Population dynamics reveal conservation priorities of the threatened New Zealand sea lion Phocarctos hookeri

Abstract

Understanding the life history basis of the population dynamics of threatened species is crucial to their assessment and recovery. Within the Southern Pacific Ocean, the decline and range contraction of New Zealand sea lions (NZSLs) (Phocarctos hookeri) are an exemplary case of a major fisheries and conservation controversy. Despite the use of bycatch reduction measures, the main population of the NZSL has declined since 1998 and its vulnerability to threats is poorly understood. Here we develop a population model for the declining population of the endangered NZSL at the Auckland Islands, New Zealand (\(50^{\circ }30^{\prime }\hbox {S}, 166^{\circ }17^{\prime }\hbox {E}\)), parameterised by mark-recapture data of 2928 female NZSLs spanning 15 years from 1998 to 2012, and provide the first comprehensive analysis of NZSL population dynamics and its relationship to life history. Our model closely predicts the observed trend in population censuses of the main breeding population. We found that the population will continue to decline without effective intervention. Furthermore, we found that the population growth rate is most responsive to changes in survival of adult female NZSLs—the demographic group that is most threatened by fishery bycatch. Nevertheless, inferences about the efficacy of NZSL bycatch reduction measures are still imprecise. Combined, this could explain why the main population of NZSLs continues to decline. Our results emphasise that reliable data on bycatch reduction measures are needed, if they are to be shown to protect key demographic groups of marine mammals.