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Abstract
The emergence of longline fishing around the world has been concomitant with an increase in depredation-interactions by odontocete whales (removal of fish caught on hooks), resulting in substantial socio-economic and ecological impacts. The extent, trends and underlying mechanisms driving these interactions remain poorly known. Using long-term (2003–2017) datasets from seven major Patagonian toothfish (Dissostichus eleginoides) longline fisheries, this study assessed the levels and inter-annual trends of sperm whale (Physeter macrocephalus) and/or killer whale (Orcinus orca) interactions as proportions of fishing time (days) and fishing area (spatial cells). The role of fishing patterns in explaining between-fisheries variations of probabilities of odontocete interactions was investigated. While interaction levels remained globally stable since the early 2000s, they varied greatly between fisheries from 0 to >50% of the fishing days and area. Interaction probabilities were influenced by the seasonal concentration of fishing effort, size of fishing areas, density of vessels, their mobility and the depth at which they operated. The results suggest that between-fisheries variations of interaction probabilities are largely explained by the extent to which vessels provide whales with opportunities for interactions. Determining the natural distribution of whales will, therefore, allow fishers to implement better strategies of spatio-temporal avoidance of depredation.
Highlights
Over the last 60 years, the world’s commercial fisheries have undergone substantial changes in distribution, intensity, regulations and technology[1]
Patagonian toothfish longline fisheries emerged as commercial fisheries in the 1980s-2000s, replacing existing bottom-trawling fisheries, and have all been subject to killer and/or sperm whale depredation interactions since the first years following their commencement[22,23,24,25,26,27,28,29]
Fisheries operating in Chile, the Falklands and PEMI predominantly use the trotline system equipped with “cachalotera”, a fish protection device developed to reduce odontocete depredation and seabird mortality[32], whereas the other fisheries use the autoline system
Summary
Over the last 60 years, the world’s commercial fisheries have undergone substantial changes in distribution, intensity, regulations and technology[1]. The present study used this hypothesis to investigate the influence of fishing patterns of different commercial fisheries in the Southern Ocean on the levels of interaction between fishing vessels and two odontocete species: killer whales (Orcinus orca); and sperm whales (Physeter macrocephalus). These commercial fisheries operating in the waters of southern Chile, and around the Falklands, South Georgia, Prince Edward and Marion islands (hereafter “PEMI”), Crozet islands, Kerguelen islands, and Heard and MacDonald islands (hereafter “HIMI”) all use demersal longlines to catch Patagonian toothfish (Dissostichus eleginoides). Most fisheries experienced substantial Illegal Unreported and Unregulated (IUU) fishing in the 1980s and 1990s, resulting in an over-harvest of local fish stocks and impacts on seabird and whale populations interacting with illegal vessels[30,33,34,35,36,37,38,39]
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