Importance of toothfish in the diet of generalist subantarctic killer whales: implications for fisheries interactions

Abstract

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.

When large marine predators feed on fisheries catches: Global patterns of the depredation conflict and directions for coexistence

“…Africa (Shaughnessy et al, 1981), Alaska (Dahlheim, 1988;Matkin, Ellis, Von Ziegesar, & Steiner, 1986), Florida (Zollett & Read, 2006) and Chile (Sepúlveda et al, 2018). Rocklin et al, 2009;Söffker et al, 2015;Tixier, Burch, et al, 2019;Tixier, Giménez, et al, 2019) and for sharks (e.g. MacNeil, Carlson, & Beerkircher, 2009;Rabearisoa et al, 2018;Ryan, Taylor, McAuley, Jackson, & Molony, 2019).…”

“…killer whales) with culturally transmitted behaviours and sometimes high levels of specialization, a switch to depredation may be conditioned by whether or not the fish caught by fishers is part of their natural diet. While a cosmopolitan distribution likely contributes to the species found interacting with many fisheries worldwide, depredation appears to occur on fish naturally predated by local populations (Esteban, Verborgh, Gauffier, Giménez, Guinet, et al, 1984;Similä, 2005;Tixier, Giménez, et al, 2019). Other biological determinants may occur at the individual level and include personality, social learning and/or sex/age-dependent factors.…”

Role of sociality in the response of killer whales to an additive mortality event

“…These studies described the social groups as composed of two to seven individuals, with a generalist diet including seals, birds, cetaceans, and fish, and using unique group hunting strategies such as intentional stranding, which was shown to be socially transmitted from old females to offspring (11,42,47,49). Later, these killer whales were reported to depredate Patagonian toothfish (Dissostichus eleginoides) from the local commercial longline fisheries that began in the mid-1990s, with a high level of interactions with fishing vessels (42 ± 14% of longline sets on average) (37,38,(51)(52)(53). From 1996 to 2002, the Crozet killer whale population experienced substantial additive mortality, with the death of individuals from multiple social groups, mainly caused by lethal responses from illegal fishing vessels that used firearms and explosives to deter killer whales during depredation interaction events (37)(38)(39)(40)54).…”

Isotopic niche differs between seal and fish‐eating killer whales (Orcinus orca) in northern Norway

“…In the study of killer whales, high variability in δ 15 N and δ 13 C values has proved effective in assessing inter and intrapopulation dietary variations and preferences (Durban, Fearnbach, Burrows, Ylitalo, & Pitman, 2017; Herman et al, 2005; Krahn et al, 2007; Reisinger et al, 2016; Samarra et al, 2017; Tixier et al, 2019). Isotopic profiles confirmed dietary segregation between sympatric resource specialists in the northeastern Pacific (Herman et al, 2005) and around the Antarctic Peninsula (Durban et al, 2017) but also revealed generalist killer whale populations adopting a mixed diet including both fish and mammal prey (Reisinger et al, 2016; Tixier et al, 2019). In these studies, a priori knowledge of sampled individuals/populations through previous field observations has been highly beneficial for meaningful interpretation of dietary patterns (Newsome et al, 2010).…”