Biology of Marine Birds - Chapter 16 docx

Seabirds provided sources of food and bait Collins 1882, fishermen used marine birds for navigational information about the locations of fishing banks and landfalls and followed birds at s

Interactions between Fisheries and Seabirds

William A Montevecchi

CONTENTS

16.1 Introduction 528

16.2 Negative Effects of Fisheries on Seabirds 529

16.2.1 Direct Effects 529

16.2.1.1 Entrapment in Fishing Gear 529

16.2.1.2 Disturbance 534

16.2.2 Indirect Effects 534

16.2.2.1 Prey Depletion 534

16.2.2.2 Competition and Predation by Scavenging Seabirds 538

16.3 Positive Effects of Fisheries on Seabirds 539

16.3.1 Direct Effects 539

16.3.1.1 Provisioning of Fisheries Discards and Offal 539

16.3.2 Indirect Effects 540

16.3.2.1 Removal of Competitors — Multispecies Interactions 540

16.3.2.2 Increase Abundance of Small Fishes 540

16.4 Negative Effects of Seabirds on Commercial Fisheries 540

16.4.1 Direct Effects 540

16.4.1.1 Interactions with Aquaculture 540

16.4.1.2 Bait Stealing 541

16.4.2 Indirect Effects 541

16.4.2.1 Prey Depletion 541

16.5 Positive Effects of Seabirds on Commercial Fisheries 543

16.5.1 Direct Effects 543

16.5.1.1 Birds as Fishing Devices 543

16.5.1.2 Birds as Indicators of Prey Location 543

16.5.2 Indirect Effects 543

16.5.2.1 Predation on Predators, Competitors, and Parasitized and Diseased Fishes 543

16.5.2.2 Guano and Nutrient Recycling 543

16.5.2.3 Prey Information 543

16.6 Interactions of Fisheries and Other Perturbations on Seabirds 544

16.6.1 Oceanographic Fluctuations 544

16.6.2 Pollution 544

16.6.3 Hunting 544

16.6.4 Cumulative Effects 544

16.7 Management and Mitigation 545

16.7.1 Misguided Management 545

16

16.7.1.1 Culls 545

16.7.1.2 Colony Displacements 545

16.7.2 Mitigation 545

16.7.2.1 Observer Programs 546

16.7.3 Marine Protected Areas 546

Acknowledgments 547

Literature Cited 547

16.1 INTRODUCTION

Since humans first inhabited coastal margins and ventured out to sea, they have exploited marine birds Seabirds provided sources of food and bait (Collins 1882), fishermen used marine birds for navigational information about the locations of fishing banks and landfalls and followed birds at sea to find schools of fishes (Nelson 1978, Montevecchi and Tuck 1987) Over millennia and perhaps most rapidly in the present century, human populations and their technological capabilities

at sea have increased many fold, and so have their demands for marine prey Human harvests have moved consistently from exploitive to over-exploitive levels with marine birds (e.g., Burger and Gochfeld 1994, Montevecchi and Kirk 1996), mammals (Laws 1985), fishes (Harris 1990), crus-taceans (Pauly et al 1998), cephalopods (Montevecchi 1993b), and shellfish (Dahl 1992) Clearly, these and other human harvests influence seabirds and other marine animals in many ways The rapid enhancement of fishing capabilities and overexploitations of fish stocks in the 19th and 20th centuries inevitably led to questions about the influences of marine predators, such as seals (Harwood and Croxall 1988), whales (Harwood 1983), and seabirds (Milton et al 1995, Cairns 1998) on commercial fishery stocks (Nettleship 1990, Tasker et al 2000) Large-scale energetics and trophic models of prey consumption by seabirds demonstrated that marine birds consume huge tonnages of prey (Furness 1978, Furness and Cooper 1982, Croxall et al 1985, Cairns et al 1990, Montevecchi 2000), mostly small pelagic fishes and crustaceans (Montevecchi 1993a) These levels of consumption are matched or well exceeded by marine mammals (e.g., Furness 1990) and dwarfed by orders of magnitude by the consumption levels of large predatory fishes (Bundy et al 2000) For example, Table 16.1 shows estimates of prey consumption by large predators in the northwest Atlantic

Interactions between seabirds and fisheries are dominated by influences of fisheries on birds (Montevecchi 1993a, 1993b; Tasker et al 2000) These influences may be direct or indirect and either negative or positive (Table 16.2) Direct effects include entrapment in fishing gear, distur-bance, and food provisioning with fishery discards and offal Indirect effects include prey depletion, increases in scavenging and predatory seabirds, decreases in large fish competitors, and increases

TABLE 16.1

Consumption Estimates (tons) of Capelin (Mallotus

villosus), a Small Pelagic Fish, by Large Predators

in the Northwest Atlantic

Taxa

Capelin Consumption

Birds 250,000 Montevecchi 2000 Seals 800,000 Stenson and Lawson 2000 Whales 700,000 Stenson et al 2000 Cod 1,000,000–3,000,000 Lilly et al 2000 Human Quota 40,000 Carscadden et al 2001

