Article

Consumption of discards and interactions between Black-browed Albatrosses (Thalassarche melanophrys) and Kelp Gulls (Larus dominicanus) at trawl fisheries in Golfo San Jorge, Argentina

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  • Centro para el Estudio de Sistemas Marinos - CONICET
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Abstract

We evaluated discard consumption by Kelp Gulls (Larus dominicanus) and Black-browed Albatrosses (Thalassarche melanophrys) associated with trawlers operating at Golfo San Jorge, Argentina, and assessed differences in their feeding behaviour, prey size preferences and foraging efficiency. Observations were made onboard hake (January 2007) and shrimp vessels (November 2008). The Kelp Gull and Black-browed Albatross were the most frequent and abundant seabirds at both fisheries, while Argentine Hake (Merluccius hubbsi) dominated the composition of discards. Kelp Gulls and Black-browed Albatrosses consumed 91% of experimentally discarded hake (n = 1236). Mean sizes of prey consumed by gulls were 22. 4 and 23. 7 cm in the hake and shrimp fisheries, respectively, while those of prey consumed by albatrosses were 28. 5 and 31. 3 cm, respectively. In both fisheries, gulls selected the smaller prey available (<25 and > 30 cm in hake and shrimp fisheries, respectively) while Black-browed Albatrosses selected the larger prey available (>25 and >30 cm in hake and shrimp fisheries, respectively). Intraspecific and interspecific kleptoparasitism were significantly more frequent than expected in Kelp Gulls and Black-browed Albatrosses, respectively. Robbing efficiency was clearly higher in albatrosses than gulls. In both species, sizes of prey consumed by direct capture were significantly smaller than those stolen, and the rate of kleptoparasitism increased with prey size. Gulls stole significantly smaller prey than albatrosses. Prey selection by Kelp Gulls is affected by the interaction with Black-browed Albatrosses, and the degree at which discards are used by both species appears to depend on the fishery considered, the food on offer and the selection of prey sizes according to seabird species-specific preferences.

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... As store space is limited and shrimp is better priced than hake, the latter was also dominant in discards composition 20 . Fishes, birds, marine mammals and crustaceans of the area were reported to feed on discards [23][24][25][26] . Discard practice makes available a surplus of food to scavengers but also to top-predators. ...
... Discard practice makes available a surplus of food to scavengers but also to top-predators. Such surplus of novel and predictable food item is particularly important for non diving bird, as the kelp gull Larus dominicanus and the albatross Thalassarche melanophrys which are the most important consumers of discards among seabirds 23 . Moreover, discards of hake, a demersal fish not accessible for non diving birds, was identified to be an important factor triggering their population increase observed since 80s' 23,27 . ...
... Such surplus of novel and predictable food item is particularly important for non diving bird, as the kelp gull Larus dominicanus and the albatross Thalassarche melanophrys which are the most important consumers of discards among seabirds 23 . Moreover, discards of hake, a demersal fish not accessible for non diving birds, was identified to be an important factor triggering their population increase observed since 80s' 23,27 . Although some insights were reported at the population level for certain species, how discard biomass impacts the local community remains unaddressed, and this phenomenon requires an integrated ecosystem approach, like the one offered by food web theory 16 . ...
Article
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Demersal fisheries are one of the top anthropic stressors in marine environments. In the long term, some species are more vulnerable to fishery impacts than others, which can lead to permanent changes on the food web. The trophic relationships between predator and prey constitute the food web and it represents a network of the energy channels in an ecosystem. In turn, the network structure influences ecosystem diversity and stability. The first aim of this study was to describe for the first time the food web of the San Jorge Gulf (Patagonia Argentina) with high resolution, i.e. to the species level when information is available. The San Jorge Gulf was subject to intense fisheries thus our second aim is to analyse the food web structure with and without fishery to evaluate if the bottom-trawl industrial fishery altered the network structure and stability. We used several network metrics like: mean trophic level, omnivory, modularity and quasi-sign stability. We included these metrics because they are related to stability and can be evaluated using predator diets that can weight the links between predators and prey. The network presented 165 species organized in almost five trophic levels. The inclusion of a fishery node adds 69 new trophic links. All weighted and unweighted metrics showed differences between the two networks, reflecting a decrease in stability when fishery was included in the system. Thus, our results suggested a probable change of state of the system. The observed changes in species abundances since the fishery was established, could represent the state change predicted by network analysis. Our results suggests that changes in the stability of food webs can be used to evaluate the impacts of human activity on ecosystems.
... As store space is limited and shrimp is better priced than hake, the latter was also dominant in discards composition 20 . Fishes, birds, marine mammals and crustaceans of the area were reported to feed on discards [23][24][25][26] . Discard practice makes available a surplus of food to scavengers but also to top-predators. ...
... Discard practice makes available a surplus of food to scavengers but also to top-predators. Such surplus of novel and predictable food item is particularly important for non diving bird, as the kelp gull Larus dominicanus and the albatross Thalassarche melanophrys which are the most important consumers of discards among seabirds 23 . Moreover, discards of hake, a demersal fish not accessible for non diving birds, was identified to be an important factor triggering their population increase observed since 80s' 23,27 . ...
... Such surplus of novel and predictable food item is particularly important for non diving bird, as the kelp gull Larus dominicanus and the albatross Thalassarche melanophrys which are the most important consumers of discards among seabirds 23 . Moreover, discards of hake, a demersal fish not accessible for non diving birds, was identified to be an important factor triggering their population increase observed since 80s' 23,27 . Although some insights were reported at the population level for certain species, how discard biomass impacts the local community remains unaddressed, and this phenomenon requires an integrated ecosystem approach, like the one offered by food web theory 16 . ...
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Demersal fisheries are one of the top anthropic stressors in marine environments. In the long term, some species are more vulnerable to fishery impacts than others, which can lead to permanent changes on the food web. The trophic relationships between predator and prey constitute the food web and it represents a network of the energy channels in an ecosystem. In turn, the network structure influences ecosystem diversity and stability.The first aim of this study was to describe for the first time the food web of the San Jorge Gulf (Patagonia Argentina) with high resolution, i.e. to the species level when information is available. The San Jorge Gulf was subject to intense fisheries thus our second aim is to analyse the food web structure with and without fishery to evaluate if the bottom-trawl industrial fishery altered the network structure and stability. We used several network metrics like: mean trophic level, omnivory, modularity and quasi-sign stability. We included these metrics because they are related to stability and can be evaluated using predator diets that can weight the links between predators and prey. The network presented 165 species organized in five trophic levels. The inclusion of a fishery node adds 71 new trophic links. All weighted and unweighted metrics showed differences between the two networks, reflecting a decrease in stability when fishery was included in the system. Thus, our results suggested a probable change of state of the system. The observed changes in species abundances since the fishery was established, could represent the state change predicted by network analysis. Our results show how changes in the stability of food webs can be used to evaluate the impacts of human activity on ecosystems.
... When feeding on discards, species composition and behaviour greatly influence feeding success (Garthe & Hüppop 1998, Maynard et al. 2020, where larger and more aggressive species have been reported to displace and/or steal (kleptoparasitism) discards from smaller and less aggressive species (Hudson & Furness 1988b, Jiménez et al. 2011. These interactions, however, can also be influenced by the type of discards (e.g., whole fish or offal) and the type of fishery (i.e., species composition and sizes discarded) owing to species-specific prey preferences of predators (González-Zevallos & Yorio 2011). ...
... For instance, Great Shearwaters are frequently observed associated with longline fishing vessels off the Brazilian coast (Bugoni et al. 2010) and have been reported to approach larger species (e.g., albatross) by lunging at them and, on a few occasions, were capable of stealing food from Pomarine Jaegers (Olmos 1997). Interestingly, kleptoparasitism events were infrequently observed in this study compared with several others (Hudson & Furness 1988a, Camphuysen & Garthe 1997, Garthe & Hüppop 1998, González-Zevallos & Yorio 2011. Less frequently observed kleptoparasitism may be related to the characteristics of the discarded prey (Camphuysen & Garthe 1997), whereby more kleptoparasitic events may occur when larger prey (2020) is discarded (González-Zevallos & Yorio 2011, Hudson & Furness 1988b. ...
... Interestingly, kleptoparasitism events were infrequently observed in this study compared with several others (Hudson & Furness 1988a, Camphuysen & Garthe 1997, Garthe & Hüppop 1998, González-Zevallos & Yorio 2011. Less frequently observed kleptoparasitism may be related to the characteristics of the discarded prey (Camphuysen & Garthe 1997), whereby more kleptoparasitic events may occur when larger prey (2020) is discarded (González-Zevallos & Yorio 2011, Hudson & Furness 1988b. This might be because individuals will usually take longer to manipulate larger prey before swallowing, as previously suggested (Garthe & Hüppop 1998, Spear et al. 2007. ...
