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South African Journal of Marine Science
ISSN: 0257-7615 (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/tams19
Observations on the spawning, development and
rearing of the South African abalone Haliotis midae
Linn.
A B Genade , A L Hirst & C J Smit
To cite this article: A B Genade , A L Hirst & C J Smit (1988) Observations on the spawning,
development and rearing of the South African abalone Haliotis�midae Linn., South African Journal
of Marine Science, 6:1, 3-12, DOI: 10.2989/025776188784480465
To link to this article: https://doi.org/10.2989/025776188784480465
Published online: 08 Apr 2010
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Citing articles: 18 View citing articles
Trang 2S.Afr. J. mar Sci.6: 3-12
/988
OBSERVATIONS ON THE SPAWNING, DEVELOPMENT AND REARING OF
THE SOUTH AFRICAN ABALONE HAL/OTIS MJDAE LINN
A description is given of the first successful controlled breeding of the South African abalone Ha/iotis midae.
Gametes of ripe abalone were obtained from spontaneous spawnings as well as by subjecting animals to
spawning stimuli The larval period is five days at 20°C and seven days at 17,5°C The first respiratory pore is
completed at a shell length of2,3 mm and the first shell ridge can be observed at approximately 2~ mm Benthic
diatoms on fibreglass plates served as food for post-larval stages to a length of 5-8 mm Larger Juveniles were
fed the seaweeds Plocamium spp and Ulvafasciata. Shell pigmentation is affected by the food consumed.
Variable growth rates for individuals from the same spawning were observed.
'n Beskrywing word gegee van die eerste suksesvolle gekontroleerde te1ing van die Suid-Afrikaanse
perlemoen Haliotis midae Gamete is van ryp perlemoen deur spontane kuitings sowel as kuitprikkels bekom.
Die larwale periode is vyf dae teen 20°C en sewe dae teen 17,5°C Die eerste respiratoriese porie word voltooi
op 'n skulplengte van 2,3 mm, terwyl die eerste skulprifby ongeveer23 mm waarneembaar is Bentiese diatomc
op veselglasplate is as voedse1bron vir postlarvale stadia tot ongeveer'n lengte van 5-8 mm aangewend, waarna
die seewiere Plocamium spp en Ulvafasciata gevoer is Die pigmentasie van die skulp word deur voeding
beinvloed 'n Groot variasie in groei is vir individue afkomstig van dieselfde kuitgroep waargeneem.
3
Of the six South African abalones (Muller 1986),
only Haliotis midae (locally known as perlemoen)
occurs in quantities sufficiently extensive to warrant
commercial exploitation It contributes substantially
to the inshore fisheries and the present whole mass
quota is 660000 kg (Chief Directorate Marine
Deve-lopment 1986) Strict conservation measures were
implemented from 1965 to curb overfishing, peak
production of 2,28 million kg being recorded in that
year Because the present supply cannot meet the
demand, commercial industry has shown an intense
interest during recent years in the possibilities for
controlled culture of H midae.This need has been
stimulated further by the advancement in abalone
cultivation techniques overseas
Japan is at present the acknowledged leader in
developing techniques for the mass production of
juveniles especially for restocking The number of
seed abalone released from hatchery-produced seed
in Japan has increased from 200 000 in 1970 to 10,7
million in 1978 (McCormick and Hahn 1983) Efforts
to culture this economically valuable shellfish are
now undertaken world-wide and, since the major
contribution made by Ino (1952), research and
development projects have been launched and
pro-duction facilities established in Australia, the United
Kingdom, Canada, Chile, France, Mexico, Taiwan
and the United States (Ebert and Houk 1984)
Whereas the production of juveniles so far has been
aimed at augmenting the supply of natural seed,
abalone producers in the United States are now
investigating a market for small or cocktail abalone
for the restaurant trade (Chew 1984)
Various aspects of the biology of H midae of importance to fisheries management have been re-searched by Newman (1966, 1967a, b, 1968, 1969), and they cover mainly the migration, reproduction, growth and distribution of natural populations No successful rearing ofH midae has been recorded to date, whereas most of the commercially important species in other countries have been cultured and larval and post-larval development documented (lno
