Variability in the diet of New Zealand sea lion (Phocarctos hookeri) at the Auckland Islands, New Zealand

Abstract

We examined the stomach contents of 121 New Zealand (NZ) sea lions (Phocarctos hookeri) caught by the squid fishery during the summer/autumn 1997–2006 around the Auckland Islands (51°S, 166°E). Dietary variation was assessed among juveniles, lactating females, nonlactating females and males, and between areas on the Auckland Islands shelf. The digested fraction of the contents consisted mostly of opalfish (Hemerocoetes spp.) (50.1% by number [N], 4.7% by mass [M]), rattail (Coelorinchus spp.) (12.0%N, 2.4%M), arrow squid (Nototodarus sloani) (14.1%N, 17.9%M), octopus (Enteroctopus zealandicus) (2.1%N, 27.8%M), and red cod (Pseudophycis bachus) (3.8%N, 4.3%M). Opalfish was found in greater proportions in the stomachs of females (lactating: 58.1%N, nonlactating: 62.4%N) and juveniles (56.9%N) than males (14.5%N). Juveniles caught smaller opalfish and rattail than adults did. Over all classes, sea lions ate larger prey in the east than in the north of the Auckland Islands shelf. The common prey—arrow squid and rattail—constitute an abundant resource at the edges of the Auckland Islands shelf, where lactating NZ sea lions forage. Although these key areas are far from the rookeries and impacted by the squid fishery, they may provide the only reliable resource able to support the cost of benthic foraging behavior in the deepest diver of all otariids.