Article
Species tend to concentrate in areas with high prey availability, which could lead to competitive interactions within a feeding assemblage as resources become depleted. In coastal Newfoundland, Canada, capelin Mallotus villosus is the focal forage fish species that many top predators feed on during the summer; however, inshore availability varies throughout the boreal summer when abundant aggregations migrate inshore to spawn. We investigated the interactions and responses of Great Shearwaters Ardenna gravis and Sooty Shearwaters A. grisea during their non-breeding season to supplemental food supply under changing natural prey availability (higher and lower capelin availability) by conducting an at-sea experiment during July–August (2015/2016) in coastal Newfoundland. Supplemental food was offered every 30 s over 10 min (‘experimental period’), which was preceded and followed by 10-min control periods (i.e., no food provided). The number of both species increased during the experimental periods, indicating that both species were attracted to the food supplementation experiment. Great Shearwaters were 7.6–13.8 times more likely than Sooty Shearwaters to land near the experimental platform and 95.2 times more likely than Sooty Shearwaters to fight over supplemental food items with individuals of the same or different species. These species-specific tendencies remained consistent as prey availability varied within years, but both species increased in their abundance and interactions with other species (including Herring Gulls Larus argentatus and Northern Fulmars Fulmarus glacialis) during prey capture at lower relative to higher prey availability, as evidenced by lower proportions of flying birds and a greater likelihood of landing on the water. Overall, we suggest that when Great and Sooty shearwaters feed in close association, Great Shearwaters are the more competitively dominant species, which may lead to higher risks of by-catch mortality, especially when the availability of natural prey decreases.
... If individual albatrosses specialize in fisheries products, we would expect their isotopic niches to differ from those of birds feeding predominantly on 'natural' prey. For example, trawlers operating on the Patagonian Shelf mainly target Argentine hake Merluccius hubbsi, kingclip Genypterus blacodes, Patagonian rock cod Patagonotothen ramsayi and hoki Macruronus magellanicus, among other demersal predatory finfish (Copello & Quintana, 2009;González-Zevallos & Yorio, 2011;Granadeiro et al., 2011). Overall, the discards of these fisheries mostly involve undersized and nontarget demersal prey mostly rejected unprocessed and offal from processed target species. ...
... We used information on carbon and nitrogen isotopic ratios of likely relevant prey species from studies carried out in the Falkland Islands and nearby Patagonian Shelf (Copello & Quintana, 2009;Weiss et al., 2009;González-Zevallos & Yorio, 2011;Granadeiro et al., 2011, Granadeiro, Brickle & Catry, unpubl. data). ...
Article
Fisheries can have profound impacts on the structure and functioning of marine ecosystems and affect seabird populations. For seabirds, impacts can include direct mortality in fishing gear, but fisheries also represent an abundant source of food that may otherwise be inaccessible. Previous studies with seabirds have revealed the occurrence of individual foraging specializations, and therefore in scavenging species some individuals may have a higher propensity to feed on fisheries discharges than the rest of the population. Here we used recently-developed techniques (spatio-temporal match of positions) to detect interactions between black-browed albatrosses Thalassarche melanophris and fishing vessels, and also stable isotope analysis of tissues with different turnovers, to investigate long-term individual specialization in fishery waste products. We combined GPS tracking data from 89 birds with Vessel Monitoring System data from the entire fleet operating around the Falklands Islands, in 2009 and 2011. Interactions with vessels (freezer/factory bottom trawlers) occurred in 15 out of 89 independent albatross trips. Among individuals tracked in both years, those that associated with fisheries in 2009 were not more likely to do so again in 2011. Carbon and nitrogen isotopic signatures in whole blood and feathers of albatrosses that interacted with trawlers were similar to those of individuals that did not. Also, we found no correlation between feather and blood isotopic ratios of carbon or nitrogen, indicating no long-term consistency in the isotopic niche of study birds. These results suggest no specialization of individual albatrosses with regard to fisheries. Studies of other albatrosses have also failed to show long-term trophic consistency, which may indicate that scavenging albatrosses, a group particularly threatened by fisheries activity, do not specialize in discards. Therefore, any management actions leading to a reduction of discards will be beneficial, decreasing the numbers of birds behind vessels and consequently the likelihood of incidental mortality.
... Although these differences may be related to the availability of sizes between the years sampled, the size difference between the two species supports the hypothesis of limited overlap in resource use. A range of studies on seabird foraging near trawl vessels has demonstrated the influence of body size and hierarchy in prey size selection González-Zevallos and Yorio 2011). ...
Article
The diet of the Kelp Gull (Larus dominicanus) was investigated at the northern limit of its distribution along the South American Atlantic coast. We used two complementary methods, pellet analysis and stable isotope analysis (SIA), to describe and compare Kelp Gull feeding ecology in freshwater and marine environments. The assimilated diet over two different time scales was investigated via SIA of plasma and red blood cells, blood components with different turnover rates. Fish composed the bulk of the diet of Kelp Gulls in both marine (White Croaker Micropogonias furnieri and Banded Croaker Paralonchurus brasiliensis) and limnetic areas (Armoured Catfish Loricariidae and La Plata Croaker Pachyurus bonariensis), despite the importance of benthic prey from the intertidal zone in samples collected from the marine environment (Wedge Clam Donax hanleyanus and the Yellow Clam Mesodesma mactroides). The fish consumed by the gulls were common discards from fisheries in both environments, and marine bivalves were found at a high density at the marine beach. Diet varied between the different time scales analysed. Conventional diet data generally agreed with stable isotope model estimates, emphasising the importance of using complementary approaches in dietary studies.
... even though multiple fisheries targeting a range of species operate in the study area, demersal fish considered in this study constitute the main discarded species. The first three above-mentioned species are common and abundant in the discards of argentinean ice (and to some extent freezer and shrimp) trawlers (aubone et al. 2004;Dato et al. 2006;gonzález-Zevallos and Yorio 2011;Seco Pon unpubl. data). ...
Article
Full-text available
Black-browed albatrosses (Thalassarche melanophris) disperse over the Argentinean Continental Shelf and neighboring waters during their non-breeding season. It is one of the most frequent seabirds attending fishing vessels and also the most common Procellariform in the bycatch of longliners and trawlers in the area. Understanding the use of fishery discards by this species is an important issue when assessing the potential effect of strategic discard management in decreasing the abundance, interactions, and mitigating mortality. In the present study, we analyzed carbon and nitrogen stable isotope compositions in the blood of Black-browed albatrosses to assess the relative contribution of discards from different fisheries to the diet of this species in winter. Samples were obtained in winter 2011 from fishing vessels operating between 41–43°S and 57–59°W. No sex differences in δ13C and δ15N were observed. Results indicate that during their non-breeding season, isotopic signatures of Black-browed albatrosses are closer to discards and offal generated by fisheries and in particular by trawlers. The large fishing effort of trawl fisheries in Argentina highlights the urgency of an exhaustive analysis to find practical and effective ways to reduce the number of seabirds attending trawlers.
... The total annual mortality of these birds associated with the trawl fleet under investigation was roughly estimated to be from several hundred to over a thousand albatrosses (Favero et al., 2011). Entanglement of penguins in trawl nets is considerable, and other inshore feeding species are doubtless at risk (although this risk is in some cases minor) in various other net fisheries (Gandini et al., 1999; González-Zevallos et al., 2007; González-Zevallos and Yorio, 2011; Seco Pon et al., 2012; Seco Pon et al., 2013). Seabirds foraging and breeding in the Southeast Atlantic are subject to many threats, such as: human disturbance of breeding colonies; destruction of breeding habitats by development (du Toit et al., 2003); predation by domestic cats and mice (Wanless et al., 2009); egg and chick predation by Kelp Gulls and Great White Pelicans (Crawford, 1997; du Toit et al., 2003); competition with commercial fisheries for food (du Toit et al., 2003). ...
Chapter
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... The total annual mortality of these birds associated with the trawl fleet under investigation was roughly estimated to be from several hundred to over a thousand albatrosses (Favero et al., 2011). Entanglement of penguins in trawl nets is considerable, and other inshore feeding species are doubtless at risk (although this risk is in some cases minor) in various other net fisheries (Gandini et al., 1999;González-Zevallos et al., 2007;González-Zevallos and Yorio, 2011;Seco Pon et al., 2012;Seco Pon et al., 2013). ...
