Estimation of fur seal (Arctocephalus forsteri) bycatch in New Zealand trawl fisheries, 2002–03 to 2008–09

Finlay N Thompson & Edward R Abraham


Thompson, F. N., & Abraham, E. R. (2010). Estimation of fur seal (Arctocephalus forsteri) bycatch in New Zealand trawl fisheries, 2002–03 to 2008–09. New Zealand Aquatic Environment and Biodiversity Report No. 61.


In the 2008–09 fishing year, 72 New Zealand fur seals (Arctocephalus forsteri) were observed captured in commercial trawl fisheries around New Zealand. Trawls targeting hoki continued to have the highest number of observed captures, with 38 (53%) observed, 20 of these on hoki trawlers in the Cook Strait alone. The highest observed fur seal capture rate of 14.17 captures per 100 tows was on southern blue whiting trawls near the Bounty Islands, with 17 fur seal captures observed. Another 16 captures were observed in a range of other fisheries: hake, squid, scampi, jack mackerel, and other middle depth species (barracouta and silver warehou).

Observer coverage of inshore fisheries increased to over 3.4% in 2008–09, the first year it has been greater than 1%. One fur seal capture was observed on an inshore tow targeting giant stargazer west of Stewart Island, the first time a fur seal capture has been observed on an inshore trawl. Despite the recent increase in inshore observer coverage, no attempt was made to estimate fur seal captures in the inshore trawl fisheries as over the whole period coverage in these fisheries remained too low.

A Bayesian capture model was developed to predict fur seal captures in commercial trawl fisheries. The parameters from the fitted model were used to estimate fur seal captures across commercial trawl effort, excluding inshore fishing, for the seven year period from 2002–03 to 2008–09. The mean estimate in 2008–09 was 550 fur seal captures (95% c.i.: 338 to 826), lower than the estimate of 710 captures (95% c.i.: 489 to 996) in 2007–08, but higher than the estimate of 501 captures (95% c.i.: 304 to 764) in 2006– 07. Trawl effort targeting hoki had the highest estimated fur seal captures amongst the different target fisheries in 2008–09, with 191 estimated captures (95% c.i.: 112 to 306). Estimated fur seal captures by hoki trawlers in Cook Strait accounted for 23% of all estimated captures. Trawls targeting middle depth species were estimated to have caught 150 fur seals (95% c.i.: 57 to 307) in 2008–09.

In 2008–09 the mean estimate of 96 fur seal captures (95% c.i.: 40 to 195) near the Bounty Islands, was greater than the mean estimate of 80 captures (95% c.i.: 47 to 125) on the west coast of the South Island. Trawl effort near the Bounty Islands, mostly southern blue whiting, reached a peak of 637 tows in 2008–09, while trawl effort on the west coast of the South Island continued a slow decline to 3726 tows. The capture rate near the Bounty Islands was estimated to be 16.16 (95% c.i.: 8.06 to 28.74) fur seal captures per 100 tows, the highest of any area.