Interactions between Fisheries and Seabirds 529

in the availability of small fishes Owing to the small biomass of birds compared to fishes in theworld’s oceans, influences of seabirds on fisheries tend to be localized, small-scale events, oftenoccurring in artificial situations involving aquaculture (Kirby et al 1996, Cairns 1998, Tasker et

al 2000) or the stocking of commercial or game fishes (Blackwell et al 1995, Roby et al 1999).The life history attributes of seabirds are such that their populations are relatively robust tointerannual variation in breeding success, but highly sensitive to slight changes in adult mortality.Seabirds are long lived, have delayed maturity (often 5 to 10 years) and recruitment to breedingpopulations, and exhibit low fecundity and high annual adult survival (on the order of 80 to 90%

or more; Furness and Monaghan 1987) Hence, poor reproduction must be long term and extensive

to decrease populations When such effects do occur they often lag well behind the environmentalfactors that caused them Seabird populations are therefore buffered from environmental perturba-tions that influence annual production (Montevecchi and Berutti 1991) Yet even slight changes inadult mortality can have profound effects on seabird populations (Furness 2000) Hence, throughoutthis review an attempt is made to differentiate potential fishery influences on reproduction fromthose on adult survival

The present chapter reviews the influences of fisheries on marine birds and also reviews theinfluences of seabirds on fisheries Interactions and cumulative effects among fisheries, oceano-graphic perturbations, pollution, and hunting are also considered Research and management rec-ommendations to protect seabirds and the large-scale natural ecosystem processes that sustain themare also provided

16.2 NEGATIVE EFFECTS OF FISHERIES ON SEABIRDS

16.2.1 D IRECT E FFECTS

16.2.1.1 Entrapment in Fishing Gear

By-catches of seabirds in fishing gear have resulted in negative population effects on birds on aglobal scale (Tasker et al 2000)

Nets — Pursuit divers, such as auks and shearwaters, are the seabirds most commonly killed

in gill nets in the North Atlantic and North Pacific (Tull et al 1972, Ainley et al 1981, King 1984,Ogi 1984, Piatt and Nettleship 1987, Petersen 1994, Artukhin et al 2000) Loons, cormorants, andgannets are also caught in high numbers with surface-feeding gulls and storm-petrels being caught

to a much lesser extent (Piatt and Nettleship 1987) As well as seabirds, seaducks, marine mammals,sharks, and sea turtles also become entrapped in fishing gear (e.g., Harwood 1983)

Before their banning in 1993, high-seas drift nets set for salmon and squid entrapped millions

of birds including shallow divers and surface-feeders More deeply set gill nets catch birds thatdive below the foraging range of these species Among the pursuit divers, birds that denselyaggregate (e.g., alcids, shearwaters) are most vulnerable to mortality in nets (e.g., Artukhin et al.2000), especially nets set near breeding colonies and migratory concentrations (e.g., Piatt and

Indirect Prey depletion

Increase populations of scavengers/predators

Increase predation by removing artificial food sources of scavengers

Remove competitors Increase abundances of small fishes

Nettleship 1987) Entrapments are often most frequent during periods of fish movements nearfishing gear (Christensen and Lear 1977, Piatt and Nettleship 1987), when both birds and targetedfish are pursuing forage fishes.

Common Murre (Uria aalge) is the species most widely affected on a global basis by mortality

in fishing nets (Melvin et al 1999) Net mortality has been implicated in population declines ofCommon Murres in northern Norway (Vader at al 1990a, Strann et al 1991) and of Thick-billed

Murres (U lomvia) in western Greenland (Evans and Waterston 1976, Evans and Nettleship 1985;

Figure 16.1) Net mortality has been implicated in negative population effects on murres in the

western Bering Sea and on the Farallon Islands as well as on Red-legged Kittiwakes (Rissa

brevirostris) on the Commander Islands (Artukhin et al 2000) and on Sooty Shearwaters (Puffinus griseus) and Short-tailed Shearwaters (P tenuirostris; DeGange et al 1993, Veit et al 1996) Net

mortality has also been associated with population declines of endangered Marbled Murrelet

(Brachyramphus marmoratus; Carter and Sealey 1984, Grettenberger et al 2000) and endangered Japanese Murrelet (Sythliboramphus antiquus; Piatt and Gould 1994) Northern Gannets (Morus

bassanus), Atlantic Puffins (Fratercula arctica), and nonbreeding Dovekies (Alle alle) are also

killed in gill nets Relationships that show that entrapments decrease with increasing distance fromcolonies (Ainley et al 1981, Piatt and Nettleship 1987) indicate that no-fishing zones aroundbreeding sites could in some circumstances benefit some seabird populations Nets set inshore for

lumpfish also catch high numbers of marine birds, especially Black Guillemots (Cepphus grylle) and Common Eiders (Somateria mollissima; Petersen 1998) Some evidence indicates that juvenile

and immature murres may be more vulnerable to net mortality than older birds, suggesting thatbirds may learn to avoid nets (Strann et al 1991, Brothers 1999), as marine mammals do (Lien

et al 1988)