Chapter
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In this chapter we refer to the area of the Atlantic Ocean south of the Equator and north of the Polar Front (Antarctic Convergence). The main topographical feature in the South Atlantic is the Mid-Atlantic Ridge which runs between Africa and South America from approximately 58° South to Iceland in the north. A rift valley is associated with the Ridge. The Ridge is of volcanic origin and the development of transverse ridges creates a number of basins: the Argentine, Brazil, Guinea, Angola and Cape Basins. The Atlantic coast of South America is influenced by three major rivers, Orinoco, Amazon and La Plata, that discharge large amounts of freshwater and sediment into the Atlantic Ocean. The Amazon discharges about one-fifth of the world’s total freshwater runoff into the Atlantic (Curtin, 1986) and it is transported offshore up to 500 km seaward (Lentz, 1995). The heavy sediment discharge (2.9 · 108 tons year)-1) is not deposited over the outer shelf, but is carried by the North Brazil Current to Guyana’s shelf, where it forms extensive mud deposits (Gratiot et al., 2008). The continental shelf is wider along its West Coast, both in the north at the Amazon (≈300 km) and in southern Argentina, where it reaches up to 600 kilometres (Miloslavich et al., 2011). The shelf is narrower along the East Coast of the Atlantic and also along the east coast of Brazil, where riverine muds give way to calcareous deposits and the shelf in some areas reaches a minimum of 8 km width (Miloslavich et al., 2011). The continental slope is cut by deep canyons connecting shelf and deep waters. High benthic richness was reported at the head of the submarine canyons, and about half of the species are shared with the shelf-break community (Bertolino et al., 2007; Schejter et al., 2014b). The ~7500 km of the Brazil coasts comprise a combination of freshwater, estuarine and marine ecosystems, with diverse but poorly known habitats in its northern part and with sandy beaches, mangrove forests, rocky shores, lagoons and coral reefs to the south (Miloslavich et al., 2011). Uruguay’s coasts are dominated by sandy beaches; a narrow rocky portion has high biodiversity (Calliari et al., 2003). The coasts of Argentina are mostly sandy beaches, with some rocky formations located mainly at Mar del Plata, Peninsula Valdes and Tierra del Fuego; pebble beaches are common in Patagonia. The coasts of South Africa are part sandy beaches, rocks and rocks mixed with sand on the upper shore and a wave-cut rocky platform (Bally et al., 1984).
... The total annual mortality of these birds associated with the trawl fleet under investigation was roughly estimated to be from several hundred to over a thousand albatrosses (Favero et al., 2011). Entanglement of penguins in trawl nets is considerable, and other inshore feeding species are doubtless at risk (although this risk is in some cases minor) in various other net fisheries (Gandini et al., 1999;González-Zevallos et al., 2007;González-Zevallos and Yorio, 2011;Seco Pon et al., 2012;Seco Pon et al., 2013). ...
... Individuals of each species were identified using binoculars (10 × 50 magnification). Seabirds attending trawler fishing were counted during hauling operations conducted at a speed of 2-4 knots and with a mean duration of 30-45 min, until the end of the discarding process, following relevant standard methodological approaches (Abelló et al. 2003;Valeiras 2003;González-Zevallos and Yorio 2011). Information on seabirds associated with fishery operations included spatial distribution within a radius of 200 m (divided into three zones of 0-50, 50-100 and 100-200 m) around the fishing vessel, as well as behaviour such as kleptoparasitism, diving, resting, etc. ...
Article
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Background: The banning of fisheries discards by imposing an obligation to land unwanted catch constitutes a key point of the Common Fishery Policy reform proposed by the European Commission. The effect of such a ban on discards on top marine predators such as seabirds is largely unknown, especially in oligotrophic systems of the Mediterranean. The current study investigates the presence of scavenging seabirds around fishing trawlers as well as the exploitation of discards produced by bottom trawlers in the eastern Ionian Sea. Methods: On-board observations were randomly conducted in May and December 2014, in order to record the presence and use of fishery discards by two common seabird species, namely, Scopoli’s Shearwater (Calonectris diomedea) and the Yellow-legged Gull (Larus michahellis). Results: A total of 3400 seabirds were counted during May of which 2190 individuals were Scopoli’s Shearwaters and 1210 were Yellow-legged Gulls. The latter species was the only scavenger observed during winter and in total, 768 individuals were counted. Differences in species abundance in the study area are related to breeding phenology and migratory movements. The number of seabirds attending bottom trawler operations during morning and afternoon hours showed no significant differences for both seabird species. Both scavenging seabirds extensively exploited fishery discards, which were mainly demersal fish, and consumed 70–80% of the total fishery discards biomass; however, they appeared to avoid poisonous species and/or large-sized fish. Yellow-legged Gulls displayed kleptoparasitic behaviour on Scopoli’s Shearwater during feeding experiments. The number of such incidents depended on the number of gulls around the fishing vessel, with more than 90% success rates. Conclusions: Considering the average annual biomass of discards estimations and the consumption rate found in this work, 106.1–117.9 t may be offered as a food subsidy to scavenging seabirds in the study area and should support a substantial part of local populations. Our results constitute baseline information on the annual amount of fishery discards and their exploitation rate by seabirds in the Ionian Sea, and suggest further work for a complete understanding of the potential impacts of the discards reform bill on seabirds.
... Prey items were selected and grouped by ecological similarities following diet descriptions based on stomach contents analysis [53][54][55][56] and combined with SIA [52]. The mixing model was constructed including four food sources: (a) benthic invertebrates with similar δ 15 N and δ 13 C values of M. gregaria benthic ecotype, P. muelleri and A. longicornis; (b) benthic fishes with similar composition of A. chiloensis, T. argentina, R. eigennmanniand and R. brasiliensis; (c) pelagic invertebrates, represented only by M. gregaria pelagic ecotype, because it is the principal pelagic prey item of these fishes [50], and (d) discards represented by hakes between 15 and 35 cm of total length, as mean discarded size is 23 cm [64]. Prey had similar δ 15 N and δ 13 C values in both environments (trawled and untrawled). ...
Article
Trawling is the main fishing practice worldwide and its ecosystem effects have been raising concern over the past decades. Long-term impacts can be monitored through changes in the trophic structure, and several studies evaluated trophic level (TL) shifts in fish populations between trawled and untrawled environments. However, published results are contrasting. We performed a metanalysis integrating all available studies that evaluated TL shifts in fishes between trawled and untrawled environments and conducted a local study comparing several features of the trophic ecology in two species of fishes. According to the metanalysis, TL does not change significantly with trawling. In contrast, the local study showed higher TLs and broader isotopic niches in the trawled environment. Diet reconstruction indicated a potential consumption of hake, the main discard component, at the trawled environment. All the studies used in the metanalysis were conducted in the Northern Hemisphere, whereas the local study represents the first data available from the Southern Hemisphere. As industrial commercial fisheries in Argentina are relatively recent, it is possible that our data are capturing the initial stage of ecological changes induced by trawling, compared with the historical fisheries located at the Northern Hemisphere. https://www.tandfonline.com/eprint/VmSeg4bFCC9y4DhcBVY2/full?target=10.1080/10256016.2019.1626381
... Finally, the success of immature gulls decreased with an increase in the number of procellarids and species simultaneously attempting capture of supplemental food. Immature gulls are less successful than adult gulls when using a kleptoparasitic strategy (Steele andHockey 2010, Sotillo et al. 2014) and when competing against adult gulls and other species (Greig et al. 1983, González-Zevallos andYorio 2011). This reduced success may have resulted in the lower willingness to compete with adult gulls or procellarids for supplemental prey items in our experiment, which is supported by the lower number of immature gulls attempting when there was a higher number of procellarids. ...
Article
While foraging, a predator can feed solitarily or in a group. The net energy gain of joining a group is predicted to vary with prey patch quality, species-specific prey capture behavior, and the size and species composition of the predator group. In coastal Newfoundland, Canada, capelin (Mallotus villosus), a key forage fish, migrates inshore to spawn during the summer, resulting in a dramatic shift in prey availability. During July–August 2015–2017, we examined the numerical and behavioral responses of procellarid (Great Shearwater [Ardenna gravis], Sooty Shearwater [A. grisea], Northern Fulmar [Fulmarus glacialis]), and gull species (Herring Gull [Larus argentatus], Great Black-backed Gull [L. marinus]) to fish offal under varying capelin availability as well as flock size and composition using an at-sea experiment on the northeast Newfoundland coast. The experiment consisted of providing offal every 30 s (10-min experimental period), along with 10-min control periods before and after. We recorded the species-specific number of birds on the water, the number of birds simultaneously attempting to capture offal, and the number of successful attempts (“foraging success”). The number of birds on the water was lower during high capelin availability for all species, except for Northern Fulmar. The number of conspecifics simultaneously attempting to capture offal increased with the number of conspecifics on the water, but plateaued at different numbers (4–17) for most species. The species-specific proportion of successful attempts (i.e. foraging success) varied with flock size and composition (i.e. number of conspecifics, heterospecifics, species). Foraging success of Herring Gulls and fulmars were moderately affected by flock size and composition, suggesting that they may be dominant competitors. Findings suggest that seabirds rely more heavily on supplemental food sources, such as fisheries discards and offal, when natural prey availability declines, potentially resulting in a higher risk of by-catch during fisheries activities as forage fish stocks decline.