1952, Leighton 1974)
The purpose of this preliminary investigation is to demonstrate that spawning, larval development and further growth of H midae can be achieved under controlled and semi-controlled conditions Facilities, which were mainly utilized for bivalve culture, at the Aquaculture Unit of the Fisheries Development Corporation of South Africa Ltd at Knysna (Cape Province) were used for this study Knysna is about
500 km east of the main commercial exploitation area of abalone, Cape Hangklip to Quoin Point (Fig I)
MA TERIALS AND METHODS
Collection and transport Mature H midaelarger than 110 mm shell length were collected during August and September 1981 and May and June 1985 from the exploitation area
• Formerly Fisheries Development Corporation Aquaculture Unit, P.O Box 346, Knysna 6570, South Africa
tSea Fisheries Research Institute, Private Bag X2, Rogge Bay, 8012, South Africa - to whom all correspondence should be addressed
Trang 3South African Journal of Marine Science 6
4
S
29 0
30 0
31 0
32 0
33 0
34 0
35 0
36 0
o
31 32o E
1988
Fig 1: The laboratory site and main area of commercial exploitation of Haliotis midae
(Fig I) The animals were removed either by
knock-ing off by hand those which had raised part of the
foot (the method used by abalone for capturing
drifting fragments of kelp) or by the conventional
levering-off method used by commercial divers The
captured animals were sexed by inspecting the colour
of the gonad, which is dark green in females and
cream in males Collection took place between IOhOO
and I3hOO
Immediately after delivery to the boat by SCUBA
divers, each abalone was placed in a 10-1 plastic bag
filled to one-third with seawater and inflated with
oxygen before being sealed off The sex was recorded
on the bag, which was then put into a polystyrene
cool-box for the six-hour journey to the laboratory
Animals collected during 1981 were placed 10 per
aerated 1600-1 tank at ambient temperature
(± 17°C), and those collected in 1985 were put in
shallow tanks with 19°C water filtered to 3 ~m
flowing at a rate of 1 1 per minute per animal In all
cases the sexes were kept separate The tanks used for
the 1985 groups were lined with clear plastic sheeting
to facilitate subsequent removal of the animals
Spawning
The groups obtained during 1981 were not exposed
to any additional stimulus, but those collected in 1985
were subjected to a stimulus procedure, more or less corresponding to that described by Chen (1984), the following morning These abalone were first exposed
to air for one hour and then placed in tanks with flowing, 17°C seawater previously exposed to ultra-violet light (2,537 A) and filtered to 3 ~m Sexes were still kept separate The water temperature was then increased by 1°C per hour for three hours, and then decreased over the same period to the original temperature Once spawning had commenced, the waterflow was reduced
Fertilization and larval cultivation
Spawned ova were collected by siphoning into 15-1 round fibreglass containers to a density that, when spread out, the ova would form a single layer on the bottom Because of their higher specific gravity than seawater, the ova would settle within approximately
10 minutes Siphoning was then easily accomplished The ova were fertilized by adding freshly released sperm (±I hour after spawning) at a concentration of
± 120000·ml-1 to the container after microscopic examination of their activity After gentle mixing, the ova were exposed to the sperm for 15 minutes, after which the water was topped up to 15 1 Decanting to remove the excess spenn,and debris began after most
of the ova had settled Refilling with fresh seawater
Trang 41988 Genade et at.: Laboratory Spawning, Development and Rearing of Abalone 5
and decanting was then repeated ten times,
approx-imately every ten minutes After the final wash the
ova were transferred to 240-t tanks for further
development at either 20°C or ambient temperature
(± 17,soC)
As the larvae of the Haliotidae are lecithotropic
(Leighton 1974), no food was added to the cultures
until the settlement stage was reached Also, there
was no water-flow or aeration during larval
develop-ment The water was changed once per day after
siphoning the larvae onto a 45-~m net Development
stages were recorded according to the classification
Leighton (op cit.)
