Study points to fishery by-catch as reason for sea lion decline

University of Otago research examining why numbers of the threatened New Zealand sea lion, Phocarctos hookeri, are declining on their main breeding ground of the Auckland Islands has identified squid fishing as the most likely cause.

Lead Researcher and Senior Lecturer in the Zoology Department Dr Bruce Robertson has just published his research findings in the International journal Mammal Review, where it is the cover story.

Pup numbers on the sub-Antarctic Auckland Islands – one of two areas in New Zealand where the species breeds- have declined by 40 per cent between 1998, when 3021 pups were produced, and 2009, when only 1501 pups were born.

This is compared to the second largest breeding area on the Campbell Island, which saw its population of New Zealand sea lions slowly increase during the same period.

Dr Robertson says previous research linked the drop on the Auckland Islands to breeding females not returning to these Islands to feed their pups or to have new pups. But it was unknown what was causing the decline. The species of sea lion is found only in New Zealand waters.

“The different population trajectories of the Auckland Islands and Campbell Island sea lion colonies provided an opportunity to evaluate various causes of the decline of pup numbers at the Auckland Islands,” he says.

The study reviewed all previous data and concluded the most plausible explanation for the decline in breeding females in the Auckland Islands was competition for resource between the squid fishery and sea lions, and that sea lions were too often casualties of fishing by-catch.

Researchers noted that the Auckland Islands has a strong squid fishing industry within range of the sea lions, where as Campbell Island has no fishery operating close enough to cause a problem for its breeding colony. The Auckland islands squid fishery is trawler based, using large towed nets.

The study investigated other possible causes, such as natural predation, disease, environmental change, genetic effects and pollution, but still found fishing impacts as the most likely causes.

“This research is not about apportioning blame, rather I was interested in what is the cause of the observed decline. Understanding the cause should allow managers to address the decline. We can now say, of all the potential causes, the most likely are human impacts associated with squid fishing around the Auckland Islands”.

The study recommended that future research should investigate the impacts of the squid fishery at least around the Auckland Islands, and also the affects nutrient stress and resource competition have on reproductive ability of the sea lions.