Increasing numbers of killer whale individuals use fisheries as feeding opportunities within subantarctic populations

Abstract

Fisheries can generate feeding opportunities for large marine predators in the form of discards or accessible catch. How the use of this anthropogenic food may spread as a new behaviour, across individuals within populations over time, is poorly understood. This study used a 16-year (2003–2018) monitoring of two killer whale Orcinus orca subantarctic populations ( regular and Type-D at Crozet), and Bayesian multistate capture–mark–recapture models, to assess temporal changes in the number of individuals feeding on fish caught on hooks (‘depredation’ behaviour) of a fishery started in 1996. For both populations, the number of depredating individuals increased during the study period (34 to 94 for regular ; 17 to 43 for Type-D ). Increasing abundance is unlikely to account for this and, rather, the results suggest depredation was acquired by increasing numbers of existing individuals. For regular killer whales, a plateau reached from 2014 suggests that it took 18 years for the behaviour to spread across the whole population. A more recent plateau was apparent for Type-D s but additional years are needed to confirm this. These findings show how changes in prey availability caused by human activities lead to rapid, yet progressive, innovations in killer whales, likely altering the ecological role of this top-predator.

Sperm and northern bottlenose whale interactions with deep-water trawlers in the western North Atlantic

“…Anecdotal evidence suggests that these interactions have continued and may have spread to or from other regions. Although some studies have shown that associations with fisheries have benefitted whales (e.g., killer whales in the Crozets; (Amelot et al 2022;Tixier et al 2017), the energetic implications and risks of interactions for cetaceans described here are unknown and need further study.…”

“…Open water may make it easier for whales to detect (i.e., either visually or acoustically) and catch escaping fish that may also be disoriented or disabled. The energetic costs of deep vs. shallow diving and differences in prey encounter rates compared to natural foraging may contribute to whales continuing and potentially increasing their associations with fishers in this and other areas (Amelot et al 2022). Karpouzli & Leaper's (1997) earlier research on interactions between sperm whales and benthic trawlers in the same area as this study showed a pronounced difference in sighting rates with trawling phases.…”

“…Open water may make it easier for whales to detect (i.e., either visually or acoustically) and catch escaping fish that may also be disoriented or disabled. The energetic costs of deep vs. shallow diving and differences in prey encounter rates compared to natural foraging may contribute to whales continuing and potentially increasing their associations with fishers in this and other areas (Amelot et al 2022).…”

“…Escapees associated with towing, particularly fish that are disoriented or disabled, may also be easier to detect (i.e., either visually or acoustically) than other naturally occurring prey. Opportunistic associations between cetaceans and fishing vessels are thought to not only increase prey encounter rates, but also reduce energetic costs associated with greater dive depths (Amelot et al 2022). Such energetic incentives are likely to contribute to increasing the number of whales that forage opportunistically in association with fishers in this and other areas (Amelot et al 2022).…”

“…Fisheries can also indirectly threaten cetaceans when overexploitation, decreasing their prey availability (Carlucci et al 2021). However, feeding in association with fishing activities may provide short term energetic benefits for cetaceans by increasing the quantity and the quality of the prey consumed, while reducing foraging costs and providing some foraging advantage (Amelot et al 2022; Fertl and Leatherwood 1997; Silva et al 2002).…”