During the 1990s, the Japanese set about 150,000 km of salmon drift nets (Artukhin et al.2000) and almost 2,000,000 km of squid drift nets in the North Pacific (DeGange et al 1993).Concurrent increases in the frequency of free-traveling, unattended nets have increased the mortality

of birds and other marine animals, and continue to do so as fixed gear is lost or discarded Manyseabirds, especially gannets and cormorants, scavenge bits of nets, rope, line, etc from the seasurface for nest material that may in turn entangle adults and chicks at nests (Montevecchi 1991)

Alcids, Northern Gannets, and Great Cormorants (Phalacrocorax carbo) collected during beach

surveys are often entangled in fishing gear (Tasker et al 2000; see Figure 16.2)

FIGURE 16.1 Common Murre holding a capelin to be delivered to a chick (Photo by W A Montevecchi.)

Interactions between Fisheries and Seabirds 531

The mortality imposed by gill nets is evident when nets are removed For instance, the closure

of the Atlantic salmon fishery (Potter and Crozier 2000) and ground-fisheries in eastern Canadaduring the 1990s removed gill nets in waters off eastern Newfoundland Concordantly, numbers ofbreeding murres, Atlantic Puffins, Razorbills, and gannets appear to be responding positively.Increases in murre populations have also been reported following closure of the drift-net salmonfishery in western Greenland (Piatt and Reddin 1984)

Even though high-seas gill nets have been banned globally, much of the fishing effort that usedgill nets subsequently shifted focus to long-lining (see Brothers et al 1999; see previous section).Thus, while populations of pursuit-diving seabirds benefited from this ban, populations of surface-feeding birds have suffered from the consequences

Long-lines — Long-line fishing (i.e., setting extensive lines of more than 100 km in length with

hundreds of thousands of baited hooks) is an old technique that is used in all of the world’s oceans(Bjordal and Løkkeborg 1996) Long-line fishing is generally conservative, in that it catches mainlytarget species and causes little disturbance to habitat (Løkkeborg 1998) Pelagic long-lining fisheriesare directed at tuna, swordfishes, and sharks, primarily in tropical and temperate oceans, anddemersal long lining is directed at deep-water fishes like cod, halibut, hakes, toothfish, and snappers

in colder waters Fisheries for large pelagic fishes operate near ocean fronts and continental shelfbreaks where marine birds forage (Croxall and Prince 1996, Robertson 1998, Brothers et al 1999).The major pelagic long-line fisheries for tuna are Japan, Taiwan, and Korea, primarily in the PacificOcean (Figure 16.3) Pelagic long-line fisheries for swordfish are smaller and carried out by Spain,the U.S.A., Canada, Portugal, Italy, Greece, and Brazil, mostly in the Atlantic (Figure 16.4).Seabirds vulnerable to long-line fisheries include those that feed at or near the surface, scavenge,

and attempt to steal bait from hooks These include petrels (e.g., Northern Fulmars, Fulmarus

FIGURE 16.2 Drowned murre in a fragment of net washed up on a beach on the south coast of Newfoundland,

Canada (Photo by W A Montevecchi.)

glacialis; White-chinned Petrel, Procellaria acquinoctialis; Giant Petrels, Macronectes spp.),

alba-trosses (e.g., Gray-headed Albatross, Diomedea chrysostoma; Black-browed Albatross, D

mel-anophris; Wandering Albatross, D exulans; Black-footed Albatross, Phoebastria nigripes; Laysan

Albatross, P immutabilis; Mollymawk Albatrosses, Thalassarche spp.), gulls, and skuas (Cherel et

al 1995, Croxall and Prince 1996, Brothers et al 1999, Tasker et al 2000; Figure 16.5) Majorby-catches of seabirds are documented in the Southern and Pacific Oceans where Brothers (1991)estimated that approximately 108,000,000 hooks are set by the Japanese tuna fisheries with anestimated annual mortality of 44,000 albatrosses Clearly, by-catches of this magnitude hold serious,nonsustainable consequences for long-lived albatrosses and petrels (Brothers, 1991, Moloney et al

1993, Robertson and Gales 1998, Brothers et al 1999) These consequences are intensified byseabird by-catches that are both adult- and sex-biased (Brothers et al 1999) Short-tailed Albatrosses

(Phoebastria albatrus) and endangered Spectacled Petrels (Pteraldroma conspicillata) are also

caught (Table 16.3; Ryan 1998, Brothers et al 1999)

Many hundreds of thousands and possibly millions of seabirds are killed by long-line fisheries

Table 16.4 summarizes the available information on fishing effort and avian mortality in the world’slong-line fisheries Much information still needs to be collected in order to assess fisheries effects

on birds, e.g., Indian Ocean, northwest Atlantic Additionally, there is little information on ulated and illegal long-line fisheries that operate in many regions (Brothers et al 1999) Birds are

unreg-FIGURE 16.3 Long-line catches of tuna (Based on data in Brothers et al 1999.)

