Estimation of the capture of New Zealand sea lions (Phocarctos hookeri) in trawl fisheries, from 1995–96 to 2007–08

Citation

Thompson, F. N., Oliver, M. D., & Abraham, E. R. (2010). Estimation of the capture of New Zealand sea lions (Phocarctos hookeri) in trawl fisheries, from 1995–96 to 2007–08. New Zealand Aquatic Environment and Biodiversity Report No. 52. 25 p. Retrieved from http://fs.fish.govt.nz/Doc/22271/AEBR_52.pdf.ashx

Summary

In this report, the number of New Zealand sea lion (Phocarctos hookeri) captures in New Zealand's trawl fisheries are estimated for the 1995—96 to 2007—08 fishing years. Over this period, Ministry of Fisheries observers recorded the capture of between 5 and 39 sea lions within the New Zealand Exclusive Economic Zone, within each fishing year. During the 2007—08 fishing year, 8 sea lions were observed captured on trawls. This count excludes 3 captures observed on the first day of the 2007—08 fishing year, in the last days of the southern blue whiting fishery. Seven of the 8 captured sea lions were retrieved dead, and 1 was released alive. Of the 8 sea lion captures, 5 were observed caught in the squid fishery around the Auckland Islands. This was the lowest number of observed captures in this fishery since the 1998—99 fishing year. Two sea lion captures were observed in the southern blue whiting fishery east of Campbell Island, a decrease from 6 captures the year before. There were no sea lion captures in the Auckland Islands scampi fishery or any other Auckland Islands trawl fishery. One sea lion was caught on an observed trawl in the hoki fishery south of The Snares and released alive.

From these observations, estimates of total captures were made for four different strata: the squid fishery near the Auckland Islands; the Campbell Island southern blue whiting fishery; other (non-squid) trawl fisheries near the Auckland Islands; and all trawl fisheries on the southern end of the Stewart-Snares shelf. Bayesian generalised linear models were fitted to data from the first two of these strata. A previous model, used for estimating sea lion captures in the 2006—07 fishing year, was re-implemented for the Auckland Islands squid fishery. Likewise, the model developed in 2006—07 for the Campbell Island southern blue whiting fishery was re-implemented, with minor changes. Ratio estimates were calculated for the remaining two strata.

Since 2001, trawl nets in the Auckland Islands squid fishery have increasingly been fitted with sea lion exclusion devices (SLEDs) that allow animals to escape from the net. The model estimated that the probability a sea lion failed to escape from a net fitted with a SLED (i.e., the retention probability) was 0.234 (95% c.i,: 0.14 to 0.38). This was similar to previous estimates, and correspondingly the predicted strike rate in 2007—08 of 5.2 sea lions per 100 trawls (95% c.i.: 2.2 to 9.7) was similar to estimates of the strike rate made previously. As SLEDs have been used more widely, the number of captures has fallen relative to the number of interactions, and in 2007—08 there were an estimated 14 (95% c.i.: 7 to 25) sea lion captures in the Auckland Islands squid fishery. For the same year, the model estimated that there were 65 (95% c.i.: 26 to 124) sea lion interactions, the lowest number of estimated interactions since 2002—03. This decrease was largely due to a decrease in effort from a peak of 2706 trawls in 2004—05 to 1265 trawls in 2007—08. The number of attributed mortalities in this fishery in 2007—08 was 42.1 (95% c.i.: 17 to 81), assuming a discount rate of 35%.

There were 552 trawls made in the Campbell Island southern blue whiting fishery in 2007—08, of which 41% were observed. The observed sea lion capture rate was 0.88 animals per 100 trawls, a decrease from the previous two years. The model estimated 4 captures during 2008 (95% c.i.: 2 to 10), and an estimated strike rate of 0.53 captures per hundred trawls (95% c.i.: 0.07 to 1.45). In other (non-squid) Auckland Islands trawl fisheries there were 12 estimated captures (95%c.i.: 7 to 18) in 2007—08, and 4 estimated captures (95% c.i.: 3 to 6) for all trawl fisheries on the southern Stewart-Snares shelf; numbers typical of the previous three fishing years for these strata.