| Summary: | The New Zealand sea lion (Phocarctos hookeri) is an endangered pinniped endemic to New Zealand (Wilkinson, Burges et al. 2003). Declared “nationally critical” under the NZ threat classification system and “vulnerable in decline” by IUCN, they have shown a 49% decline in pup production since 1995 (Chilvers, Wilkinson et al. 2010). I investigated the role of long-term changes in diet on the population ecology of NZ sea lions. I was interested in the role played by long-term changes in diet into the observed decline of the sea lion population. My study is set apart from others in that it spans 13 years of routine sampling, and represents one of the longest timelines of diet data for any pinniped species. I used scat and regurgitate samples from New Zealand sea lions that were collected at the Auckland Islands between the summer of 1995/1996 and 2012/2013. I identified 11 main prey types from hard parts including otoliths, beaks and other diagnostic bones. In scats these main prey types were opalfish (Hemerocoetes spp.), rattail (Macrouridae), red cod (Pseudophycis bachus), octopus (Octopus sp.) and arrow squid (Ommastrephidae), Ling (Genypterus blacodes), smallscaled cod (Paranotothenia sp.), hoki (Macruronus novaezelandiae), triplefin (Tripterygiidae), fur seal (Arctocephalus forsteri) and giant octopus (Enteroctopus zealandicus). Main prey types found in regurgitates were similar but there was a higher proportion of cephalopods than in scat samples. When assessing long-term trends in occurrence of main prey species I found that smaller prey types such as opalfish and Octopus sp. have been increasing in the diet over time, while larger species hoki and giant octopus have been decreasing. The ratio of fish:cephalopods in NZ sea lion diet has also been decreasing with time, possibly indicating an overall reduction in diet quality. I used catch per unit effort as a proxy for prey availability in the environment and compared this to frequency of occurrence (%FO) of main prey types over time. The best models for ...
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