To test the effect of "GABA" (')I-aminobutyric
acid), a neurotransmitter, on the settling rate of
advanced veligers, the following experiment was
designed, as used by Morse et al. (1979) for H.
rufescens: 15 X 4-t Pyrex beakers were placed in a
waterbath at 20°C, in 5 groups of 3 beakers each All
the beakers were filled to 4t and treated with
penicillin at 33 ppm "GABA" was then added to each
group at increasing concentrations, i.e 3 with no
addition (control), 3 at 10-6molar, 3 at 10-5molar, 3
at 10-4 molar and 3 at 10-3 molar Some 3 000
advanced larvae (digitate cephalic-tentacle stage)
were added per beaker and the reaction of the larvae
was recorded 2 and 18 hours later
Settlement and juvenile cultivation
Benthic diatoms scraped from the bottom of the
outflow channel of the oyster nursery in the same
laboratory were used to seed corrugated fibreglass
plates of 25 X 30 em, which had been matured
previously in seawater for three weeks A slurry was
made after washing the diatoms (± 70 per cent
Navicula) through a 50 ~m net The cleaned settling
plates were suspended overnight in the diatom
mixture, and the next morning, covered with a thin
film of benthic diatoms, they were placed on the
bottom of the 240-t tanks with advanced larvae
"GABA" was added to the tanks at 10-6molar
After settlement, the plates were left in the tanks
for a further five days, during which time the tanks
received constant lighting, slight aeration and a daily
water exchange To encourage further diatom growth,
the plates were periodically removed and
resus-pended in a section of the oyster nursery which
received a constant water flow No temperature
control was administered, but regulating natural light
by occasional screening with a shade mesh was
necessary to control diatom growth
At an approximate length of 5-8 mm, the seed
abalones were transferred to tanks 2,0 m long X
0,5 m wide X 0,25 m deep, into which chopped pieces
of seaweed about 1,0 cm were introduced daily at
17hOO.Initially, pieces of Plocamium sp., which was collected at low springtides on the coast and stored in
a freezer, were used, but because of the distance from the nearest source, the feed was changed after a few months to Viva fasciata. This species could be obtained easily from close to the laboratory Shelters,
in the form of halved 100-mm PVC and ceramic pipes, which covered approximately 50 per cent of the bottom of the tanks, were also introduced into the tanks
Unfiltered seawater of salinity 30-35 X 10-3and ambient temperature (1O-27°C) was supplied at a flow rate of 61·min-l·tank-1 or 24 m£·min-1·animat' The tanks were also aerated Stocking density was ±
250 animals·m-2 (bottom surface) at a shell length of approximately 20 mm
The outlets were screened with an appropriate mesh to prevent animals from escaping during night migration Food residues and faeces were siphoned off every second day At no stage was any attempt made to grade the abalones to prevent competition
RESULTS
Collection and transport Specimens knocked off by SCUBA divers experi-enced little or no damage, but the foot of those levered from the substratum was injured in more than
40 per cent In intact abalone, mortality was less than
5 per cent two weeks after transfer Injured animals showed signs of extreme stress and more than 50 per cent mortality within the same period No injury was caused to the abalone when removed from the plastic bags because of the ease with which the plastic could
be released from the adhering foot On arrival, oxygen concentration in the bags varied between 8,0 and 12,0 ppm and pH between 7,2 and 7,6
Spawning Natural spawning had commenced in the 1981 groups before 08hOO the morning after transfer and continued, in the August group, sporadically until IOh30 The temperature at spawning for the August group was 13,5°C and that of the September group 17,0°C, in both cases the ambient temperature Because of the number of abalone in each tank, no accurate estimate of the number that spawned could
be made, though probably in excess of 80 per cent of both sexes spawned spontaneously
