A nascent breeding population of New Zealand sea lions at Otago, southeast South Island

Abstract

New Zealand sea lions, Phocarctos hookeri, are one of the least abundant pinniped species and practically all breeding is restricted to the Auckland Islands and Campbell Island. They were extirpated from the New Zealand mainland by hunting following human colonisation. The philopatric nature of sea lions restricts their capacity to colonise new areas. New Zealand sea lions were declared a threatened species in 1997. The primary aim of the population management plan is to achieve non-threatened status within 20 years, which is contingent on new breeding locations (>35 mature females exhibiting breeding philopatry) being established outside the subantarctic population base. Otago, southeast South Island, is the only location north of the Auckland Islands with sustained annual breeding. Breeding was initiated at Otago by a solitary migrant female in the 1993/1994 season.

Otago beaches were surveyed from December 2003 to June 2004 and during the 2004/2005, 2005/2006 and 2006/2007 breeding seasons. Individuals were identifiable by flipper tags and diagnostic natural body features. A total of 28-29 pups were born at Otago Peninsula (all in the lineage of the founding female) from 1993/1994 to the 2005/2006 breeding season. Results from this study indicate the initiation of colonial breeding since all breeding females and their pups were based at the same sites for the 13 years since breeding began and all Otago-born juvenile females remained in contact with mothers and pups. Two pups were born in the Catlins in the 2005/2006 and 2006/2007 seasons to a new resident immigrant female indicating potential for a new breeding location to develop.

Results from a Leslie matrix model predicted the Otago population will reach >35 breeding females between 2018-2034: 21-37 years from 1997, at rates of annual increase of 6-15%. The average age of primiparity was earlier at Otago than at the Auckland Islands. At stable population demography in the model, four year old females at Otago made up 13-17% of the breeding population compared to 1% for four year olds at the Auckland Islands. Survival of Otago born pups to one year was high (86% to 93%). Mothers at Otago had shorter attendance pattern and foraging cycle durations than mothers at the Auckland Islands indicating that Otago is a more favourable foraging environment. The combination of these findings leads to the conclusion that Otago represents a superior breeding location for New Zealand sea lions compared to the Auckland Islands in terms of environmental conditions.