Seasonal variation in the diet of New Zealand fur seals (Arctocephalus forsteri) at Otago Peninsula, New Zealand

Abstract

We examined the diet of New Zealand fur seals (Arctocephalus forsteri) at a rookery on Otago Peninsula, New Zealand, by analysing diagnostic remains from 500 faecal samples and 84 regurgitates collected between July 1993 and September 1994. In total, 27 taxa (species or genera) were identified. Small fish were numerically dominant in faecal samples, with five fish species (three species of lanternfish (Myctophidae), ahuru (Auchenoceros punctatus) and juvenile red cod (Pseudophycis bachus)) accounting for over 90% of prey items. Regurgitates occurred only seasonally and contained predominantly cephalopod remains (99% of prey items). Numerical analyses may prove misleading as an indicator of the composition of seal diet because the contribution of large fish that dominate estimates of prey biomass are underrepresented. Estimates of prey biomass were made by combining estimates from both faecal and regurgitate samples. Possible biases created by using this method are considered. A seasonal variation in prey composition was apparent, with arrow squid (Nototodarus sloanii) dominant in summer and autumn, replaced by a combination of barracouta (Thrysites atun), mackerel (Trachurus sp.) and New Zealand octopus (Octopus maorum) in winter and spring.