The two groups collected during 1985 responded
to stimulation as follows:
May group - 40 per cent of the females reacted at
Trang 56 South African Journal of Marine Science 6 1988
Table I: Times at which developmental stages (after Table II: Achieved stages of development of H midae larvae Leighton 1974) of H midae larvae were first at different ambient temperatures
observed at 20°C
Settlement and juvenile cultivation
Table III: Percentage settlement in 4-1 beakers at 20°C after
addition of "GABA" to 4-day-old larvae
Time Control 10- 3 molar 10-· molar IO- l molar 10- 6 molar After 2
After 18
Settling of larvae was advanced and stimulated by addition of "GAB A ", as shown by the results in Table III Settlement was on the sides as well as at the bottom of the beakers
Larvae held at 17,5°C in 240-£ tanks reacted well to the addition of "GABA" only on Day 7, with a maximum settlement of l·cm-2 on Day 8 However, many larvae were still swimming actively at this stage Complete settlement was observed on Day 9 (Fig 2g)
3 4 5
5 6 7
Day on which stage achieved
Stage of development
M idformed cephalic tentacle Digital cephalic tentacle Crawling and settling
in Table I No specific differences in larval features from those recorded by other authors for haliotid embryology (Ino 1952, Leighton 1974) could be observed (Fig 2b-f) The green pigmentation of the egg yolk is retained by the trochophores and veliger larvae This phenomenon has also been recorded for other species by Leighton (1972) The behaviour of trochophores and veligers is also typical of Haliotis.
Trochophores tend to concentrate at the surface of containers (negative geotaxis), and healthy veligers congregate in vertical columns, then tumble to the bottom where they scatter and regroup again to form new columns This cycle is repeated every few minutes Trochophores measure approximately
164 J.lm X 190 J.lm just after hatching and fully developed veligers 207J.lm X 265 J.lm
Development from cleavage through to settlement
at the two temperature regimes, 17,5 and 20,0°C, appeared to be quite normal A marked difference in rate of development was, however, recorded as shown in Table II
When achieved after
Hours ±Days
2 Free-swimming trochophore 22
3 Cap-shell, early veliger' 24 I
4 Infiatc-shell veliger, torsion 31
5 Early operculate veliger,
6 Incipicnt cephalic tentacle,
7 Mid-formed cephalic tentacle 86 3
8 Digitate (branched) cephalic
10 Total metamorphosis (loss of
cilia, but no mouthparts or
feeding yet observed) 145 6
Fertilization and early development
A high rate of fertilization (± 80 per cent) was'
obtained for the naturally spawning groups of August
and September 1981 However, the June 1985 group
showed a relatively low percentage, ranging from 11
to 25 per cent The addition of excess sperm to this
group, ova of which already showed cleavage, did not
improve the percentage successfully fertilized but
rather resulted in the rupture of many egg
mem-branes
The progress in larval development is summarized
least slightly only after five hours
All ova were prematurely released
No males responded
June group - 20 per cent of the females reacted
within three hours and produced well formed "mature" ova similar to those obtained during 1981
Ejaculation of ova and sperm occurred mainly
through the first four respiratory pores In contrast to
the ova, which settled to the bottom within a few
minutes, the sperm stayed in suspension
Egg counts were made of two females 150 mm long
in the June spawning Both animals still had a
substantial reserve after spawning and released I 245
X 106and 0,750 X 106ova respectively The spherical
ova (Fig 2), which are enveloped by a relatively thick
gelatinous membrane, varied in diameter between
212 and 222 J.lm (x =214 J.lm), and the yolk varied
between 172 and 192 J.lm (x = 181 J.lm)
Trang 61988 Genade et al.: Laboratory Spawning, Development and Rearing of Abalone 7
