Tracking data reveal both indirect and direct interactions between killer whales and fisheries in subantarctic waters

Abstract

Conflicts between large marine predators and fisheries often involve both indirect (competition for fish stocks) and direct negative interactions (bycatch or depredation). However, the extent and the mechanisms of these conflicts are hampered by lack of data on the behavior of predators in natural situations (absence of fishing vessels) and in response to fishing activities. For killer whales Orcinus orca in the remote subantarctic waters of the southern South Atlantic and Indian Oceans, this lack of understanding is particularly problematic since negative interactions with industrial fisheries targeting toothfish affect the conservation of populations. In this study, we combined data from 36 satellite tags deployed on killer whales in these regions between 2011 and 2024 with tracking (AIS) data of toothfish fishing vessels to i) assess the overlap between killer whale offshore foraging areas and fishing areas, and ii) examine the factors influencing the decision of individuals to engage in depredation. Through kernel utilization distributions and statistical models we show that killer whales foraged in offshore areas used by fishing vessels to catch toothfish, but this overlap varied greatly across individuals within populations. We found that killer whales changed their trajectories and headed toward fishing vessels as far as > 100 km from them, possibly to engage in depredation. However, this behavior was not systematic and differed across individuals and areas. The behavior was detected only 55% of the times killer whales entered a 60 km range from a vessel. By highlighting areas of co-occurrence of killer whales and fisheries and the extent to which killer whales are able to change their behavior in response to fishing activities, our findings provide information that can be used to mitigate the negative impacts of interactions.