

ratio of body depth to mesh opening for marbled sole was

greater than 0.5 This indicates that most of the fish entered

through the separator to the cod end, because the contact

selection probability based on the relative girth is 0 when

they encountered the 400-mm-mesh front panel of the

separator When the silver croaker, melon seed, and

mar-bled sole encountered the 200-mm-mesh rear panel of the

separator, some large-sized individuals may not have been

able to pass the separator due to its contact selection

However, our results showed that some portion of each fish

species except for yellow croaker were excluded through

the outlet (Table2) Moreover, the difference in the length

distributions of silver croaker and melon seed that were

caught in the cod end and cover net was not statistically

significant This means that fish were excluded through the

outlet not only by the contact selection of separator but also

by its available selection including their avoidance

behavior [21,28]

In conclusion, our results show that a constant function

is the best-fit model for the selectivity of the separator net

for individual fish species, because their exclusion ratio

was independent of their length In addition, the exclusion

ratios, which represent the selectivity of the separator,

differed among fish species, suggesting that the escape

behavior of fish is species specific However, when giant

jellyfish encountered the conical separator in the jellyfish

exclusion device, they were considered to be excluded by

contact selection of the separator, because there is no

expectation of escape behavior Further research is needed

to elucidate the species-specific behavior of fish shoals

with respect to the mesh size of the separator net in the

jellyfish exclusion device We anticipate that optimized

conical jellyfish exclusion devices will significantly reduce

the negative impact of jellyfish on the fishing industry

Research and Development Institute (grant no RP-2010-FE-022).

The authors are grateful for the support and assistance of the officers

and crew of the R/V Tamgu 3rd and anonymous reviewers for their

insightful comments about this manuscript.

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O R I G I N A L A R T I C L E Fisheries

Biomass fluctuation of two dominant lanternfish Diaphus garmani

and D chrysorhynchus with environmental changes in the East

China Sea

Seiji Ohshimo•Tohya Yasuda•Hiroshige Tanaka •

Chiyuki Sassa

Received: 6 June 2011 / Accepted: 5 October 2011 / Published online: 5 November 2011

Ó The Japanese Society of Fisheries Science 2011

Abstract Acoustic surveys have been conducted for

estimating the biomass of commercially important fish

(e.g., anchovy, jack mackerel), lanternfish (Diaphus

garmani and D chrysorhynchus), and pearlside

(Mauroli-cus japoni(Mauroli-cus) in summer in the East China Sea (ECS)

since 1997 The biomass of lanternfish and pearlside was

2.26–19.16 times that of commercially important fish, and

these species represented substantial biomass in the ECS

Though there were no correlations between biomass of

pearlside and environmental indices, significant

correla-tions between biomass of lanternfish and southern

oscilla-tion index (SOI) in March (positive correlaoscilla-tion), arctic

oscillation (AO) in March (negative) and October

(posi-tive), monsoon index (MOI) in February (posi(posi-tive), and

Kuroshio flow mass in winter (positive) were observed

Weak AO and strong MOI would cool down the sea

temperature and would lead to increased primary and

secondary production in the ECS, thereby enhancing larval

survival of lanternfish The SOI would affect the Kuroshio

meander in the ECS, and strong SOI and Kuroshio flow

mass would transport larvae of lanternfish to the present

survey area This is the first report on the lanternfish

standing stock and its fluctuation in the ECS

Keywords Acoustic survey  Biomass  Diaphus 

Environmental change

IntroductionMyctophidae (lanternfish) is a dominant family of meso-pelagic fish in the ocean Although mesopelagic fish are notgenerally fished commercially, they represent a substantialbiomass in oceanic waters and are a critical but poorlyunderstood intermediate trophic link between the meso-zooplankton and higher trophic levels including fishes,seabirds, and marine mammals [1 5] Lanternfish (Diaphusspp.) are distributed in the shelf break area in the EastChina Sea (ECS), and pearlside (Maurolicus japonicus) aredistributed in the Tsushima Straight area [6] Generally,stock size of small pelagic fish is assessed by cohortanalysis based on biometric and catch data Because therewas no information on the biological characteristics such asgrowth and maturation of the mesopelagic fish, the stocksize of these mesopelagic fish was unknown based oncohort analysis in the ECS Though biomass indexes ofDiaphus spp and pearlside in the ECS were estimatedbased on acoustic survey [6], biomass of micronekton inthe ECS was not estimated because the target strength ofthe species was not reported Subsequently, Yasuma et al.[7] reported the target strength of Diaphus garmani and

D chrysorhynchus, and therefore the biomass of themicronekton in the ECS could be estimated in the presentstudy The first purpose of this study was to determinewhether the biomass of mesopelagic fish in the ECS issubstantial or not based on the acoustic method

