The diet of New Zealand fur seals (Arctocephalus forsteri) in southern New Zealand
I used information from 574 scats and 56 regurgitants to investigate diet of New Zealand fur seals (Arctocephalus forsteri) in southern New Zealand between June 1995 and March 1997. The five study sites were: Long Reef at Martin's Bay in northern Fiordland, Solander Island, Codfish Island, and Bench Island in the Foveaux Strait area, and The Snares. In total, identifiable remains from 38 prey taxa (identified to family, genus, or species) were found, representing about 178 kg of prey. Eighteen prey taxa not previously recorded in the diet of fur seals in the New Zealand region were found, including four squids, 14 fish, and one diving petrel. Of these species six were recorded at more than one study site, highlighting the necessity of collecting data on the diet of New Zealand fur seals throughout their range. Fur seals fed mainly on mesopelagic schooling prey over the outer shelf and upper slope. Octopus (Octopus maorum) was the only benthic prey item important in diet. Fur seals consumed a wide variety of prey taxa, taking advantage of seasonally and locally abundant prey species. Substantial seasonal and regional variation in prey composition was apparent. Arrow squid (Nototodarus sloanii) were an important component of prey at Long Reef in summer, Codfish Island in winter and summer, and Bench Island in summer and spring. The lanternfish Lampanyctodes hectoris was recorded in at least one season at all study sites, and was the most numerous prey item over the whole study (49% of total number of prey items). Hoki (Macruronus novaezelandiae) were important prey at Long Reef, in autumn and spring. Mackerel (Trachurus sp. probably T. murphyi) were important prey in winter at all study sites. Barracouta (Thyrsites atun) were important prey at Codfish Island and Bench Island in spring. Octopus were important prey at Codfish Island in winter and autumn, and at Bench Island in autumn. The size of squid represented in scats was significantly smaller than those from regurgitants. Fish remains including vertebrae of barracouta and large otoliths from hoki were also found in regurgitants. The importance of arrow squid as a prey item and the presence of remains from large fish prey emphasises the need to collect regurgitants as well as scats. Composition of diet by number of prey items was substantially different to that determined from prey mass. Results from this study were consistent with previous studies of diet using information from scats and regurgitates. Information on diet and foraging indicates that New Zealand fur seals do not compete with inshore commercial or recreational fisheries. Overlap occurs with two of New Zealand's largest offshore fisheries, for hoki and arrow squid. Whether this constitutes significant competition is difficult to demonstrate, partly because of uncertainties about seals (e.g. abundance), but more because the consumption of fisheries species by other predators is unquantified.
Advisor: Dawson, Steve; Lalas, Chris
Degree Name: Master of Science
Degree Discipline: Marine Science
Publisher: University of Otago
Research Type: Thesis