Estimation of fur seal bycatch in New Zealand trawl fisheries, 2002–03 to 2007–08

New Zealand fur seals (Arctocephalus forsteri) have been caught in commercial trawl fisheries operating around New Zealand’s EEZ. Ministry of Fisheries observers recorded 141 fur seal captures in trawl fisheries in 2007—08, and 72 in 2006—07. Fur seals were the most frequently observed non-fish bycatch species in 2007—08. Captures have been reported from trawlers operating throughout New Zealand’s EEZ, with the exception of the north and east coasts of the North Island. Most captures have been observed in the hoki fishery, with 57 fur seals being observed caught in 2007—08. In contrast, few captures were reported from deepwater or scampi fisheries.

A Bayesian generalised linear model was developed using the observed trawl effort. The model was fitted to data from six fishing years, 1 October 2002 to 30 September 2008. The model was then used to estimate fur seal captures on unobserved tows. Trawl effort from the north and east of the North Island, and from the eastern end of the Chatham Rise, was excluded because there were no observed fur seal captures in those areas. Trawl effort from tows targeting inshore species was also excluded as observer coverage in inshore fisheries was very low, and no fur seals were observed caught. The model included covariates for fishing area, target species, day of the year, and distance to the shore. These covariates, which have been identified in previous work, were selected because they explained much of the variation in the capture rate.

In 2007—08, an estimated 714 (95% c.i.: 465 to 1130) fur seals were caught in trawl fisheries (with trawl fishing targeting inshore species not being included). The estimated capture rate was 1.55 (95% c.i.: 0.99 to 2.50) fur seals per 100 tows. The estimated capture rate for 2007—08 was contained within the confidence intervals of estimated rates for each of the six years, and there was no evidence of a trend in the fur seal capture rate. In 2006—07, 488 (95% c.i.: 288 to 826) fur seals were estimated to have been caught, and the capture rate was 0.98 (95% c.i.: 0.59 to 1.63) fur seals per 100 tows.

Fur seal captures followed a strong seasonal pattern, which was seen in all areas and targets. The estimated capture rate peaked to about five times the mean annual rate in the winter months of July, August, and September, and dropped to around one fifth the mean annual rate in the summer months of December, January, and February. The reduced capture rates in the summer coincided with the fur seal breeding season. Previous work identified the light condition as important, with more fur seals caught in the dark. It was found, however, that once a day of year effect was included, there was little evidence of a light condition effect. Distance to shore was also related to the fur seal capture rate. The capture rate was reduced when the fishing was more than 90 km from shore.