The present survey area is one of the important fisherygrounds for commercial fishes in the ECS, and lanternfishare also distributed in the same area Lanternfish migrate tothe upper or surface sea layer at night and sometimes mixwith pelagic fish in the surface layers [6] Lanternfish feed

on zooplankton and fish larvae (Tanaka, unpublished data,2011) It is assumed that there is species competition

Seikai National Fisheries Research Institute, FRA,

1551-8 Taira-machi, Nagasaki 851-2213, Japan

e-mail: oshimo@affrc.go.jp

DOI 10.1007/s12562-011-0424-x

between mesopelagic fish and small pelagic fish in the

ECS The biomass fluctuations of small pelagic fish, such

as mackerels Scomber japonicus and S australasicus and

sardine Sardinops melanostictus, in the ECS and adjacent

waters were studied, and these fluctuations were affected

by environmental factors [8, 9] Decadal change in

abun-dance of myctophid fish, including Diaphus spp., in the

Kuroshio region was reported, and the abundance was

affected by the Kuroshio flow volume [10] However, there

are no studies of biomass fluctuation of mesopelagic fish in

the ECS It is necessary to understand the mechanisms of

fluctuation of biomass of mesopelagic fish in the ECS for

analyzing the species interactions between commercially

small pelagic fish and mesopelagic fish In this study, the

second purpose was to estimate the decadal biomass of

Diaphus spp and analyze the relationship between biomass

fluctuation and environmental factors

Materials and methods

Acoustic surveys were conducted during August and

September from 1997 to 2010 (14 years), using an EK505 or

EK60 (Simrad, Norway) echo sounder operating at 38 kHz

Vessel speed was about 10 knots, and the echo sounder was

calibrated by using a copper sphere prior to the survey The

survey area was to the north and west off Kyushu in Japan,

covered by parallel transects spaced 10 nautical miles apart

(Fig.1a) Total transect length was about 830 nautical miles

in the present study Midwater trawls were conducted at

night to allocate the daytime acoustic data to each species

group Nighttime sampling was mainly due to the significant

avoidance of the net by the fish during the day

The area back-scattering coefficient (Sa; m2/NM2) for

each 1 nautical mile (NM) was stored on a personal

com-puter The Sa is defined by the following formula:

Sa¼ 4p  18522

Zz2

z1

Sv dz;

where Sv is the spontaneous volume back-scattering

coefficient and z is the depth The Sa values were recorded

at depths from 10 to 250 m in the present study We

cat-egorized the data into each fish species group using

soft-ware BI500 (Simrad, Norway) when fish schools appeared

The depth of schools of mesopelagic fish was about

200 m or more in the daytime, and the other fish, such as

anchovy, sardine, and mackerel, distributed in shallower

layers in the daytime The lanternfish only appeared in the

waters to the west of Kyushu, and pearlside appeared in

the waters of the Tsushima Straight The school shape of

lanternfish (Fig.1b) is horizontally elongated at the

con-tinental shelf break area, allowing distinction between

lanternfish and pearlside on the echogram The fish schools

in the shallower waters could be categorized into twogroups: (1) anchovy Engraulis japonicus, round herringEtrumeus teres, and sardine and (2) jack mackerel Tra-churus japonicus, mackerel, and scad Decapterus spp [11].Ohshimo [6] estimated the biomass index of differentspecies groups including (1) anchovy, round herring, andsardine, (2) jack mackerel, mackerel, and scad, (3) lan-ternfish, and (4) pearlside The biomass index value wasstandardized to cumulative Sa value for each group by totallength of transects of each survey The biomass index (BI)value was calculated as follows:

BI¼

PSai

L ;where i and L represent group and total length (miles) oftransect, respectively Total length of transects was almostthe same in this study

chrysorhynchus and Diaphus garmani (b), and typical echogram of Diaphus spp (c)

We calculated biomass of each species using Sa and

target strength (TS) values for clupeid fish [12], jack

mackerel, mackerel, and scad [13], lantern fish [7], and

pearlside [14] The species caught by midwater trawls were

identified, and each species measured onboard The Sa

value of each species using the catch weight of midwater

trawling was determined as follows:

Sai¼SaPn wi ri

i wi ri ;

where wi and ri represent catch weight of midwater

trawling and TS of species i, respectively Target strength

(in dB) of clupeid fish, jack mackerel, mackerel, and scad,

D garmani, D chrysorhynchus, and pearlside was

calculated as follows:

Clupeid fish [12]: TS = 20 log BL - 64.0

Jack mackerel, mackerel, and scad [13]: TS = 20

log FL - 66.0

D garmani [7]: TS = 34.5 log SL - 83.5

D chrysorhynchus [7]: TS = 30.5 log SL - 96.3

Pearlside [14]: TS = 10.0 log SL - 61.3

where BL, FL, and SL represent the body, fork, and

standard lengths (cm), respectively The riwas calculated

as follows:

ri¼ 4p10ðTSi=10Þ:

The density (q: individuals/NM2) was calculated as

q = Sa/r, and the biomass was calculated using the mean

body weight of each species and area (NM2)

Correlations between the biomass of micronekton

(lan-ternfish and pearlside) and environmental indices were

analyzed, in order to verify whether environmental factors

affect the biomass fluctuation of mesopelagic fish in the

ECS The analyzed environmental indices were the monthly

North Pacific index (NPI, http://www.cgd.ucar.edu/cas/

jhurrell/Data/npindex.mon.asc), the annual Aleutian low

pressure index (ALPI,http://www.pac.dfo-mpo.gc.ca/science/

species-especes/climatology-ie/cori-irco/indices/alpi.txt), the

monthly Pacific decadal oscillation (PDO, http://jisao

washington.edu/pdo/PDO.latest), the monthly Arctic

oscil-lation (AO,http://www.cpc.ncep.noaa.gov/products/precip/

CWlink/daily_ao_index/monthly.ao.index.b50.current.ascii),

the monthly monsoon index (MOI), the monthly southern

oscillation index (SOI, http://www.bom.gov.au/climate/

current/soihtm1.shtml), and the seasonal Kuroshio flow

mass (http://www.data.kishou.go.jp/shindan/b_2/kuroshio_

flow/kt137.txt, 9106m3/s)

Results

The BI values of each fish group are shown in Fig.2a The

cumulated values fluctuated, and high values of cumulated

BI were observed in 1999, 2005, and 2007 High values of

BI of pearlside were also observed in the 1999, 2005, and

2007 The mean ± standard deviation of BI for each groupwas 84.0 ± 47.5 for anchovy, round herring, and sardine;24.3 ± 22.7 for jack mackerel, mackerel, and scad;138.2 ± 89.3 for lanternfish; and 86.5 ± 97.5 for pearl-side Percentage of BI of lanternfish ranged from 17.8 to79.3%, and the mean percentage was 41.1% Additionally,the mean percentages of biomass index of (1) anchovy,round herring, and sardine, (2) jack mackerel, mackerel,and scad, and (3) pearlside were 26.5, 8.3, and 24.0%,respectively (Fig 2b)

The percentages of D garmani and D chrysorhynchusbased on individual numbers by midwater trawling (28sites) were 70.9 and 19.6%, respectively (Fig.3) Theannual mean standard length of the two species was notsignificantly difference (ANOVA), therefore all measuredindividuals were pooled for estimating biomass The meanstandard lengths of D garmani and D chrysorhynchuswere 50.7 and 74.3 mm, respectively (Fig 4a, b), andthe relationship between body length and body weight(Fig.4c, d) was as follows:

D garmani: BW = 5.53 9 10-59 BL2.64(r2= 0.853)

D chrysorhynchus: BW = 1.15 9 10-59 BL3.02(r2= 0.984)

sardine, (2) jack mackerel, mackerel, and scad, (3) lanternfish, and (4) pearlside (a), and the percentage of each group (b)

where BW represents body weight in grams The mean

body length and body weight of pearlside were 28.8 mm

and 0.47 g (n = 25), respectively In the present study, we

assumed that the percentages of D garmani and D

chrysorhynchus were stable during survey periods The

mean lengths of anchovy, round herring, and sardine during

all annual surveys were 105, 138, and 140 mm,

respec-tively Additionally, the mean lengths of jack mackerel,

mackerel, and scad were 125, 230, and 125 mm,

respectively

We calculated biomass by species (Table1) The

bio-mass of lanternfish was higher than that of pearlside,

ranging from about 17,100 to 354,700 tons The biomass ofpearlside ranged from 1,300 to 44,400 tons The biomass ofanchovy, round herring, and sardine ranged from 200 to20,100 tons, from 100 to 28,300 tons, and from 0 to 2,100tons, respectively (Table1) The biomass of jack mackerel,mackerel, and scad ranged from 1,100 to 30,900 tons, from

100 to 13,300 tons, and from 0 to 2,400 tons, respectively.The biomass of lanternfish and pearlside was 2.26–19.16times that of small pelagic fish Significant positive cor-relations between biomass and BI for each species wereobserved

There were no significant correlations between ronmental factors and biomass of pearlside In contrast,significant correlations (P \ 0.05) between SOI in March(positive, r2= 0.303), AO in March (negative, r2= 0.314),

envi-AO in October (positive, r2= 0.294), MOI in February(positive, r2= 0.364), and Kuroshio flow mass in winter(positive, r2= 0.307) and biomass of lanternfish wereobserved (Table2; Fig.5) There were no correlationsbetween biomass of lanternfish and NPI, ALPI, and PDO