Fig 2: Developmental stages of Haliotis midas (a) fertilized eggs of±214 Ilm, sperm attached to membrane; (b) freshly hatched trochophore larva of 164 X 190 Ilm; (c) cap-shell veliger larva; (d) inflate-shell veliger (torsion) of 207 X 2651lm with retractor muscle; (e) operculate veliger (pre-eye spot); (f) operculate veliger
(pre-eye spot) with retracted velum; (g) crawling and settling stage; (h) peristomial growth; (i) circular-shell post-larva of ±600 Ilm; (j) whole shell pink with no respiratory pores; (k) juveniles showing dietary
pigmentation; (I) juveniles of approximately 1 year of age
Trang 7South African Journal of Marine Science 6 1988
Fig 4: Radial ridges on the shells of H midae (largest
specimen ±36 mm)
other Baliotis species (Table V) The effect of temperature on growth (and pore formation) at this early stage was quite clear The August 1981 group took 65 days (I2-17°C ambient temperature) and the September 1981 group 48 days (I7-22°C ambient temperature) to display this feature
Respiratory pores are continuously being formed and previous ones sealed off as growth advances (Fig 3) The first sealing off already starts at a size of
3 mm, where just two of the three pores are func-tional At 25 mm a total of 27 pores would have been formed but only five of these were open for respir-ation A definite difference in pore formation in H midae from that of the smaller Japanese H diversi-color supertexta (Oba 1964) can be observed after a length of some 15 mm (Fig 3)
Although the effect of light intensity on growth was not tested at any stage during development, it was observed that juveniles (2-3 mm long) on plates receiving less light were generally larger (by up to 50 per cent) than those exposed to brighter light The shell colour is affected by the food consumed For instance, when feeding on benthic algae, a
o
25
0
" Open pores
• ~ Closed pores
Fig 3: Relationship between length and number of pores
8
30
Vl 25
UJ
C>::
0
0->- 20
C>::
~
C>::
c: 15
Vl
UJ
C>::
0 10
C>::
UJ
a:l
~
::::l 5
Z
An extremely variable pattern of settlement took
place over the plates
Some features of post-larval development after
transfer from the settling tanks and at different
ambient temperature are given in Table IV
Peri-stomial growth (Fig 2h) was apparent the first day
after transfer to substrate with benthic algae At a size
of 600 ~m, one juvenile could clear algae from an area
of up to 16 mm2 in 24 hours and could move 1,0 cm
At 700 ~m, the shells were fully round with a pink tint
(Fig 2i, j) and movement of 2 cm in 6 hours was
recorded under nursery conditions At this stage they
were already extremely light-sensitive and would
immediately move away from a bright
stereo-micro-scope light
In H midae, the notch stage is reached at a size of
2,1-2,2 mm and completion of the first pore at
2,3 mm, similar to that of the American west coast
pink abalone H corrugata, but larger than for some
Table IV: Post-larval development features after transfer to
ambient temperature
Time taken to achieve Table V: Shell length of post-larval Haliotis spp at formation Description of stage stage (days) of first respiratory pore
12-I7°C 17-22°C
Species Shell length Source Virtually fully round, length up
(mm)
Fully round, pink tint to shell, 70011m 26 H corrugata 2,0-2,5 Leighton (1974) Whole shell pink, no respiratory pore 40 H.fulgens 1,7-2,0 Leighton (1974) Development of first respiratory pore H sorenseni 2,0-2,1 Leighton (1974)
Trang 81988 Genade et al.: Laboratory Spawning Development and Rearing of Abalone 9
50
40
30
20
n=300
25
20
UJ
'"
;:) 15
'«
'"
UJ
<l.
~
UJ I-10
"'" '. • • 0"
~ 10
>
u
;:)
9
'"
u
n=300
5
Average
-Maximum "' 4
Minimum , '.
Fig 5: Size frequency of H midae at (a) one and (b) two
years of age
turquoise pigmentation was produced, but when fed
mainly on Plocamium spp., a brick-red colour was
apparent (Fig 2k, I)
The formation of radial ridges, which are formed
parallel to the growing margin and are most probably
used for shell strengthening, can be observed at a
length of 23-24 mm Ridges are continuously being
added thereafter, so that an abalone of 36 mm would
have approximately five of these (Fig 4) Inmature
40
30
20
10
.:.:-:.:.:.:.:.
