Abstract
© 2019 Inter-Research. All Rights Reserved. Fisheries may generate new feeding opportunities for marine predators, which switch foraging behaviour to depredation when they feed on fish directly from fishing gear. However, the role of diet in the propensity of individuals to depredate and whether the depredated resource is artificial or part of the natural diet of individuals is often unclear. Using stable isotopes, this study investigated the importance of the commercially exploited Patagonian toothfish Dissostichus eleginoides in the diet of generalist subantarctic killer whales Orcinus orca depredating this fish at Crozet (45°S, 50°E). The isotopic niche of these killer whales was large and overlapped with that of sperm whales Physeter macrocephalus from the same region, which feed on toothfish both naturally and through depredation. There was no isotopic difference between killer whales that depredated toothfish and those that did not. Isotopic mixing models indicated that prey groups including large/medium sized toothfish and elephant seal Mirounga leonina pups represented ~60% of the diet relative to prey groups including penguins, baleen whales and coastal fish. These results indicate that toothfish are an important natural prey item of Crozet killer whales and that switching to depredation primarily occurs when fisheries facilitate access to that resource. This study suggests that toothfish, as a commercial species, may also have a key role as prey for top predators in subantarctic ecosystems. Therefore, assessing the extent to which predators use that resource naturally or from fisheries is now needed to improve both fish stock management and species conservation strategies.
Highlights
Anthropogenic activities may influence the dietary niche width of wild populations by modifying the availability of resources (Van Valen 1965, Newsome et al 2015)
This study investigated the importance of the commercially exploited Patagonian toothfish Dissostichus eleginoides in the diet of generalist subantarctic killer whales Orcinus orca depredating this fish at Crozet (45° S, 50° E)
Knowing the importance of the depredated resource in the natural diet of predators is needed to assess the effects of depredation and fisheries on wild populations, fish stocks and ecosystems as a whole
Summary
Anthropogenic activities may influence the dietary niche width of wild populations by modifying the availability of resources (Van Valen 1965, Newsome et al 2015). The depredated resource may already be part of the natural diet of predators, with access facilitated by humans, or it may be an entirely artificial resource which would not otherwise have been used by predators in natural conditions. This distinction is critical in understanding the underlying behavioural mechanisms of predators switching from a natural to a depredated resource (Boitani & Powell 2012). Knowing the importance of the depredated resource in the natural diet of predators is needed to assess the effects of depredation and fisheries on wild populations, fish stocks and ecosystems as a whole. By feeding on fish caught in fishing gear, marine predators may cause increased and difficult-to-quantify mortality for fish stocks, thereby increasing the catches needed for fisheries to reach their quotas (Gilman et al 2013, Gasco et al 2015, Mitchell et al 2018)
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