FIGURE 16.4 Long-line catches of swordfish (Based on data in Brothers et al 1999.)

OCEAN ATLANTIC PACIFIC INDIAN

0 50 100 150 200 250 300

also killed by ingesting hooks in discarded offal and by-catch, as well as by sport fishers (Figure16.6) Moreover, long-lining crews commonly shoot birds to discourage bait-stealing (Brothers et

al 1999)

Seabird mortality can be reduced by using streamers trailed on lines behind vessels to scarebirds, by releasing baited hooks under water line and at night, by increasing their sinking rates,and by avoiding discards (Brothers 1991, Cherel et al 1995, Løkkeborg 1998, Robertson 1998,Brothers et al 1999) However, mitigative procedures are not currently widely used (Croxall andPrince 1996) and require the cooperation of fishers for effective implementation (Robertson 1998).Interestingly, Løkkeborg (2000) showed that streamers on lines trailed behind vessels (see Figure

2 in Tasker et al 2000) significantly reduce bird by-catch and bait-loss and increase target fish

catch This effect could motivate fishers to incorporate these techniques

16.2.1.2 Disturbance

Shellfish aquaculture sites remove potential habitat from use by seaducks, while their dense culturedfood sources also attract them Cormorants, gulls, diving ducks, and other birds are attracted to siteswhere marine fishes are held in cages or holding pens (Kirby et al 1996), and to rivers and estuarieswhere hatchery-reared fishes are released (e.g., Wood 1985, Kalas et al 1993, Cairns 1998) Many

cormorants (Phalacrocorax spp.), Shags (P aristotelis), and herons are shot in these situations (e.g.,

Carss 1994) Birds also are disturbed by fishers and fishing vessels working near concentrations ofnonbreeding birds and near colonies where they at times store gear (e.g., lobster pots)

16.2.2 I NDIRECT E FFECTS

16.2.2.1 Prey Depletion

Negative effects of fisheries on seabirds are expected when fisheries target the same species andsize-classes that birds consume In contrast, when fisheries take fishes larger than those that birdsprey on, the effects of fisheries on seabirds can be positive (see below) Instances of the formerare associated with industrial fisheries that exploit abundant, highly aggregative species for fishmeal and oil that are used for animal feeds, aquaculture, and other industrial uses (Aikman 1997).Catches by industrial fisheries doubled in the last 30 to 40 years (Aikman 1997), consistent withpatterns of overfishing stocks (“raw material”) to commercial extinction Industrial fisheries accountfor about a third of the world catch of marine fish (Coull 1993, Aikman 1997) As inappropriate

as it seems, sandlance catches in the North Sea are essentially unregulated (Aikman 1997).Another complication of fishery effects on the depletion of seabird prey involves the by-catch

of nontarget species For example, the by-catches of larval and forage fishes in small-mesh shrimptrawls are very substantial (Alverson and Hughes 1996, Aikman 1997), at times exceeding shrimp

TABLE 16.3

Endangered and Critically Endangered Seabird Species That Are

Killed by Long-Line Fishing Activities

Tristan Albatross, Diomedea dabbenena Endangered South Atlantic

Northern Royal Albatross, D sanfordi Endangered Southern Ocean

Amsterdam Albatross, D amsterdamensis Critically Endangered Southern Ocean

Chatham Albatross, Thalassarche eremita Critically Endangered Southern Ocean

Spectacled Petrel, Pterodroma conspicillata Endangered Southern Ocean

Note: International Union for the Conservation of Nature (IUCN) Criteria.

Estimated Numbers of Hooks Set, Catch Rates and Estimated Seabird Mortality by Different Long-Line Fisheries in Different

Oceanographic Regions (based primarily on Brothers et al 1999)

Fishery (years)

Oceanographic Region

Estimated No.

of Hooks Set ( × 10 6 )

Catch Rates/1000 Hooks (range [median])

Estimated

Patagonian toothfish; king klip hake,

tung, shark (1988–1997)

Southern 125 <0.01–1.90

[0.32]

0.043 (mitigation) 0.02 (night)

32,268–40,200 White-chinned Petrel, Albatrosses (Grey-headed, Black-browed,

Wandering, Shy, Antipodean, Chatham, White-capped, footed, Buller’s, Yellow-nosed, Sooty, Campbell), Petrels (Giant, Grey, Giant-winged, Westland, Black, Pintado), Cape Gannet, Subantarctic Skua, Penguins (Gentoo, Macaroni), Shearwaters (Short-tailed, Wedge-tailed, Flesh-footed, Sooty)

