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Fishing nets a major cause of death in declining New Zealand sea lion population

A mother and baby sea lion nuzzling noses

Photo: John Burke, New Zealand Sea Lion Trust.

Getting caught in fishing nets remains a major cause of death for the increasingly endangered New Zealand sea lion even after the addition of “sea lion exclusion” devices, according to new research from the University of Toronto and New Zealand’s University of Otago and Massey University.

To investigate the role commercial fishing has played in the sea lions’ decline, Martin Krkosek of the Department of Ecology & Evolutionary Biology in the Faculty of Arts & Science at U of T and his colleagues in New Zealand analyzed government data of the sea lion population and fisheries “bycatch.” Bycatch describes the fish or other marine species that are caught unintentionally while targeting other species.

Since 2001, sea lion exclusion devices have been used in arrow squid fishing around the sub-Antarctic Auckland Islands, a main breeding area for the sea lions. While the observed bycatch has declined, the researchers say the numbers may be misleading and the devices have not successfully removed the bycatch threat.

“Based on the information we have, it seems that the devices may be allowing dead sea lions to fall out of the nets at sea or be causing injury to them that reduces their life expectancy or reproductive ability,” says Krkosek. “If the problem isn’t addressed, it is likely putting the New Zealand sea lion population at risk of extinction.”

“We’d also urge a wider assessment of bycatch exclusion devices used in global trawl fisheries,” Krkosek says.

The international research team includes lead author and population ecologist Stefan Meyer and Bruce Robertson, both of the Department of Zoology at the University of Otago, and Louise Chilvers of Massey University.

The New Zealand government has committed to halting the decline of New Zealand sea lions within the next five years, aiming for a stable or growing population within the next 20 years.

“The government’s current focus is on reducing pup deaths. Fishing impacts are, incorrectly, thought to be only minor,” says Meyer.

The research team hopes the study might lead to more meaningful management.

“The good news is there are a range of options open to the government to reduce the impact of fishing on the sea lion population, while still allowing commercial fishing in the New Zealand sub-Antarctic,” says Robertson.

The research – published in the international journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) – was funded by the Institute of Veterinary, Animal and Biomedical Sciences, the Natural Sciences and Engineering Research Council of Canada, a Sloan Fellowship in Ocean Science, the Canada Research Chair in Population Ecology and the New Zealand Department of Conservation’s Conservation Services Program.

With files from the University of Otago.