Diet of New Zealand sea lions (Phocarctos hookeri) at their southern breeding limits

Abstract

The New Zealand (NZ) sea lion (Phocarctos hookeri) is endemic to NZ and is listed as “Vulnerable” by the International Union for Conservation of Nature. Approximately 99 % of pups are born at the Auckland Islands (a declining population) and Campbell Island (a growing population). The causes of contrasting population trajectory are not well understood, though spatial and temporal variations in prey availability have frequently been implicated for other pinnipeds. This is the first published diet study of the Campbell Island population, located at the species’ southern breeding limit. Prey species were identified and quantified from scats and regurgitate samples collected in March 2013 (n = 159 and 7, respectively). An array of prey taxa was identified, though two species were particularly dominant in terms of reconstituted diet mass (M) of fishes and cephalopods: small-scaled cod (Notothenia microlepidota), which dominated scat samples (50 % M); and yellow octopus (Enteroctopus zealandicus), which dominated a small sample of regurgitates (72 % M). The diet lacked many of the key prey taxa of the Auckland Islands, and we hypothesise that the key prey identified here provides a highly available food source for the growing population of NZ sea lions at Campbell Island. These differences are likely to reflect spatial heterogeneity in prey availability and may be one of the main causes of an increase in NZ sea lion population size at Campbell Island in contrast to a decrease at the Auckland Islands.