The effect of prey abundance and fisheries on the survival, reproduction, and social structure of killer whales (Orcinus orca) at subantarctic Marion Island

Most marine apex predators are keystone species that fundamentally influence their ecosystems through cascading top‐down processes. Reductions in worldwide predator abundances, attributed to environmental‐ and anthropogenic‐induced changes to prey availability and negative interactions with fisheries, can have far‐reaching ecosystem impacts. We tested whether the survival of killer whales (Orcinus orca) observed at Marion Island in the Southern Indian Ocean correlated with social structure and prey variables (direct measures of prey abundance, Patagonian toothfish fishery effort, and environmental proxies) using multistate models of capture–recapture data spanning 12 years (2006–2018). We also tested the effect of these same variables on killer whale social structure and reproduction measured over the same period. Indices of social structure had the strongest correlation with survival, with higher sociality associated with increased survival probability. Survival was also positively correlated with Patagonian toothfish fishing effort during the previous year, suggesting that fishery‐linked resource availability is an important determinant of survival. No correlation between survival and environmental proxies of prey abundance was found. At‐island prey availability influenced the social structure of Marion Island killer whales, but none of the variables explained variability in reproduction. Future increases in legal fishing activity may benefit this population of killer whales through the artificial provisioning of resources they provide.