DiscussionThe stock sizes of commercially important fish, such asmackerel and sardine, were calculated using cohort anal-ysis in the ECS [8,9] The biomass of small pelagic fish

midwater trawling

of Diaphus garmani (a) and

Diaphus chrysorhynchus (b),

and relationships between body

length and body weight of D.

garmani (c) and D.

chrysorhynchus (d)

was estimated in the present study (Table1), and we could

compare the biomass size between pelagic fish and

meso-pelagic fish in summer in the ECS Ohshimo [6] reported

on the BI (biomass index) value for each species in the

same survey area The BI value was standardized to the

total Sa values in the survey area by the transect line

length Then the BI value was assumed to be the relative

stock size in summer off the north and west of Kyushu in

Japan (Fig.1a) In the present study, we estimated the

biomass using target strength [7, 12–14], and significant

positive correlations between biomass and BI for each

species were observed The biomass of lanternfish and

pearlside was 2.26–19.16 times that of small pelagic fish

Biomass of lanternfish was also higher than pearlside,

although the BI value of pearlside was sometimes higherduring the survey periods The reason for differences inbiomass and BI value of lanternfish and pearlside could bethe TS values of the species Ohshimo [6] reported thatlanternfish occurred in the deeper waters, and the BI oflanternfish may have been underestimated In the presentstudy, the range of recorded data was from 10 to 250 m,and biomass of lanternfish was also underestimated Weconclude that the biomass of mesopelagic fish in summer inthe ECS was higher than that of small pelagic fish, andmicronekton (lanternfish and pearlside) represent a sub-stantial biomass in the ECS

The stock size or recruitment sizes of small pelagic fishare affected by environmental factors in the ECS [8,9] Inthis study, correlations between biomass indices of lan-ternfish or pearlside and environmental factors were ana-lyzed There was no significant correlation between thebiomass of pearlside and environmental factors Pearlsideare widely distributed in the Sea of Japan, and the species

is dominant in the mesopelagic layers [15] The survey inthe present study was carried out to the edge of the Sea ofJapan, thus the variance in the estimated biomass ofpearlside would be large whether the schools of pearlsideappeared or not Therefore the survey area should beenlarged in the Sea of Japan for analyzing correlationsbetween environmental factors and fluctuations of pearl-side biomass Fujino [16] reported the relationshipsbetween egg abundance of pearlside in the Sea of Japanand the Tsushima warm current index He discussed thatthe warm periods in the Sea of Japan (strong Tsushimawarm current index) were suitable for pearlside because theamount of zooplankton increased during warm-waterperiods

lanternfish and environmental factors

Significant positive correlations between biomass of

lanternfish and SOI, MOI, and Kuroshio flow mass in

winter periods, and negative correlation between biomass

and AO in winter were observed (Fig.5) A significant

correlation was observed between the biomass of

lantern-fish and AO in October Basically, AO was considered to

affect the water temperature in the winter season [17], and

the interaction between biomass and AO in October was

previously undetermined

Weak AO and strong MOI in winter would lead to

a strong northwest wind around Japan, and the sea

temperature would cool down in the Sea of Japan [9] Weanalyzed the correlations between water temperature andenvironmental factors in the ECS Though there were nosignificant correlations between AO in March and seasurface temperature (SST) in the northern part of the ECS,including the present survey area (http://www.data.kishou.go.jp/kaiyou/db/nagasaki/nagasaki_warm/areaC_SST.txt),significant positive (P \ 0.1) and negative (P \ 0.05)correlations were found between AO during January toMarch and SST and between MOI in February and SST,respectively

Generally during the strong northwest wind in winter,primary and secondary production, such as phytoplanktonand zooplankton, are at a high level [18], and survival offish larvae would correspondingly be high The SOI indi-cates an ENSO (El Nin˜o southern oscillation) event and theENSO event associated with the Kuroshio meander in theECS [19] Hwang and Kao [20] reported that the Kuroshiovolume transport northeast of Taiwan has a positive cor-relation with ENSO with a 1-month lag Lanternfish werenot observed in the present survey area were not observed[21], whereas the larvae of Diaphus spp were observed inthe waters near Taiwan [22] and in the shelf break area inthe ECS [23] in winter Adults of lanternfish might migrate

to southern areas in the autumn or winter season, andtherefore it is necessary to conduct the acoustic surveyannually in the ECS Increasing the Kuroshio flow masswould increase the recruitment of the lanternfish larvae tothe present survey area We assume that the high primaryand secondary production and transport pattern of larvae oflanternfish cause the fluctuation of biomass in the presentsurvey area

Yoko-maru and Hokko-maru, and thank Dr Funamoto, Dr Takasuka, and Mr Yukami for help with the acoustic survey and sampling of lanternfish The authors thank for Dr C P Norman for critical reading of this paper This study was funded by Fisheries Research Agency.

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