Feeding ecology of the New Zealand sea lion (Phocarctos hookeri) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor Philosophy in Zoology at Massey University, Palmerston North, New Zealand
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Date
2009
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Massey University
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Abstract
The New Zealand (NZ) sea lion Phocarctos hookeri is the only pinniped endemic to NZ
with a population of approximately 12,000 individuals. Its breeding range is currently
restricted to NZ sub-Antarctic islands, and it has failed to recolonise its pristine
distribution around the NZ main islands despite its protection since 1881. The current
hypothesis is that the population growth of this pinniped is limited by the distribution of
suitable prey on the Auckland Islands (50°30'S, 166°E) shelf, and by the direct and
indirect pressure exerted by the arrow squid Nototodarus sloani fishery. However, this
hypothesis has not been fully tested to date as there has been limited information on the
diet of the NZ sea lion and their potential prey. The objective of this thesis is to analyse
the diet of NZ sea lions over several years with particular emphasis on the most
reproductively important segment of the population: lactating females.
This thesis provides the first quantification by percentage mass of the diet of NZ sea
lion using a combination of stomach content analysis, qualitative fatty acid (FA)
analysis, and quantitative FA signature analysis (QFASA). Stomach contents and
blubber FAs were analysed from 121 individuals incidentally caught (by-caught) in the
southern arrow squid fishery from the years 1997 to 2006. The blubber FAs of 78 freeranging
lactating females captured at Enderby Island, Auckland Islands, were also
examined during January and February of 2000 to 2005.
Data obtained from both stomach analysis and QFASA indicate that arrow squid,
rattails Macrouridae, hoki Macruronus novaezelandiae and red cod Pseudophycis
bachus are key prey species for NZ sea lions in the Auckland Islands region. Because
these prey species live mostly at depths greater than 200 m, lactating females must
undertake long foraging trips and dive regularly to greater depths than other sea lion
species. Data from QFASA indicates that this foraging pattern is conducted over an
extended period through the summer and autumn. The daily food requirement of a
lactating female was estimated by a simple energetic model to be greater than 20% of its
body mass. During years of low arrow squid recruitment such as 1999 and 2001, the
amounts of squid required by the NZ sea lion population may have been similar to the
amount harvested by the fishery, suggesting that resource competition is likely to occur
between the arrow squid fishery and NZ sea lions in years of low squid abundance.
Half of the fishing activity of the southern squid fishery occurs in the north of the
Auckland Islands shelf where NZ sea lions forage, leading to incidental captures every
year. This research emphasises that management of the NZ sea lion must not only
consider the direct interactions with the arrow squid fishery, but also the likelihood of
food resource competition between the fishery and NZ sea lions.
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Keywords
Arrow squid, Red cod, Auckland Islands, Fishing