... However, stable isotope analysis showed that diet composition was similar between sexes with fishery discards represented by demersal species such as Argentine Hake and Banded Cuskeel being the main prey, implying a higher use of this anthropogenic food resource than suggested by tracking results obtained in a shorter time scale. A higher use of this alternative food resource by larger and dominant males was also expected, as foraging behind vessels can be highly competitive (Arcos et al. 2001;Camphuysen 1995) and studies in coastal Patagonia show a high degree of social interactions among Kelp Gulls feeding on discards (Bertellotti and Yorio 2000;González-Zevallos and Yorio 2011). However, the spatial association with vessels and consumption of demersal prey was similar between sexes. ...
Article
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Sexual segregation in feeding strategies has been widely reported in seabirds. Most seabirds occupy wide breeding distributional ranges, and dissimilar ecological settings may result in distinct environmental pressures on males and females leading to geographical differences in sexual segregation. Using GPS loggers and stable isotope analysis of whole blood (δ¹⁵N, δ¹³C), we assessed the occurrence of sexual differences in Kelp Gull (Larus dominicanus) foraging trip parameters, habitat use, isotopic niche and diet during the incubation period at three breeding locations in Argentina characterized by different foraging contexts. At Islote Arroyo Jabalí Este, sexual differences were found in trip parameters and habitat use mainly associated with a significantly higher use by males of a refuse dump as foraging site. However, their isotopic niches were similar with both sexes consuming mostly recreational fish waste, suggesting it was mainly obtained by males at the dump and by females at shoreline areas used by fishers. At Punta Tombo, there were no sex differences in trip parameters nor in the main prey consumed consisting of fishery discards, although females used more shoreline areas and males presented a significantly larger isotopic niche width. In contrast, at Isla Vernaci Este, males and females showed similar trip parameters, habitat use, and isotopic niche width and diet composition. Results show geographical differences in the degree of sexual segregation, which may result from differences in the availability of local food resources, particularly anthropogenic food subsidies.
Article
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Although fisheries discards are recognized as a key food source for many seabirds, there have been few thorough assessments of their importance relative to natural prey, and of their influence on the trophic structure of pelagic seabird communities during the non-breeding period. Competition for resources in Procellariiformes appears to be reduced mainly by avoiding spatial overlap, which is supposed to influence diet composition. However, artificial food sources provided by fisheries might relax niche partitioning, increasing trophic niche overlap. Using bycaught birds from pelagic longline fisheries, we combined the conventional diet and stable isotope analyses to assess the importance of fishing discards in the diet of eight species of Procellariiformes. Both methods revealed the high contribution of trawl discards to the non-breeding diet of three neritic species and a moderate contribution in several other species; discards from pelagic and demersal longline fisheries were considerably less important. There was a clear contrast in diets of neritic vs. oceanic species, which are closely related taxonomically, but segregate at sea. Niche partitioning was less clear among neritic species. They showed an unexpectedly high level of diet overlap, presumably related to the large volume of trawl discards available. This is the first study combining the conventional diet and stable isotope analyses to quantify the importance of fishery discards for a community of non-breeding seabirds, and demonstrates how the super-abundance of supplementary food generates high levels of overlap in diets and allows the coexistence of species.
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• Commercial fishing has been identified as one of the main threats affecting the survival of most seabird species. Although seabird mortality in Argentine longline and demersal trawl fisheries has already been characterized and quantified, the interactions with pelagic trawl fisheries targeting anchovy (Engraulis anchoita Hubbs & Marini, 1935) remains unknown. • The goal of this study was to characterize seabird assemblages attending pelagic trawl vessels and to analyse their interactions (i.e. contact of the birds with the vessel and/or fishing gear and by‐catch). Data were obtained by on‐board observers during three consecutive fishery runs, 2011–2013. • From a total of 333 observations, seabird abundance averaged 157.3 ± 229.7 birds per haul (totalling 23 species). Procellariiform followed by Charadriiform birds were the more frequent and abundant groups. The black‐browed albatross (Thalassarche melanophris (Temminck, 1828)), shearwaters (Ardenna spp. and Puffinus spp.), white‐chinned petrel (Procellaria aequinoctialis Linnaeus, 1758), and the kelp gull (Larus dominicanus Lichtenstein, 1823) were the most frequent and abundant attending species. • The seabird abundance increased when the swell and the number of neighbouring vessels decreased. • Seabird interactions with the vessel and/or fishing gear occurred in approximately 70% of the observations, with most of these representing interactions with the net (92%). The estimated contact rate was 16.7 birds h⁻¹ per haul. A total of 121 birds were by‐caught and the average mortality rate was 0.55 birds h⁻¹ per haul. Shearwaters and Magellanic penguins (Spheniscus magellanicus (Forster, 1781)) were the main by‐caught species (101 and 12 individuals, respectively). Lower levels of mortality were recorded in black‐browed albatrosses and white‐chinned petrels. • The interactions increased in the presence of fishing discards and during haulback operations. • This study is relevant to the implementation of the Argentine National Plan of Action – Seabirds, as well as for the continuing certification process in the anchovy fishery.
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At least about 70 species of seabirds feed in the waters of the argentinean continental shelf, which is probably the area with the highest biomass of albatrosses in the world. the distribution of these birds at sea generally coincides with commercial fisheries in areas with high marine productivity, so their interactions can be frequent and diverse. the study of the argentine sea considering seabirds as part of an integrated approach represents a challenge. some aspects of the biology of these birds could act as environmental indicators to understand systems, assess their conservation status and analyze their relationships with global processes. although interactions between seabirds and some fishing fleets operating on the patagonian shelf have not yet been addressed, the knowledge of the ecology, behaviour and conservation of this group of birds has increased significantly in the last decade. including ecosystem considerations into fisheries management should contribute to long-term food security and ensure effective conservation and sustainable use of the ecosystem and its resources.
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We evaluated seabird attendance and incidental mortality at coastal trawl vessels targeting Argentine red shrimp (Pleoticus muelleri) in the Isla Escondida fishing area, Argentina, during 2006–2007 and 2007–2008. Eight seabird species attended vessels, and the most frequent and abundant seabird (percent occurrence, mean number per haul) in the two seasons was the Kelp Gull (Larus dominicanus) (100%, 112.3 and 100%, 263.4, respectively), followed by the Black-browed Albatross (Thalassarche melanophris) (85%, 17.6, and 90%, 32.4, respectively). Eleven Magellanic Penguins (Spheniscus magellanicus) and one Imperial Shag (Leucocarbo atriceps) were killed in nets with a mean capture rate of 0.003 and 0.0003 birds per haul, respectively. The estimated total number of birds killed was 53 penguins and five shags considering the total number of hauls made by the fishery in the two seasons. No contacts between seabirds and warp cables were recorded. Coastal shrimp vessels generally operated between 15 and 20 km offshore, at a mean distance from the main Kelp Gull colony (Punta Tombo) of 43.9 km. At least 100 fish and invertebrate species were discarded, mostly Argentine hake (Merluccius hubbsi). Total amount discarded per season by this coastal fishery in the two seasons was estimated at 3,284 and 6,590 tonnes, respectively. The coastal shrimp fishery in the Isla Escondida area appears to have a small impact on seabirds in terms of incidental mortality but provides significant amounts of supplementary food during the breeding season of the Kelp Gull.
