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Review of evidence for indirect effects of commercial fishing on New Zealand sea lions (Phocarctos hookeri) breeding at the Auckland Islands (PDF, 642K)

Summary

The New Zealand (NZ) sea lion (Phocarctos hookeri) is a rare pinniped species with a highly restricted geographic distribution. It mainly breeds on New Zealand’s sub-Antarctic islands and currently about 71% of all births occur on the Auckland Islands. Pup production at the Auckland Islands colonies has been declining. Sea lions have been recently uplisted to ‘Nationally Critical’ under the New Zealand threat classification system.

Sea lions are incidentally drowned each year in southern commercial trawl fishing operations, particularly those targeting arrow squid (Nototodarus sloanii). Satellite tagging studies indicate that foraging areas of juveniles and lactating females from the Auckland Islands overlap with commercial trawl fisheries, including arrow squid. Arrow squid are eaten by sea lions. Therefore, there may be indirect fishery effects on the NZ sea lion population through resource competition.

Scientific literature pertaining to indirect effects was reviewed. Although there is overlap in the distribution of adult female sea lions with commercial fisheries, this conclusion is based on data from only the first month of a 10-12 month lactation period. A short-term study of the foraging distribution of juveniles has been conducted at only one colony. Reliable estimates of diet, including the contribution of arrow squid, are not available, although the main prey species consumed are probably known.

Despite the opportunity of indirect fishery effects, there is no evidence that competition with the squid fishery for food is negatively affecting the NZ sea lion population. Reliable estimates of diet, prey abundance, and a better understanding of the foraging distribution of adult females later in lactation and juveniles is needed. Simultaneously testing competing hypotheses (including indirect fishery effects on food) about the causes of population decline may be a more effective approach than attempting to evaluate single factor explanations for the decline.

Publication Information

Author: W.D. Bowen

This project was conducted under contract to the Deepwater Group Ltd.