Cod, ling, haddock, redfish

(1980s)

NE Atlantic ~1000 [1.75]

0.04 (scaring)

1,750,000 ? Fulmar, Gannet Wolffish, swordfish, tuna (1996) NE Atlantic 0.49 (underwater

sets)

Skua, Gulls (Glaucous, Great Black-backed, Lesser Black-backed, Herring)

Cod, tusk, haddock, halibut, plaice,

saithe, hake, tuna, swordfish (1996)

[7.6]

? Petrels (White-chinned, Spectacled), Albatrosses (Wandering

Black-browed, Yellow-nosed), Shearwaters Halibut, pollock, cod, sablefish, turbot,

rockfish, flounder, tuna, sharks,

Tuna, swordfish, sharks, snappers, hake,

king klip, skate (1994–1995)

Central Pacific 7+ 0.083–0.41

[0.214]

1,898 Albatrosses (Yellow-nosed, Laysan, Chatham, Black-browed,

Petrels (White-chinned, Spectacled), Shearwaters (Great, Cory’s)

© 2002 by CRC Press LLC

catches by an order of magnitude or more (e.g., Pender et al 1992; see Pauly et al 1998) feeding seabirds (e.g., gulls, terns) and shallow-diving species (e.g., puffins) are generally consid-ered the most vulnerable to the over-fishing of small pelagic fishes (see Furness and Ainley 1984,

Surface-Monaghan et al 1992), though deep divers such as murres can also be negatively affected (e.g.,Vader et al 1990a)

There are many demonstrations of the negative effects of intense and over-exploitive fishingpressures on the reproduction and populations of seabirds (Table 16.5) For example, the breeding

FIGURE 16.6 Brown Pelican hooked by a sports fisher in Florida (Photo by E A Schreiber.)

TABLE 16.5

Associations between Intense Fishing Pressures and Breeding Failures or Population Declines

of Seabirds

Jackass Penguin, Cape Gannet Pilchard Benguela 1956–1980 Burger and Cooper 1984, Crawford et al

1985 Peruvian Brown Pelican,

Black-legged Kittiwake,

Arctic Tern, Common Tern,

Common Murre

Sandlance Herring

Shetland North Sea

1986–1990 Furness 1990, Monaghan et al 1989,

1992, Uttley et al 1989, Huebeck 1989; Hamer et al 1991; Bailey et al 1991, Klomp and Furness 1992, Monaghan 1992

Atlantic Puffin Herring Norway

North Sea

Anker-Nilssen 1987, 1992, Barrett et al

1987, Vader et al 1990b; Anker-Nilssen and Røstad 1993

Atlantic Puffin Capelin NW Atlantic 1981 Brown and Nettleship 1984

Common Murre Capelin N Norway 1985–1987 Vader et al 1990a, b

Interactions between Fisheries and Seabirds 537

population of Cape Gannets (Sula capensis) in southern Africa decreased by about half from 1956

to 1980 and has been attributed to a fishery-induced collapse of pilchard (Sardinops ocellata) and reduced availability of anchovies (Engraulis capensis; Crawford et al 1980, 1983) The over-

exploitation and crash of anchoveta fishery and the resultant population declines of Peruvian guanobirds have been well documented (Schaefer 1970, Paulik 1981) The fishery-induced collapse ofAtlanto-Scandian herring stock along the Norwegian coast resulted in 14% per annum declines inthe breeding population of Atlantic Puffins in Lofoten (Anker-Nilssen and Røstad 1993) Breedingpopulations of Common Murres in the Barents Sea were reduced by 80% in the mid-1980s due to

a drastic reduction in capelin stocks induced at least in part by industrial fishery catches (Vader et

al 1990a, b) Changes in the diets of seabirds are also associated with fishery catches For example,due to economic incentives from Asian markets, there was intense fishing pressure on short-finned

squid (Illex illecebrosus) during the late 1970s in the northwest Atlantic (Figure 16.7) These squid

subsequently disappeared from both seabird diets and commercial catches for at least two decades(Montevecchi and Myers 1995, 1996, unpublished)

Some of the complexities of potential effects of fisheries on seabirds were pointed out by Burgerand Cooper (1984) They suggested that purse-seine fisheries targeting pelagic pilchards off southernAfrica negatively affected pursuit-diving penguins through prey depletion but also positively affectedsurface-foraging gannets by providing fishery discards (see also Duffy et al 1987; see below) Furness(2000) suggested that the huge harvests of sandeel fisheries in the North Sea may not have negativelyaffected seabird populations because fishery catches and seabird consumption occur in different

regions and because the population of major predators of forage fish (mackerel, Scomber scombrus)

was greatly reduced (Camphuysen and Garthe 2000) Over-fishing large pelagic fishes in tropicaloceans can have an opposite effect on marine birds dependent on these fishes to drive small fishes

to the surface where birds can access them (Au and Pitman 1988, Safina and Burger 1985)