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We know that juvenile birds can be less proficient than adult birds in their selection of foraging sites, search patterns, recognition and selection of food, and in capture and handling techniques. In all species that have been examined, age-related differences n foraging proficiency have found in at least one aspect of foraging behavior. We frequently do not know the relative importance of these differences. Unfortunately, for only a few species we know the rate at which foraging proficiency improves, the factors influencing this rate, and whether predictable species differences occur in foraging development rates. Experience, necessary for both cognitive and motor skills, is the factor most likely to delay the juvenile's acquisition of adult levels of foraging proficiency. Sometimes, juveniles compensate for their inept foraging skills and differ from adults by foraging in different sites, selecting different sizes and types of food, using different capture methods, resorting to piracy, and/or joining feeding groups. While these age-related differences in foraging differences have been reasonably well documented, we generally do not know the conditions in terms of costs and benefits which encourage or prevent these juvenile foraging differences. Because juveniles are usually subordinate to adults they are sometimes displaced from the prime foraging sites, prevented from taking the most profitable prey, and/or more likely to be victims of piracy. Therefore, adult dominance may interact with the juvenile's inept foraging skills, and contribute to obtain an optimal diet or maintain a positive energy budget. Untangling the effects of adult dominance and the juvenile's lack of foraging skills is a difficult task for field workers concerned with identifying the factors contributing to high juvenile mortality rates and has been accomplished in only a few studies with marked individuals. Finally, experimentation and laboratory studies have yet to fully contribute to solving problems associated with the acquisition of foraging proficiency in juvenile birds.
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Between November 1994 and May 1996, observations on the interactions between seabirds and trawl vessels were conducted on five Patagonian coastal fisheries between 41 and 52 S. Twenty- three seabird species used food made available by fishing operations. Mean number of species recorded per day varied between 2.5 and 6, depending on the fishery and was significantly lower in the Bahia Engafio area. The highest number of species (17) was observed at the Golfo San Matias area. Kelp Gulls (Larus dominicanus) and Black-browed Albatrosses (Diomedea melanophris) were the most important seabirds, both in frequency of occurrence and numbers, followed by White-chinned Petrels (Procellaria aequinoctialis) or South American Terns (Sterna hirundinacea), depending on the fishery. Kelp Gulls were present between 91.6 and 100% of days at all fisheries except for Bahia Grande (28.6%), with numbers which varied between a few and 600 birds (means between 148.2-178.8 birds). Black-browed Albatrosses were present between 92.2 and 100% of days at all fisheries except for Bahia Engafio (7.3%), with numbers that varied between one and 375 birds (means between 53.7-123.7 birds). Most other species were either present in small numbers or were rarely observed. Seabirds obtained prey during discarding activities or from the net. Seabird mortality as a result of incidental captures was very low, with only one Imperial Cormorant (Phalacrocorax atriceps) and one Magellanic Penguin (Spheniscus magellanicus) killed during 394 hauls on 124 trips in the five fishing areas. Coastal trawl fisheries appear to have a small impact on Patagonian seabirds in terms of incidental mortality, but might have a significant effect through the provision of fishing waste, especially for Kelp Gull and Black-browed Albatross populations.
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Silver Gulls (Larus novaehollandiae) stole fish from Crested Terns (Sterna bergiz) at One Tree Island, Great Barrier Reef, Australia. They attempted to steal larger fish of several kinds more often than smaller ones up to a point, but did not try to steal the largest fish most frequently. Silver Gulls preferred disc-like fish 8-l 0 cm long and cone-like fish 14-l 6 cm long. The likelihood that gulls would try to steal a fish was influenced by such factors as its length, weight, shape, and availability, of which weight appeared to be particularly important. Robbing success differed significantly with length in only three of the 11 types of prey available. Gulls robbed in various ways, the success of which depended on a tern' s maneuverability, method of evasion, and speed of reaction to pirates. Many birds steal food from other birds (see Brockman and Barnard 1979 for a recent re-view). Many variables appear to affect the fre-quency and success of such behavior: (1) the number of potential victims and pirates in a colony (Dunn 1973, Veen 1977); (2) the rela-tive size of the pirate and its victim (Corkhill 1973, Hulsman 1976); (3) interference from conspecifics (Hulsman 1977~); (4) the number of pirates chasing a victim (Hatch 1970, 1975; Hulsman 1976) and the duration of the chase (Fumess 1978); (5) the victim' s reactions to a pirate (Furness 1978); (6) the method used by a robber to steal a food item (Hulsman 1976); (7) the size of the food item (Hopkins and Wiley 1972, Dunn 1973, Hulsman 1976, Veen 1977); and (8) weather conditions (Hatch 1975, Veen 1977). However, the effect of prey type on the frequency and success of such piracy has not been examined in situations where a robber can choose between several types of food. In addition, the stimulus (or stimuli) to which a robber responds has not been deter-mined, although the length (Hulsman 1976) and the weight (Dunn 1973) of a prey item seem to be important.
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The robbing behaviour of some larids at One Tree Island, Great Barrier Reef, was observed. Terns usually were moderately successful when robbing members of their own species but stole only a few fish brought into a colony. Between species, Roseate Terns rarely succeeded though they often tried to rob Black-naped Terns whereas Silver Gulls frequently succeeded and often tried to rob Lesser Crested and Crested Terns. The number of attempts by most species was reatest during high tide when the most pirates were round the colonies. The success of Silver Gulls varied in time and place. Some of the variables that affected the success were the number of Gulls attacking a tern, the length of the fish, the size of the pirate relative to its victim and the strategy used.
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The distribution and abundance of scavenging seabirds and their utilization of discards and offal between June and December 1998 were studied in the western Baltic Sea. Herring gulls were clearly the most numerous scavenging species in all areas and all seasons, followed by great black-backed gulls, lesser black-backed gulls and mew gulls. High percentages of discarded gadoids (cod, whiting), clupeids (herring, sprat), scad, rockling and offal were consumed by seabirds during experimental discarding on fishing boats, whereas the percentages of flatfish consumed were extremely low. There was a clear effect of cod length on total and species-specific consumption by birds but this pattern was hardly evident for clupeids or dab. By combining official discard and offal statistics and our experimental discarding, we estimate that 6500 t of fish discards and 16 000 t of offal were consumed annually by seabirds in the Baltic Sea. Bivalves, especially blue mussels Mytilus edulis, were the most frequently represented food item in herring gull pellets. Fish identified in the pellets consisted mainly of gadoids, in particular cod. The proportion of discards in herring gull pellets was on average 1.6% (range: 0-4.5%) at Laboe and 17.5% at Warnemunde (range: 9.4-25.5%), but pellets bias diet assessment as offal and other soft prey (including clupeids) will be under-represented. Scavenging on discards and offal is a widespread phenomenon in the Baltic Sea as it is in other shelf areas of Europe, but the number of bird species involved is generally lower and strongly biased towards gulls, especially herring gulls.
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This study investigated the level of seabird mortality caused by the domestic trawl fleet (freshies) for hake (among other less important targets) operating in waters off central Patagonia (37–48°S), analyzing the effect of environmental and operational variability on the level of seabird interactions. With a total of 135 vessels, the fleet is one of the largest in Argentina. Specifically tasked seabird observers were placed onboard trawlers during the summer and winter seasons of the years 2006 and 2007. The type and number of seabird interactions (i.e. contacts with fishing gear) were recorded during shooting and hauling operations, covering 72 days of observation and 328 trawls. Black-browed albatrosses, white-chinned petrels Procellaria aequinoctialis, southern giant petrels Marconectes giganteus and southern royal albatrosses Diomedea epomophora were the most abundant species interacting with trawlers. Confirmed mortalities of black-browed and southern royal albatrosses were the result of collisions and entanglement with the warp cable while birds were scavenging. The estimated total mortality rate was 0.017 birds h−1 and 0.105 birds per vessel per day. The intensity of interactions (in terms of the number of contacts per unit time) was largely explained by the distribution of the fishing effort. Seasonality and the incidence of discards were the strongest factors explaining the occurrence of seabird interactions. The total annual mortality in the trawl fleet under investigation was roughly estimated to be from several hundred to over a thousand albatrosses. However, these figures should be considered preliminary due to the limited spatial and temporal coverage of data and the fact that estimations were based on a low number of observed mortalities. The implementation of a strategic discard management may significantly reduce the number of seabird mortalities from collisions with warp cables or improve the effectiveness of other complementary mitigation methods. Urgent implementation of mitigation measures is needed in this fleet to reduce the mortality of albatrosses and petrels along the Patagonian shelf.