:-:.:.:.:.:.:-:
~??~~)
SHELl LENGTH (mm)
40
MONTH
Fig: 6: Average monthly temperature of the nursery section
of the aquaculture unit
specimens, the shell surface would have an irregular corrugated surface, as described by Day (1974), as a result of the moulding of these ridges
The size frequency (length) after one and two years
is given in Figure 5 From this Figure it is clear that a wide variation in growth rate, originating from a specific spawning group, can be expected under culture conditions After 12 months a size range
of 5-30 mm was recorded (x = Il,8 mm), and at
an age of 24 months the range was 10-45 mm
(x = 34,4 mm) Another notable feature was the stunted growth displayed by certain animals These results vary from those obtained by New-man (1968, 1969) for natural populations at Stony Point near Cape Hangklip (estimated growth rate of 21,7 mm per annum) and Port Elizabeth (28,8 mm) Newman (1969) attributed this difference to vari-ations in the mean annual temperature at the two sites, 15,8°C at Stony Point and I7,4°C at Port Elizabeth The mean monthly temperature (recorded
by thermograph for the semi-closed nursery system used for ongrowing) is given in Figure 6 The recorded mean for the II months February-Decem-ber is identical to the annual mean for Port Elizabeth
Trang 910 South African Journal of Marine Science 6 1988
1.25
Fig 8: Relationship between shell length and shell weight
of juvenile H midae
especially to the foot Although infection can be treated by soaking in an antibiotic solution such as penicillin, such treatment probably only adds addi-tional stress and may influence results
The reaction to spawning stimuli and successful fertilization depends mainly on the state of ripeness Basically, all methods used throughout the world (Morse et al. 1979, McCormick and Hahn 1983, Chen 1984) encompass a form of stress to stimulate gamete release (e.g temperature shock, air exposure, ultra-violet irradiation, addition of hydrogen per-oxide, or a combination of these)
The results indicate that the stress caused by the duration and mode of transport is adequate to bring
on spawning by ripe H midae. Although the August
1981 group spawned while the temperature in the tanks was declining, the September 1981 group were not exposed to much temperature fluctuation and also spawned spontaneously It is doubtful whether the high concentration of oxygen during transporta-tion could have stimulated spawning, but this aspect needs further clarification
Although specific stimuli were only applied to groups collected in May and June 1985, it is con-cluded that this method should be adequate for abalone with more advanced gonads which are approaching the natural spawning period of Octo-ber-December, as established by Newman (l967b) The addition of hydrogen peroxide (Morse et al.
1978) as a spawning inducer was not used, although
H midae is listed by Morse (1984) as one 'of the species which reacts to this stimulus To ensure the
30
1.0
0.25
>-:r:
Ij 0.75
w 3
~
UJ
i13 0.5
10
30
25
35
E
:r:
>-Ij
~
:j 15
w
:r:
<f)
Fig 7: Relationship between shell length and shell breadth
for juvenile H midae
SHELL BREADTH (mm )
DISCUSSION
5
given by Newman (1969) A difference of 5-IO°C
between the maximum and minimum within a single
month, mainly caused by changing air temperatures,
was recorded
The relationship between shell length and shell
breadth for juveniles is given in Figure 7, and it
mirrors the linear relationship determined by
New-man (1968) for abalone 20-40 mm shell length in the
wild However, the relationship between shell length
and shell weight is exponential for juvenile H midae
(Fig 8)
If mature animals are to be collected for further
keeping and specific studies on maturation, the
conventional method of levering off is not
recom-mended because of the injuries that can be inflicted
Trang 101988 Genade et al.: Laboratory Spawning, Development and Rearing of Abalone 1/
availability of ripe animals for longer periods,
methods for out-of-season conditioning should be
investigated for H midae. These methods could
include inter alia saturation-feeding (Morse et al op.
