Abstract
Fisheries modify prey availability for marine predators by extracting resources but also by providing them with new feeding opportunities. Among these, depredation, which occurs when predators feed on fish caught on fishing gear, is a behavior developed by many species as a way to acquire food through limited foraging effort. However, the extent to which depredated resources from fisheries contribute to the energetic requirements and affect the demography of depredating individuals is unknown. We investigated the contribution of Patagonian toothfishDissostichus eleginoidesdepredated on longlines to the energetic requirements of killer whalesOrcinus orcaaround the Crozet Islands (southern Indian Ocean) over the period 2007-2018. Our results indicate that during days when depredation occurred, depredating individuals fulfilled on average 94.1% of their daily energetic requirements with depredated toothfish. However, the contribution varied from 1.2 to 13.3% of the monthly energetic requirements and from 2.4 to 8.8% of the yearly energetic requirements of the total population. Together, these findings suggest that intake of depredated toothfish can be substantial at a fine scale (daily and individually), potentially leading to temporary provisioning effects and changes in predation pressures. These effects become minor (<10%), however, when considering the full population over a whole year. The contribution of depredated fish to the annual energetic requirements of the population has increased in recent years, likely due to larger fishing quotas and greater opportunities for whales to depredate, which stresses the importance of accounting for depredation in ecosystem-based management of fishing activity.
Highlights
The increase in exploitation of marine resources over the past 60 yr has intensified interactions between fisheries and large marine predators (Northridge 1984, 1991, Pauly et al 1998, DeMasterPublisher: Inter-Research · www.int-res.comMar Ecol Prog Ser 668: 149–161, 2021 fish caught on fishing gear, and fishery discards or baits are often used by seabirds (Grémillet et al 2008, Votier et al 2013, Tixier et al 2021)
With killer whale DPER2009 estimated at 995 MJ d−1, the daily Ci during days when depredation occurred was estimated to be 94.1% for depredating individuals (Table 3)
When considering the daily energetic requirements of all killer whales of the population in 2009 (n = 89 weaned individuals, comprising 17 weaned juveniles, 19 adult males and 53 adult females; see Table S1), the mean daily Ci was estimated at 8.5%
Summary
The increase in exploitation of marine resources over the past 60 yr has intensified interactions between fisheries and large marine predators (Northridge 1984, 1991, Pauly et al 1998, DeMasterPublisher: Inter-Research · www.int-res.comMar Ecol Prog Ser 668: 149–161, 2021 fish caught on fishing gear, and fishery discards or baits are often used by seabirds (Grémillet et al 2008, Votier et al 2013, Tixier et al 2021). While depredation has socio-economic impacts on fishing industries through reduced catch rates and damage to fishing gear, it has effects on biodiversity and ecosystem functioning (Gilman et al 2006, 2008, Hamer et al 2012, Mitchell et al 2018). Among these effects, bycatch and injuries to the predators from direct interactions with fishing gear have been documented in many fisheries (Werner et al 2015). Quantifying these changes, which may alter both predator population dynamics and the natural predatory role of depredating species in ecosystems, is crucial to assess the full trophic impacts of fishing and inform fishery management
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