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Discards refer to that part of the catch which is returned to the sea during commercial fishing operations. Organisms that do not survive the discarding process can provide an additional food source to scavenging species. The aim of this study was to determine whether the quantity and quality of discarded material from the intensively fished English Nephrops norvegicus fishery is such that it has a positive effect on marine scavenger populations. Field studies were used to identify marine scavenger species and estimate their abundance. Discard experiments combined with data from commercial vessels provided estimates of the partitioning of discards between aerial and marine scavengers and the spatial distribution of discarding. A bioenergetic model was devised to evaluate the importance of discards to marine scavengers. Seabirds utilised an estimated 57 % of the discarded material; most discarding (83 %) took place over the fishing grounds. Species identified as marine discard scavengers included Liocarcinus depurator, Asterias rubens, Neptunea antiqua, Pagurus bernhardus, Carcinus maenas, Cancer pagurus and Myxine glutinosa. The hagfish M. glutinosa was the most abundant scavenger, and made up 79 % by weight of all identified marine scavengers on the fishing grounds. The energy available from discards could potentially provide the identified marine discard scavengers on the fishing grounds with 37 % of their energetic requirements during the fishing season. This level of contribution is probably sufficient to allow larger populations of these scavenging species to exist than would otherwise be possible.
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Albatrosses have among the most remarkable travelling capacities of any extant animal. However, previous studies regarding their movements at sea have mainly focused on breeding birds commuting between the nest site and offshore feeding grounds. In this study, we compare the move ment patterns and at-sea activity of breeding and inter-breeding black-browed albatrosses Diomedea melanophris from the Falkland Islands. Data were recorded via global location and activity sensors for 26 incubating birds (during single foraging trips lasting 6.8 d on average) and 6 inter-breeding individuals (during non-stop offshore journeys of 127.5 d on average). Our results showed that foraging black-browed albatrosses utilise vast offshore areas (the average foraging area was 102000 +/- 132 000 km(2) by incubating birds and 1552 000 +/- 970 000 km(2) by inter-breeding birds). However, mean foraging range was similar in both groups (691 +/- 330 km and 680 +/- 192 km by incubating and interbreeding birds, respectively) as were their core foraging areas and their at-sea activity patterns. Our results thus indicate that black-browed albatrosses from the Falkland Islands, which represent the largest albatross population world-wide (ca 800 000 individuals), mainly rely on marine resources avail able within the Patagonian Shelf area. Although this highly productive continental shelf is the largest of the Southern Hemisphere, rapid development of industrial fisheries currently results in the removal of over 1.4 million tonnes of fish and squid per year in this zone. As our data also show significant spatio-temporal overlap between human and albatross fishing activities within the Patagonian Shelf, we anticipate major detrimental effects on the albatross population in terms of competition for food and additional mortality caused by bird bycatch.
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Most types of fishery produce discards and offal in considerable quantities which are fed upon by seabirds. This paper demonstrates the importance to seabirds of fishery waste in the North Sea. The total amount of fishery waste in the North Sea region is estimated at 62 800 t of offal, 262 200 t of roundfish, 299 300 t of flatfish, 15 000 t of elasmobranchs and 149 700 t of benthic invertebrates per year, representing 4% of the total biomass of fish and 22% of the total landings. This equals an energy value of about 3.4 x 10(12) kJ. Beam trawl fisheries discharge discards at the highest rates of all fishing fleets. Their discard fraction is dominated by flatfish which are less favoured by seabirds because of their shape. In contrast, the amounts of discards from pelagic and gadid fisheries are less, but fish species and lengths are more appropriate as food for seabirds. The number of seabirds potentially supported by fishery waste in the North Sea is estimated to be roughly 5.9 million individuals in an average scavenger community (composition in proportion to the seasonal abundance of scavenging species). During experimental discard studies, the proportions of fishery waste consumed by seabirds was calculated. We estimated that the mass of discards and offal consumed by birds during our study amounted to 55 000 t of offal, 206 000 t of roundfish, 38 000 t of flatfish, 2000 t of elasmobranchs and 9000 t of benthic invertebrates.
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Cayenne (Thalasseus sandvicensis eurygnathus) and Royal Terns (Thalasseus maximus) breed in mixed colonies in Argentina. This paper presents the first detailed information on their breeding diet and assesses differences and overlap between species in the type and size of prey. During 2004 and 2005, observations of prey delivered to mates and chicks werecarried out at Punta León,Patagonia.The dietof CayenneandRoyal Ternscomprised nine and10 preyspecies respectively. Fish comprised >99% of the prey of both species of tern in the two study years. Both species had a fairly specialised diet based on pelagic schooling fish, mostly Argentine Anchovy (Engraulis anchoita) and two species of silversides (Odontesthes spp.). Despite observed overlap in trophic resources, our results showed that Argentine Anchovy was the main prey for Royal Terns, whereas the two species of silversides together with Anchovy comprised the bulkof the diet for Cayenne Terns. Fish delivered by Cayenne Terns were significantly smaller than those by Royal Terns. Results suggested that both type and size of prey may be important factors permitting food partitioning between the two species of tern during the breeding season.
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We studied discard use and incidental mortality of seabirds attracted to high-sea trawl vessels operating in the Golfo San Jorge, Argentina, during the height of the fishing season in 2003 and 2004. Fourteen seabird species ate food made available by fishing operations. The most frequent and abundant seabirds (percent occurrence, mean number per haul) were the kelp gull Larus dominicanus (98.9%, 207.0), the black-browed albatross Thalassarche melanophrys (98.9%, 94.2) and the white-chinned petrel Procellaria aequinoctialis (91%, 8.4). Flock sizes for the 3 species var- ied from a few to a maximum of 1600 birds. Total seabird abundance varied significantly between stages of the fishing operation, being higher during discarding and haulback than during towing. Incidental capture of seabirds in nets was recorded in 37% of 89 hauls, with a mean capture rate of 1.2 birds per haul. Species incidentally caught were the great shearwater Puffinus gravis, the imper- ial cormorant Phalacrocorax atriceps and the Magellanic penguin Spheniscus magellanicus, with rates that varied between months and years. Considering the fishery's fishing effort, the estimated total numbers of birds killed during the study were 2254 great shearwaters (CV = 1.1), 1233 imperial cormorants (CV = 1.1) and 35 Magellanic penguins (CV = 2.4) in 2003, and 311 imperial cormorants (CV = 1.7) and 1516 Magellanic penguins (CV = 1.1) in 2004. Black-browed albatrosses and kelp gulls were also struck by the warp cable while feeding on discards from the surface, and drowned when they were dragged underwater. The results obtained in this study show that the hake trawl fishery operating in the Golfo San Jorge may have a significant effect on some seabird populations through the provision of fishing discards and incidental mortality.
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We studied age-related differences in feeding behaviour and foraging success of Kelp Gulls Larus dominicanus feeding on fishery discards at trawl vessels in northern Patagonia, Argentina. Kelp Gulls consumed fish by direct capture and intraspecific kleptoparasitism. Direct capture rate increased significantly with age. Juveniles dropped a higher proportion of handled fish than older birds. Intraspecific kleptoparasitism involved gulls of all age-classes and was recorded in all hauls. The percentage of successful attempts was similar among different age-classes. However, juveniles and immatures attempted to steal prey more frequently and therefore obtained more prey than expected. The proportion in which each feeding method (direct capture and kleptoparasitism) contributed to overall fish consumption varied significantly among age-classes. Juveniles obtained most of the food through kleptoparasitism (72%), while immature, subadult and adults consumed prey mostly through direct capture (77, 91 and 92%, respectively). Using both feeding methods, Kelp Gulls swallowed 87.6% of the discarded fish that they handled. However the percentage of fish swallowed over total handled fish were different among age-classes Guveniles 45.5%, immatures 70.8%, subadults 83.3%, adults 94.8%). Kleptoparasitism may be used as an alternative strategy to compensate the lower efficiency of young birds.
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The breeding distribution of the Kelp Gull Larus dominicanus in coastal Argentina ranged along over 3600 km of coastline, from Claromecó, southern Buenos Aires (38º45'S, 59°28'W) to Bahía Ushuaia, Tierra del Fuego (54°51'S, 68°16'W). A total of 105 breeding colonies was identified at 55 localities. Median colony size was 184 pairs, ranging from two to 12 260 breeding pairs. Total population size for the Argentine coast, considering only the 94 colonies for which the number of breeding pairs is available (89% of total sites), was estimated as 74 360 breeding pairs. Most colonies were on islands (81.9%). A total of 40 Kelp Gull colonies was located within coastal protected areas, representing over 60% of total population size. All Kelp Gull colonies for which we had size estimations for more than one season or for which previous information was available (n = 7) showed an increase in numbers of breeding pairs. Available information suggest that Kelp Gulls are feeding generalists taking advantage of artificial food sources resulting from human activities such as refuse tips, sewage outfalls, slaughter houses, and fisheries bycatch. As a result of their concentration close to cities, Kelp Gulls may result in hazards to aircraft and threats to human health. The opportunistic feeding habits and flexibility in nesting requirements of the Kelp Gull make this widely distributed and abundant species a probable key factor in the structuring of seabird assemblages and a problem to other coastal wildlife if populations continue to increase. Similarly, threats to human populations are likely to increase under current Kelp Gull population trends. Management and research requirements are presented.