In considerations of seabird population fluctuations and their associations with fisheries, it isnotable that both marine bird and fish populations often change significantly in the absence offisheries (Cushing 1982, Hatch et al 1993, Carscadden et al 2001) Extreme population fluctuationsoccurred well before fisheries were initiated (e.g., Soutar and Isaacs 1974) Climatic and oceano-graphic changes can induce significant fluctuations in avian populations (e.g., Schreiber andSchreiber 1984, 1989, Montevecchi and Myers 1997; see Chapter 7), fishes (Welch et al 1998,

2000, Carscadden et al 2001), and other marine animals Both physical perturbations and fisheriesimpacts can at times induce regime shifts, i.e., ecosystem-wide changes in community structures

FIGURE 16.7 Catches of short-finned squid from different areas in eastern Canada (After Montevecchi

1993b.)

100 80 60 40 20

and food webs (Steele 1996, 1998) Because physical oceanographic influences interact with fishingand pollution, it is often very difficult to partition effects attributable to either natural or anthro-pogenic factors (Duffy and Schneider 1994, Steele 1996, Tasker et al 2000; cf Hutchings andMyers 1994).

16.2.2.2 Competition and Predation by Scavenging Seabirds

In many locations, populations of scavenging gulls, kittiwakes, skuas, fulmars, and gannets havebecome dependent on food sources associated with fishery discards and offal (see next section).Many of these scavenging birds include species that are often highly predatory on other seabirds.Owing in large part to their exploitation of discards, populations of scavenging gulls and skuashave increased sharply during the 20th century (Furness et al 1992, Howes and Montevecchi 1992)and are having negative effects on other seabirds For example, gulls displaced tern colonies in theGulf of St Lawrence (Howes and Montevecchi 1993), the mid-Atlantic U.S coast (Burger andGochfeld 1991), and the Wadden Sea (Becker and Erdelen 1987)

The potentially negative, indirect effects of fishery waste generation can be intensified whendisposal is reduced or eliminated (Furness 2000) The use of larger mesh sizes permits the escape

of smaller target species, reduces by-catch, and decreases the number of discards available toscavenging birds (Hudson and Furness 1988) The closure of the eastern Canadian ground-fisheryfrom 1992 through 1999 essentially eliminated discarding and offal production in the northwest

Atlantic and imposed severe food-stress on populations of Herring Gulls (Larus argentatus) and Great Black-backed Gulls (L marinus; Regehr and Montevecchi 1997) Consequently, these gulls greatly increased predation pressure on Leach’s Storm-Petrels (Oceanodroma leuchoroa), Black- legged Kittiwakes (Rissa tridactyla), and Atlantic Puffins (Russell and Montevecchi 1996, Regehr

and Montevecchi 1997, Stenhouse and Montevecchi 1999) Similar effects of discard removal were

reported for Great Skuas (Catharacta skua; Phillips et al 1999), Yellow-legged Gulls (Larus

cachinnans), and Audouin’s Gulls (L audouinii; Oro and Martinez-Vilalta 1994, Oro et al 1995).

If the elimination of discards and offal continues in the longer term, then scavenger (e.g., gull)populations are expected to follow them in a density-dependent manner (Figure 16.8) However,

as scavenger/predator populations decline to supportable levels, extreme pressure will be exerted

on prey species by food-stressed predators

FIGURE 16.8 Populations of Herring and Great Black-backed Gulls in the Gulf of St Lawrence off western

Newfoundland, Canada, and local fishery landings used as indices of fishery discards and offal production (W A Montevecchi, unpublished.)

Interactions between Fisheries and Seabirds 539

16.3 POSITIVE EFFECTS OF FISHERIES ON SEABIRDS

16.3.1 D IRECT E FFECTS

16.3.2 Provisioning of Fisheries Discards and Offal

Offshore fishing vessels generate huge tonnages of discarded fish, scraps, and waste from demersalfishes and invertebrates that would otherwise be unavailable to marine birds (Table 16.6), creating

a global feeder-at-sea program for avian scavengers Otter trawlers and stern trawlers attract highernumbers of birds compared to beam trawlers and purse seiners, because they produce the higherlevels of discards (Camphuysen et al 1995)

