Extreme diving of females at the largest colony of New Zealand sea lions, Phocarctos hookeri

Abstract

The diving behaviours of marine predators are thought to be coupled with species demographics. Species that forage at or close to their physiological limits will be limited in their ability to respond to natural or anthropogenic induced changes in their environment. This is the case for the New Zealand sea lion (Phocarctos hookeri). A previous study from Enderby Island, Auckland Islands, the third largest colony for the endangered species, showed lactating females exhibit extreme diving behaviour, and that individuals have either: a benthic diving ecotype or a deeper, more varied mesopelagic diving ecotype which are maintained throughout their adult life. The ecological significance and implications for individual foraging specialisation has major consequences for this species as mesopelagic foragers are more likely to suffer mortality from fisheries than benthic foragers. We investigated the diving behaviour of lactating females from Dundas Island, Auckland Islands, the largest breeding colony for New Zealand sea lions. The diving behaviour of 24 lactating females was examined and confirmed that both diving ecotypes exist and that females at this colony are also operating at the upper limit of their physiological capability. Enderby and Dundas Island are the largest breeding colonies for the New Zealand sea lion and together represent 69% of this species pup production. This population has significantly declined since 1998, predominantly due to direct and indirect fisheries interactions. Measurable conservation and management steps are therefore critical to mitigate the impacts of fisheries on this population.