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We studied kleptoparasitic behavior of Kelp Gulls (Larus dominicanus) feeding on fishery waste at trawl vessels in northern Patagonia, Argentina, by experimentally discarding fish. Intraspecific kleptoparasitism by gulls of all age-classes was recorded at all hauls. During experimental discarding, Kelp Gulls attempted to steal fish from other gulls in 23.5% of 1.915 cases. The length of fish handled differed between age-classes, tending to be larger in younger age classes. The size of fish being carried by victims of kleptoparasitic attempts was similar between age-classes. On the other hand. adult and sub-adult were more frequently attacked when they carried larger fish, while juveniles were victims of kleptoparasitic attempts independent of the size of the fish carried by them. The lengths of Successfully stolen fish were similar between victims of different age-classes. Adult and sub-adult were successfully robbed when they carried larger fish, while juveniles were successfully robbed irrespective of the size of the fish carried by them. Gulls of different age-classes were victims of kleptoparasitic attempts, in direct proportion to the number of fish handled by each age-classes. However, juveniles were successfully robbed more often than other age-classes, juveniles lost prey ill 83% of kleptoparasitic attempts, while sub-adults and adults lost their fish in 41% and 42% of cases, respectively. Although selection of juvenile hosts to attack could be more profitable, attack rate on young birds was not higher than expected, suggesting attacks at trawlers were at random. Feeding where there is a high density of potential victims may be advantageous for juveniles compared to feeding elsewhere because they are not selectively harassed or displaced by older, foraging gulls. If so, this could contribute to juvenile survival which could contribute to an increased population size.
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Utilisation of fishery waste by Kelp Gulls was studied onboard trawl and longline vessels in Golfo San Matias, Patagonia, Argentina, between November 1996 and May 1997. Individuals of all age-classes were present in all counts and in similar proportions in both fisheries, although flocks consisted mainly of adults. Mean maximum number per day was significantly larger in the trawl than in the longline fishery (299.2 vs 197.6). In both fisheries, the number of gulls increased as the fishing operation progressed, with maximum numbers observed during discarding and/or gutting. Kelp Gulls consumed a fraction (67.0%) of the experimental discards as they selected prey according to species, size and shape. The Argentine Hake (Merluccius hubbsi) was the preferred fish. Longtail Hake (Macruronus magellanicus), Sea Salmon (Pseudopercis semifasciata) and flounders (Paralictchys isosceles and Xystreurys rasile) were negatively selected. Rejected species were deep bodied with respect to their length, had strong dorsal fin spines, were flat shaped or had caudal spines, making them more difficult to handle and swallow. The proportion of fish consumed for all species decreased with increasing fish length. Kelp Gull selection of discards according to species and size suggests that care should be taken when evaluating the availability of this food source for seabirds. Considering the fraction of fishery waste consumed by gulls obtained in this study, fish discards produced by coastal fisheries at Golfo San Matias may support a population of more than 30 000 Kelp Gulls. This abundant and high quality food source could have an important effect on the population expansion of the Kelp Gull.
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2000. Age-related feeding behaviour and foraging efficiency in Kelp Gulls Larus dominicanus attending coastal trawlers in Argentina. Ardea 88(2): 207-214. We studied age-related differences in feeding behaviour and foraging suc-cess of Kelp Gulls Larus dominicanus feeding on fishery discards at trawl vessels in northern Patagonia, Argentina. Kelp Gulls consumed fish by direct capture and intraspecific kleptoparasitism. Direct capture rate incre-ased significantly with age. Juveniles dropped a higher proportion of han-dled fish than older birds. Intraspecific kleptoparasitism involved gulls of all age-classes and was recorded in all hauls. The percentage of successful attempts was similar among different age-classes. However, juveniles and immatures attempted to steal prey more frequently and therefore obtained more prey than expected. The proportion in which each feeding method (direct capture and kleptoparasitism) contributed to overall fish consump-tion varied significantly among age-classes. Juveniles obtained most of the food through kleptoparasitism (72%), while immature, subadult and adults consumed prey mostly through direct capture (77, 91 and 92%, respective-ly). Using both feeding methods, Kelp Gulls swallowed 87.6% of the dis-carded fish that they handled. However the percentage of fish swallowed over total handled fish were different among age-classes Guveniles 45.5%, immatures 70.8%, subadults 83.3%, adults 94.8%). Kleptoparasitism may be used as an alternative strategy to compensate the lower efficiency of young birds.
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Discard utilization by seabirds in the North Sea has been quantified by so-called discard experiments on board fishery research vessels. In this paper we evaluate possible biases in this method, although discard practices vary considerably between commercial fishing fleets, seasons and areas. Whereas the total consumption of discards remained relatively unaffected by the different phases of the fish catch procedure, the length of fish discarded, the different methods of compiling the data and the quantities of fish discarded at once all significantly influence the results. The differences were particularly apparent between single- and multi-item trials in rapidly sinking fish. Correction factors for single-item experiments are suggested. Even more obvious were differences between the methods when focusing on the proportions obtained by the different bird species. Correcting the results of discard experiments on research vessels as presented in this paper appears necessary when quantifying the consumption of discards by seabirds in the North Sea.
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Competition between the yellow-legged gull Larus cachinnans and Audouin's gull Larus audouinii while foraging at commercial fishing vessels off the Ebro Delta (NW Mediterranean) was assessed in 1997 and 1998. Observations were performed on board two kinds of fishing vessels with different timetables: bottom trawlers (diurnal activity) and purse seiners (nocturnal activity). Three situations were distinguished with respect to the season and the fishing regime: (1) breeding season, both fleets operating; (2) breeding season, only purse seiners operating due to a trawling moratorium; (3) non-breeding season, both fleets operating. Overall, the yellow-legged gull behaved as an opportunist species and exerted pressure over Audouin's gull through kleptoparasitism and agonistic interactions (i.e. contest competition). Despite this, Audouin's gull was more efficient at capturing discards through scramble competition and was able to take profit from fishing vessels when the capture of fish required high skill, in accordance with its higher specialisation. Competition varied in intensity according to the fishing fleet and the season. Indeed, Audouin's and the yellow-legged gulls only interfered at trawlers, since only Audouin's gull attended purse seiners. During the breeding season competition at trawlers was not severe and Audouin's gull preferentially attended these vessels. Purse seiners acted as a secondary food resource and only attracted important numbers of Audouin's gulls during trawling moratoriums. Out of the breeding season the number of Audouin's gulls strongly declined in the area. Furthermore, the intensity of kleptoparasitism increased at trawlers, and the average size of the fish discarded was larger and less suitable. In parallel with these changes, Audouin's gull shifted to attend purse seiners preferentially, thus avoiding the high levels of competition at the trawlers. The lower representation of Audouin's gull in other breeding areas in the Mediterranean, as well as the less important fishing fleets in these areas, would probably reduce the attractiveness of trawlers for this species, even during the breeding season. Moreover, changes in fishing policies aimed to reduce discarding practices would lead to a globally less favourable situation for Audouin's gull.
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We studied the interaction between seabirds and warp cables in the high-seas Argentine hake Merluccius hubbsi trawl fishery operating in Golfo San Jorge, Argentina, and tested the efficacy of a simple mitigation measure designed to reduce mortality at warp cables. Observations were made onboard hake trawlers during the height of the fishing season, between December 2004 and April 2005. Thirteen seabird species used food made available by fishing operations. The most frequent and abundant seabirds (% occurrence, mean maximum number per haul) were the Kelp gull Larus dominicanus (98.1%, 348.5) and the Black-browed albatross Thalassarche melanophrys (96.1%, 132.2). Contacts with warp cables were recorded for six species in 81.4% of hauls, with a mean number of contacts per haul of 14.4 ± 23.8 (range = 0–127). A total of 53 individuals were killed due to interactions with nets and cables, resulting in a total cable mortality rate of 0.14 birds/haul. Considering the fishery’s fishing effort, the estimated total number of birds killed during the study was 2703 (CV = 0.8), of which 306 (CV = 0.9) were killed due to contacts with warp cables (255 Kelp gulls and 51 Black-browed albatross). The tested device consisted of a plastic cone attached to each warp cable. In hauls with mitigation device, the number of contacts was reduced by 89% and no seabirds were killed. Mean distances between seabirds and cables were significantly larger in hauls with than without mitigation device (2.6 vs 0.9 m). The proposed device could be easily applied in this and other trawl fisheries operating in Argentine waters. Increased effort should be placed in implementing mitigation measures and the monitoring of cable related mortality associated to high-seas trawlers operating in the Argentine Continental Shelf.