cit.), temperature control plus adequate nutrition
(Uki and Kikuchi 1984) and photoperiods
The importance of dense, natural spawning
popu-lations is emphasized by the observation that spawned
ova stay in suspension for only a few minutes The
chances of contacting viable sperm in nature will be
substantially reduced where populations are too
sparse
This investigation also confirms the speculation by
Newman (1969) that the planktonic larval stage of
H midae should be more or less within the time
confines of that of other abalone species Because of
the short free-swimming period, the distribution of
H midae is expected to be fairly limited Prevailing
inshore currents and water temperature will be
important factors in determining the possible areas
for settlement When establishing abalone reserves,
this fact should be taken into consideration
No reliable data were obtained on larval mortality
during the course of the experiments Effective water
and culture management, which would include the
combating of contamination, are considered to be
essential for successful rearing The transfer of weak
and dead larvae during water changes should be
prevented and, as in all culture programmes, weak
larvae should be eliminated
Settlement and subsequent cultivation
The most critical stage in the life history of benthic
organisms is recognized to be the availability of
suitable substrates for settling larvae The complexity
of the metamorphic process and the factors which
regulate settlement response are discussed by
Had-field (1984) It is generally accepted that both physical
and chemical characteristics of substrates playa role
in settlement Because it is convenient from a
cultu-rist's point of view to have synchronous settling and
metamorphosis, the only chemical thus far known to
achieve such synchrony, -y-aminobutyric acid (Morse
et al 1979), was used Although a positive settlement
response was recorded, the effect on metamorphosis
in H midae is still unknown. Future work should
include detailed studies of both aspects
The observation that shell colouration is affected
by diet can be applied to distinguish cultured from
natural abalone or to identify cultured groups by
biological colour-coding
Although no specific attention was given to the
causes of mortalities during rearing of juveniles,
observations were made on:
(i) excessive losses, mainly inflicted by micropred-ators during the first two months after settle-ment;
(ii) mortalities just after handling of plates or indivi-duals;
(iii) predation by polyclads
It is concluded that all these factors can be eliminated
by proper management techniques
The slower growth recorded under culture condi-tions when compared with the results obtained by Newman (1969) could have been due to a combi-nation of food availability and type, or to competi-tion or fluctuacompeti-tions in light intensity and temperature The effect of these parameters on growth has been illustrated by many authors for other species, e.g Leighton (1974), Cken (1984), Ebert and Houk (1984) Variable growth rates for individuals of a specific spawning were also recorded by Leighton (1974), who documented this tendency for four
Haliotis species with identical parentage, age and environment
ACKNOWLEDGEMENTS
The authors thank the management of the Fish-eries Development Corporation of South Africa Ltd for their encouragement during the course of this study The technical assistance from former col-leagues Messrs B A Andrews and D J. Krebser is also gratefully acknowledged
LITERATURE CITED
CHEN, H.-C 1984- Recent innovations in cultivation of ediblc molluscs in Taiwan, with special reference to the small abalone Haliatis diversicalar and the hard clam Meretrix
CHEW, K K 1984 - Recent advances in the cultivation of molluscs in the Pacific United States and Canada Aquacul-ture39(1-4): 69-81
CHIEF DIRECTORATE MARINE DEVELOPMENT 1986 Annual Report for the year 1985.Rep Chief Dir mar dev.
DA Y,J. H 1974 - A Guide 10Marine Life on South African
EBERT, E E and J J HOUK 1984 - Elements and innovations
in the cultivation of red abalone Haliotis rufescens
HADFIELD, M G 1984 - Settlement requirements of mollus-can larvae: new data on chemical and genetic roles
INO, T 1952 - Biological studies on the propagation of Japanese abalone (genus Haliotis) Bull Takai reg Fish Res Lab 5:
102 pp