527 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 © 2002 by CRC Press LLC 528 Biology of Marine Birds 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 (tons) Source 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 © 2002 by CRC Press LLC Interactions between Fisheries and Seabirds 529 in the availability of small fishes. Owing to the small biomass of birds compared to fishes in the world’s oceans, influences of seabirds on fisheries tend to be localized, small-scale events, often occurring 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 to interannual 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 breeding populations, 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 environmental factors that caused them. Seabird populations are therefore buffered from environmental perturba- tions that influence annual production (Montevecchi and Berutti 1991). Yet even slight changes in adult mortality can have profound effects on seabird populations (Furness 2000). Hence, throughout this review an attempt is made to differentiate potential fishery influences on reproduction from those on adult survival. The present chapter reviews the influences of fisheries on marine birds and also reviews the influences 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 them are also provided. 16.2 NEGATIVE EFFECTS OF FISHERIES ON SEABIRDS 16.2.1 D IRECT EFFECTS 16.2.1.1 Entrapment in Fishing Gear By-catches of seabirds in fishing gear have resulted in negative population effects on birds on a global 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, and gannets 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 that dive below the foraging range of these species. Among the pursuit divers, birds that densely aggregate (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 TABLE 16.2 Influences of Fisheries on Marine Birds Negative Positive Direct Entrapment in fishing gear Disturbance Provide food via fisheries discards and offal Indirect Prey depletion Increase populations of scavengers/predators Increase predation by removing artificial food sources of scavengers Remove competitors Increase abundances of small fishes © 2002 by CRC Press LLC 530 Biology of Marine Birds Nettleship 1987). Entrapments are often most frequent during periods of fish movements near fishing gear (Christensen and Lear 1977, Piatt and Nettleship 1987), when both birds and targeted fish 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 of Common 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 from colonies (Ainley et al. 1981, Piatt and Nettleship 1987) indicate that no-fishing zones around breeding 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 that birds 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. Many seabirds, especially gannets and cormorants, scavenge bits of nets, rope, line, etc. from the sea surface 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.) © 2002 by CRC Press LLC 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 Canada during the 1990s removed gill nets in waters off eastern Newfoundland. Concordantly, numbers of breeding 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 salmon fishery in western Greenland (Piatt and Reddin 1984). Even though high-seas gill nets have been banned globally, much of the fishing effort that used gill 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 mainly target species and causes little disturbance to habitat (Løkkeborg 1998). Pelagic long-lining fisheries are directed at tuna, swordfishes, and sharks, primarily in tropical and temperate oceans, and demersal 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 shelf breaks 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 Pacific Ocean (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.) © 2002 by CRC Press LLC 532 Biology of Marine Birds 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). Major by-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 an estimated 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 by seabird 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’s long-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 unreg- ulated and illegal long-line fisheries that operate in many regions (Brothers et al. 1999). Birds are 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 CATCH (1000s TONNES) 0 50 100 150 200 250 300 OCEAN ATLANTIC PACIFIC INDIAN CATCH (TONNES) 0 5 10 15 20 25 30 35 40 © 2002 by CRC Press LLC Interactions between Fisheries and Seabirds 533 FIGURE 16.5 Long-liner setting hooks with Laysan and Black-footed Albatrosses taking the bait. (Dra wing by J. Zickefoose.) © 2002 by CRC Press LLC 534 Biology of Marine Birds also killed by ingesting hooks in discarded offal and by-catch, as well as by sport fishers (Figure 16.6). 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 scare birds, 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 and Prince 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 cultured food sources also attract them. Cormorants, gulls, diving ducks, and other birds are attracted to sites where marine fishes are held in cages or holding pens (Kirby et al. 1996), and to rivers and estuaries where 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 of nonbreeding birds and near colonies where they at times store gear (e.g., lobster pots). 