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There is great variation in discarding practice among fisheries in different parts of the world. Management systems result in some fisheries discarding mostly fish offal, much of which is macerated into small chunks, while other fisheries discard large (ca. 25 cm) whole fish. Scavenging seabirds consume high proportions of most categories of discarded fish and offal (typically 60 to 80% of discarded roundfish, 70 to 95% of discarded offal), but tend to avoid discarded benthic invertebrates and fish that are difficult to swallow, such as species with long spines or large flatfish. Amounts and composition of fishery discards and offal reaching benthic scavenging communities are clearly very strongly influenced by the intense but selective consumption by seabirds, and this alteration will depend strongly on details of the fishery management regulations and customs, such as whether or not waste is macerated. There is scope to adjust fisheries management practices to reduce the impact of offal and discards on scavenger communities
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Arctic Terns (Sterna paradisaea) and Common Terns (S. hirundo) are similar in many aspects of their breeding ecology, but Common Terns generally lay three eggs per clutch whereas Arctic Terns lay two. In our study, Common Terns had a higher rate of food delivery and energy supply to the nest and higher nest attendance, indicating that they made trips of shorter average duration. This suggests that the number of chicks raised by these two species was primarily limited by the rate at which parents could supply food. However, estimated daily metabolizable energy intake of chicks was about 30% higher in Common Terns than in Arctic Terns. Common Tern chicks apparently spent a higher proportion of daily energy intake on maintenance of body temperature. It remains unknown whether this difference was because Common Tern parents could not brood three chicks as effectively as Arctic Terns brooded two or because the energy requirements for heat production in the third-hatched Common Tern chick were particularly high. If brooding did play a less important role in the energy budgets of Common Terns, the number of chicks that Arctic Terns could raise may have been limited not only by the rate at which parents could supply food to the nest but also by the requirements of chicks for brooding. We suggest that more detailed studies on the role of brooding constraints in limiting brood size in these species are required to clarify this matter.
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The behavioral ecology of Black-headed Gulls Larus ridibundus kleptoparasitizing terns on Coquet Island, Northumberland, was investigated between 1992 and 1994. Black-headed Gulls demonstrated behavioral adaptations that probably optimized returns for energy invested in chases. Gulls selectively attacked terns carrying larger fish, probably because the energetic content of a fish increases exponentially with length. They also selected terns carrying more than 1 fish, as the success rate was higher. Sandwich Terns Sterna sandvicensis were selected preferentially to Common Terns Sterna hirundo, which were selected more frequently than Arctic Terns Sterna paradisaea. This selection was independent of fish size; there was no difference in success rate between species. Host species selection may be due to the relative proximity of the different tern colonies to the gull colony. The number of swoops made during an attack was associated with a decrease in the likelihood of success: most gulls made only a single attack on a host. Black-headed Gulls often attacked in groups. Larger group size was associated with increased success, although the estimated average energy return for each gull was lower than for gulls foraging individually. By joining in chases initiated by another, gulls may reduce costs of chasing and be in a better position than the initiator of the chase to collect the food once it is dropped.
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Summary The authors estimate that between 17.9 and 39.5 million tons (average 27.0 million) of fish are discarded each year in commercial fisheries. These estimates are based on a review of over 800 papers. The highest quantities of discards are from the Northwest ...
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Arctic Terns (Sterna paradisaea) and Common Terns (S. hirundo) are similar in many aspects of their breeding ecology, but Common Terns generally lay three eggs per clutch whereas Arctic Terns lay two. In our study, Common Terns had a higher rate of food delivery and energy supply to the nest and higher nest attendance, indicating that they made trips of shorter average duration. This suggests that the number of chicks raised by these two species was primarily limited by the rate at which parents could supply food. However, estimated daily metabolizable energy intake of chicks was about 30% higher in Common Terns than in Arctic Terns. Common Tern chicks apparently spent a higher proportion of daily energy intake on maintenance of body temperature. It remains unknown whether this difference was because Common Tern parents could not brood three chicks as effectively as Arctic Terns brooded two or because the energy requirements for heat production in the third-hatched Common Tern chick were particularly high. If brooding did play a less important role in the energy budgets of Common Terns, the number of chicks that Arctic Terns could raise may have been limited not only by the rate at which parents could supply food to the nest but also by the requirements of chicks for brooding. We suggest that more detailed studies on the role of brooding constraints in limiting brood size in these species are required to clarify this matter.
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Among the different types of fishing vessels around Shetland, whitefish trawlers attract the largest numbers of scavenging seabirds and provide the most food. Offal was almost all consumed by seabirds, predominantly by Fulmars Fulmarus glacialis, which excluded other species by their aggression. Fulmars generally ignored discarded whole fish, which were mainly taken by Great Black-backed Gulls Larus marinus, Gannets Sula bassana and Great Skuas Catharacta skua. Although flatfish were usually ignored because seabirds found them difficult to swallow and they sank faster, most discarded roundfish were consumed. Herring Gulls L. argentatus, Lesser Black-backed Gulls L. fuscus and Kittiwakes Rissa tridactyla were rarely able to obtain offal or discards. Herring Gulls and Lesser Black-backed Gulls spent much time on the periphery of feeding flocks while Kittiwakes rarely attempted even to join these. Most of the birds at trawlers were in adult plumage, and it is suggested that the low proportion of immature birds present was a further reflection of the highly competitive feeding conditions at trawlers. We suggest that likely changes in fishing practice and seabird population sizes in the immediate future may result in Herring Gulls, Lesser Black-backed Gulls and Great Skuas finding feeding on waste around trawlers increasingly difficult, so they may be further displaced by Fulmars, Gannets and Great Black-backed Gulls.
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Multinational fisheries operating in the vicinity of the Falkland Islands currently take c. 90,000 tonnes of true fish (“finfish” as opposed to squid) per annum, including hakes Merluccius spp., Southern Blue Whiting Micromesistius australis, Hoki Macruronus magel-lanicus and Red Cod Salilota australis, and generate substantial quantities of fisheries waste in the form of discards and offal. This paper examines the use made of this waste by scavenging Black-browed Albatrosses Diomedea melanophris which breed in the Falklands. The various types of waste produced are described and their consumption by scavenging albatrosses is quantified. The results indicate that Black-browed Albatrosses obtain c. 8000 tonnes of food per annum from this source, of which two-thirds is offal and the remainder whole discards. The energy content of this waste is equivalent to 4.4% of the estimated total annual energy requirements of the Falklands Black-browed Albatross population. However, as the fishery is a greater predator of finfish stocks than are the albatrosses, its long-term impact may be detrimental to these birds.
Article
In the past decade, a major trawl fishery for the squid Loligo gahi has developed in the vicinity of Beauchêne Island, an internationally important breeding site for the Black-browed Albatross Diomedea melanophris. The breeding season diet of this albatross in the Falklands and its use of discards generated by the Loligo fishery were investigated. Albatross chicks are fed extensively on commercially exploited species of squid and fish including Loligo gahi and southern blue whiting Micromesistius australis. The quantity of waste generated by the Loligo fishery amounts to c. 5% of the reported catch and just over 50% of this waste, mainly Loligo and nototheniid fish, is scavenged by adult Black-browed Albatrosses. The total quantity scavenged during the chick rearing period amounts to 1000–2000 tonnes per year. This is equivalent to 10–15% of the total food requirement of the breeding Black-browed Albatross population on Beauchene Island during the period when the fishery is operating. Although the Loligo fishery currently provides a significant quantity of food to these albatrosses, its net effect may be detrimental to them, as it is a much greater predator of Loligo stocks than the albatrosses are estimated to have been prior to the fishery's development.
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Kleptoparasitism refers to the interspecific stealing of already procured food, but this paper shows that intraspecific food-stealing is effectively the same behaviour. A comprehensive review of the literature shows that certain orders of birds contain a disproportionate number of kleptoparasitic species. Birds in these orders occupy a limited range of ecological niches and are most commonly either predatory or dietary opportunists. Kleptoparasitism is particularly associated with certain ecological conditions, such as the availability of hosts feeding on large, visible food items and periods of food shortage. Birds show a wide range of socially parasitic feeding interactions of which kleptoparasitism is one extreme. The parasitic pattern of food-stealing is likely to involve frequency-dependent selection and may be an example of an evolutionarily stable strategy.
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