The population decline of the New Zealand sea lion Phocarctos hookeri: a review of possible causes

The population of the New Zealand sea lion (Phocarctos hookeri), New Zealand's only endemic pinniped, has declined significantly at the Auckland Islands since 1998, with pup production dropping by 40%—from 3021 pups in 1998 to just 1501 in 2009. This decline is primarily driven by philopatric females not returning to breeding areas, contrasting with a stable or slightly increasing population on Campbell Island. After evaluating nine potential causes—ranging from disease and predation to fisheries-related impacts—the study identifies fisheries-induced resource competition and by-catch mortality as the most plausible explanations for the Auckland Islands' decline. While other factors like bacterial epizootics were considered, their impact is unlikely given naturally high pup mortality rates. The research underscores the need for further investigation into how fisheries may be stressing sea lion populations through resource scarcity and direct mortality.

ROBERTSON, B. C., & CHILVERS, B. L. (2011). The population decline of the New Zealand sea lion Phocarctos hookeri: a review of possible causes. Mammal Review, 41(4), 253–275. https://doi.org/10.1111/j.1365-2907.2011.00186.x