16.2.2 INDIRECT EFFECTS 16.2.2.1 Prey Depletion Negative effects of fisheries on seabirds are expected when fisheries target the same species and size-classes that birds consume. In contrast, when fisheries take fishes larger than those that birds prey on, the effects of fisheries on seabirds can be positive (see below). Instances of the former are associated with industrial fisheries that exploit abundant, highly aggregative species for fish meal 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 with patterns of overfishing stocks (“raw material”) to commercial extinction. Industrial fisheries account for 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 shrimp trawls 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 Species IUCN Status Ocean 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. © 2002 by CRC Press LLC Interactions between Fisheries and Seabirds 535 TABLE 16.4 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 Mortality Species killed 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, Flesh- 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) Tuna, swordfish Indian ~154 ? ? Albatrosses 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) NW Atlantic 200+ ? ? Fulmar, Shearwaters, Gulls Tuna, swordfish (1987–1994) Atlantic ? 0.8–15 [7.6] ? Petrels (White-chinned, Spectacled), Albatrosses (Wandering Black- browed, Yellow-nosed), Shearwaters Halibut, pollock, cod, sablefish, turbot, rockfish, flounder, tuna, sharks, swordfish (1996) NE Pacific 510 0.059–0.087 [0.073] 13,042–37,230 Fulmar, Gulls, Shearwaters, Albatrosses (Laysan, Black-footed, Short-tailed) Pollock, cod, halibut NW Pacific 270+ ? ? 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 536 Biology of Marine Birds catches by an order of magnitude or more (e.g., Pender et al. 1992; see Pauly et al. 1998). Surface- 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, 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 fishing pressures 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 Bird Fish Location Date Sources Jackass Penguin, Cape Gannet Pilchard Benguela 1956–1980 Burger and Cooper 1984, Crawford et al. 1985 Peruvian Brown Pelican, Guanay Cormorant, Peruvian Booby Anchoveta Humboldt Current 1950s–1970s Duffy 1983 Brown Pelican, Elegant Tern Anchovy S. California 1969–1980 Anderson et al. 1982, Anderson and Gress 1984, Schaffner 1986 Shag, Great Skua, 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 © 2002 by CRC Press LLC [...]... general orientations First, bioenergetic models of prey consumption indicate that birds, like other marine © 2002 by CRC Press LLC 542 Biology of Marine Birds FIGURE 16. 10 Aggregative responses of surface-feeders (Black-legged Kittiwakes) and pursuit-divers (Marbled Murrelets Brachyramphus marmoratus) and all piscivorous birds to concentrations of hatchery-released pink salmon (Oncorhynchus gorbuscha)... 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.) © 2002 by CRC Press LLC Interactions between Fisheries and Seabirds 539 16. 3 POSITIVE EFFECTS OF FISHERIES ON SEABIRDS 16. 3.1 DIRECT EFFECTS 16. 3.2... Applications of marine refugia to coastal fisheries management Canadian Journal of Fisheries and Aquatic Sciences 50: 2029–2042 ELSON, P F 1962 Predator-prey relationships between fish-eating birds and Atlantic salmon Bulletin Fisheries Research Board of Canada 47: 39–53 ELLIOT, R D 1991 The management of the Newfoundland turr hunt Pp 29–35 in Studies of High-Latitude Seabirds 2 Conservation Biology of Thick-billed... some of which prey on commercial species (e.g., cunners on cod) No other direct evidence of prey depletion by seabirds is available, and due to the small biomass of birds in marine ecosystems, none is expected on spatial scales relevant to commercial fisheries 16. 5 POSITIVE EFFECTS OF SEABIRDS ON COMMERCIAL FISHERIES 16. 5.1 DIRECT EFFECTS 16. 5.1.1 Birds as Fishing Devices There are a few instances of. .. and offal comprise about 30% of the food of seabirds in the North Sea (Tasker and Furness 1996) Up to 70% of the diet of adult Great Skuas and about 30% of the food fed to their chicks in Shetland consists of discards (Furness and Hislop 1981) These percentages increased when the abundance of sandlance declined (Hamer et al 1991) However, when the proportion of discards increased at the expense of sandlance... DIRECT EFFECTS 16. 3.2 Provisioning of Fisheries Discards and Offal Offshore fishing vessels generate huge tonnages of discarded fish, scraps, and waste from demersal fishes 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 higher numbers of birds compared to beam trawlers... minimize avian mortality from long-line fisheries Nocturnal line-settings can reduce avian by-catch by 60 to almost 100%, though nocturnal species like White-chinned Petrels are often hooked during night sets (Cherel et al 1995, Brothers et al 1999) Nocturnal settings are less © 2002 by CRC Press LLC 546 Biology of Marine Birds effective on moonlit nights, and are often not possible at high latitudes... 1963 1968 1973 1978 1983 YEAR FIGURE 16. 7 Catches of short-finned squid from different areas in eastern Canada (After Montevecchi 1993b.) Some of the complexities of potential effects of fisheries on seabirds were pointed out by Burger and Cooper (1984) They suggested that purse-seine fisheries targeting pelagic pilchards off southern Africa negatively affected pursuit-diving penguins through prey depletion... Polar Biology 11: 525–531 FURNESS, R W 1978 Energy requirements of seabird communities: a bioenergetics model Journal of Animal Ecology 47: 39–53 FURNESS, R W 1990 A preliminary assessment of the quantities of Shetland sandeels taken by seabirds, seals, predatory fish and the industrial fishery in 1981–1983 Ibis 132: 205–217 FURNESS, R W 1993 Birds as monitors of pollution Pp 84–143 in Birds as Monitors of. .. types of plastic in gannet nests in the northwest Atlantic Canadian Journal of Zoology 69: 295–297 MONTEVECCHI, W A 1993a Birds as indicators of change in marine prey stocks Pp 217–266 in Birds as Monitors of Environmental Change (R W Furness and J J D Greenwood, Eds.) Chapman & Hall, London MONTEVECCHI, W A 1993b Seabird indication of squid stocks in the Northwest Atlantic Journal of Cephalopod Biology . 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. 545 16 © 2002 by CRC Press LLC 528 Biology of Marine Birds 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. 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