Many avian species scavenge at trawlers (Hudson and Furness 1988, Furness et al 1992, Blaber

et al 1995) Fulmars and kittiwakes are the most common and can aggregate so densely at trawlersthat their concentrations have been referred to as “blizzards” (R.G.B Brown pers comm.) Theutilization of fish discards and offal from vessels and from fish plants influences populations ofscavenging birds (Oro et al 1995, Oro 1996, Garthe et al 1996, Hüppop and Wurm 2000) For

example, during the 20th century, populations of fulmars, large Larus gulls, kittiwakes, skuas, and

gannets have increased greatly and expanded in association with increasing levels of fishery discardsthroughout the North Atlantic (Fisher 1952, Kadlec and Drury 1968, Drury and Kadlec 1974, Harris

1970, Howes and Montevecchi 1993, Camphuysen et al 1995, Stenhouse and Montevecchi 2000),the South Atlantic (Burger and Cooper 1984), and elsewhere

Discards and offal comprise about 30% of the food of seabirds in the North Sea (Tasker andFurness 1996) Up to 70% of the diet of adult Great Skuas and about 30% of the food fed to theirchicks in Shetland consists of discards (Furness and Hislop 1981) These percentages increasedwhen the abundance of sandlance declined (Hamer et al 1991) However, when the proportion ofdiscards increased at the expense of sandlance in the chicks’ diet, their growth was reduced (Furness1987) The tonnages of discards and offal produced (Table 16.6) could potentially support morethan 6 million birds in the North Sea (Garthe et al 1996)

Most seabirds attending fishing vessels are adults, and scavenging levels are highest in winterwhen there is more competition for scraps Discards are partitioned by size and shape, and byfeeding technique among seabirds (Garthe and Hüppop 1998) In food-stealing interactions atfishing vessels, the largest species (gannets, Great Black-backed Gulls, skuas) fare best (Hudsonand Furness 1988, Garthe and Hüppop 1998) Hence, if discards are reduced and competitionincreases, smaller species are expected to fare worse (Tasker et al 2000)

Multiple influences of fisheries on seabirds are common For example, purse-seine fisheries forpelagic fishes (e.g., pilchards) off southern Africa appear to have positively affected surface-foraging

TABLE 16.6

Estimated Discards and Offal Produced by Fishing Vessels in the

North Sea and Estimated Consumption by Marine Birds

Item Discarded Tonnage

Energy Density (kJ/g)

Estimated Consumption by Birds

gannets by providing discards while at the same time negatively affecting pursuit-diving penguinsthrough reduction of pelagic fish stocks (Burger and Cooper 1984) For wide-ranging species such

as albatrosses, petrels, and fulmars, discarding can change their distributions at sea (Abrams 1983,Ryan and Moloney 1988) The consequences of such changes may be either positive as in thebreeding range expansions of fulmars (Fisher 1952, Stenhouse and Montevecchi 1999) or potentially

negative as in the case of large numbers of Black-browed and Shy Albatrosses (Diomedea cauta)

being attracted to trawling sites well outside of their “normal” foraging areas in the southernBenguela region (Abrams 1983) Some birds including penguins, cormorants, and petrels oftenavoid feeding aggregations at trawlers (Ryan and Moloney 1988)

16.3.2 I NDIRECT E FFECTS

16.3.2.1 Removal of Competitors — Multispecies Interactions

Ecosystem interactions are often less straightforward than they appear initially, and concepts ofsurplus production and predator release are tenuous (May et al 1979, Lavigne 1996) The over-harvesting of large predators has, however, been associated with increases in the abundance of

forage fishes used by seabirds The depletion of herring (Clupea harengus) and mackerel in the

North Sea resulted in increases in the abundances of sandlance and sprats (Sherman et al 1981;see also Springer et al 1986, Hatch and Sanger 1992) Over-fishing Atlantic Cod (Gadus morhua)

in the northwest Atlantic (Hutchings and Myers 1994) removed a major predator of the primary

prey, capelin (Mallotus villosus), of marine birds and mammals (Table 1) Hence, the depletion of

cod can be expected to enhance food conditions for birds in the northwest Atlantic during the nextdecade (see Table 16.1) Factor in the removal of inshore fishing gear during the eastern Canadianground-fishery closure (1992–1999), and the circumstances for seabirds in the northwest Atlanticappear even better Perhaps the greatest ecosystem concern is that there is no indication of populationincreases by capelin (Carscadden and Nakashima 1997), though situations may well be beneficialfor birds

Increases in the population of Chinstrap Penguins (Pygoscelis antarctica) in the Southern Ocean

since about 1950 were linked to the depletion of baleen whales by commercial whaling and asubsequent increase in krill abundance (Conroy 1975, Coxall et al 1984) This relationship wasquestioned, however, and growing penguin populations are attributed to climate warming and lessextensive sea ice cover that in turn gave breeding penguins easier access to foraging sites (Fraser

et al 1992)

16.3.2.2 Increase Abundance of Small Fishes

As indicated above, over-fishing large predatory fish has at times resulted in increased abundance

of their prey, including small plantivorous fishes and crustaceans (Sherman et al 1981, Hamre1988) Increased numbers of juveniles (small fish) of commercially exploited species can be markedamong species such as cod and pollock that are cannibalistic When such events occur they canbenefit seabirds and be reflected in increases in their populations

