Abstract:
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.