16.4 NEGATIVE EFFECTS OF SEABIRDS ON COMMERCIAL FISHERIES

16.4.1 D IRECT I NFLUENCES

16.4.1.1 Interactions with Aquaculture

Aquaculture is a major global growth industry that accounts for about 15% of current world fisheriesproduction (FAO 1995) Successful aquaculture ventures can lead to complacency about the state

of wild fisheries Predation by cormorants on channel catfish (Ictalurus punctatus) farms in the

Interactions between Fisheries and Seabirds 541

southeastern U.S is a problem (Glahn and Stickney 1995, Glahn and Brugger 1995) However,even though their predation effects may be negligible on the whole (e.g., Kalas et al 1993, Wootenand Dupree 2000), they can impact individual farming operations Permits are issued to controlavian predators (such as cormorants, shags, anhingas, grebes, seaducks, herons, and kingfishers)feeding at fish pens and shellfish farms in the U.S (Belant et al 1998, Kirby et al 1996) Thepredation tends to be size-selective, and in situations where avian predators take many fish fromholding pens, the vast bulk tend to be small fingerlings (10 to 20 cm length; Glahn and Stickney,1995) There are some means to mitigate predator effects (see below) About five times the number

of waterbirds were killed at fish farms in Arkansas compared to Mississippi, even though there isalmost twice as much area taken up by catfish farms in Mississippi (Belant et al 1998) This isbecause there are many more baitfish farms in Arkansas (Belant et al 1998) The permitted killing

of waterbirds at aquaculture sites is increasing (Figure 16.9), and with rapidly expanding tural industries, fish farming and bird conservation are clearly proceeding on a head-on collisioncourse Belant et al (1998) argue that the levels of bird killing at catfish farms in the southeasternU.S do not influence local avian populations as indicated by Christmas Bird Counts (CBC) They

aquacul-do not, however, point out that the number of Great Egrets killed by catfish farmers (1000s) is anorder of magnitude higher than number of birds sighted on Arkansas CBC (100s) They report thatfarmers shot only about 60% of the number of waterbirds that they were permitted to kill Thissuggests that kill permits may be excessive, though these are also circumstances in which under-reporting would be expected

16.4.1.2 Bait Stealing

Bait stealing by seabirds can pose problems for long-lining fisheries (Brothers 1991, Løkkeborg1998) Techniques for scaring birds, for releasing baited hooks at night and underwater, as well asfor increasing the sinking rates of baited hooks may benefit the fishery directly and help minimizeseabird mortality

predators, consume substantial tonnages of prey (Table 16.1), at times representing up to 30% ofthe estimated production in a localized area (see Montevecchi 1993a) The question then becomes,what consequences, if any, do these consumption levels hold for commercial fisheries? Second,estimates of the consumption of fresh-water fishes, especially those that spend significant parts

of their life cycle at sea, are used in considerations of avian “impacts” on both commercial andsport fisheries

A recent bioenergetic modeling exercise led to the suggestion that predation by Northern

Gannets could negatively impact the population of Atlantic salmon (Salmo salar) in the northwest

Atlantic (D Cairns and W Montevecchi unpublished) Interestingly, the salmon are not an importantprey for gannets, comprising less than 3% of their diet on average (W Montevecchi and D Cairnsunpublished) Hence, while gannets may negatively influence the population dynamics of salmon,there are no ecologically responsible management options to address this interaction

Avian predators have been considered to limit salmon production (Elson 1962) crested Cormorants eat salmon and trout, and also damage fish that they do not kill (Kirby et al

Double-1996, Cairns 1998) Levels of avian consumption of commercially exploited fish are greater inrestricted freshwater environments (rivers, streams, lakes) than in the open ocean, though localconsumption can be inappropriately generalized to larger scales and populations (Scheel andHough 1997) Birds often show aggregative responses to hatchery-released fishes (Figure 16.10),but not to natural smolt runs (Wood 1985, Bayer 1986, Mullins et al 1999) This is possiblyrelated to differences in the behavior of hatchery- and wild-reared smolts Many seabirds, includ-ing cormorants, gulls, murrelets, and terns, prey on concentrations of hatchery-released salmonids.While birds consume many hatchery-reared fishes, they do little to negatively impact populations(Scheel and Hough 1997), except in localized situations (see Cairns 1998) Assessments of

FIGURE 16.10 Aggregative responses of surface-feeders (Black-legged Kittiwakes) and pursuit-divers

(Mar-bled Murrelets Brachyramphus marmoratus) and all piscivorous birds to concentrations of hatchery-released pink salmon (Oncorhynchus gorbuscha) and chum salmon (O keta) in Prince William Sound, Alaska (After

Scheel and Hough 1997.)