Estimation of the capture of New Zealand sea lions (Phocarctos hookeri) in trawl fisheries, from 1995–96 to 2008–09

Citation

Thompson, F. N., & Abraham, E. R. (2011). Estimation of the capture of New Zealand sea lions (Phocarctos hookeri) in trawl fisheries, from 1995–96 to 2008–09. New Zealand Aquatic Environment and Biodiversity Report No. 66. 25 p. Retrieved from http://fs.fish.govt.nz/Doc/22903/AEBR_66.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 2008—09 fishing years. Ministry of Fisheries observers reported the capture of three New Zealand sea lions (Phocarctos hookeri) during the 2008—09 fishing year. This was the lowest number of observed captures in the period covered by the data. The captures were reported from fisheries around the Auckland Islands: two sea lions were caught by trawlers targeting squid, and one sea lion was caught on a tow targeting scampi. Five sea lion captures were reported by fishers, from vessels targeting squid near the Auckland Islands. Two of these animals were among those reported by the observers, resulting in a total of six reported sea lion captures during the 2008—09 fishing year.

From the observed captures, estimates of total captures were made for four different strata: trawl effort targeting squid near the Auckland Islands, other trawl effort near the Auckland Islands, trawl effort targeting southern blue whiting near Campbell Island, and all trawl effort on the southern end of the Stewart-Snares shelf. Estimates of sea lion captures were calculated for each of these strata, and a single total estimate was calculated by combining the output from all strata. The statistical methods used to make the estimates followed those used for estimating captures in 2006—07 and 2007—08.

There were 1925 tows targeting squid in the Auckland Island fishery in 2008—09, a 52% increase from the 2007—08 fishing year. In 2008—09, it was estimated that 9 (95% c.i.: 3 to 21) sea lions were captured in this fishery. This was the lowest number of estimated captures in the period covered by the data. Trawlers fishing for squid near the Auckland Islands (within the SQU 6T area) use Sea Lion Exclusion Devices (SLEDs) to reduce the number of captures. These consist of a grid fitted before the codend of the net, with an open hole above the grid that sea lions can pass through. The model also estimated the number of interactions, the number of sea lions that would have been caught if no SLEDs were used. In 2008—09, the number of interactions in the Auckland Islands squid fishery was estimated at 56 (95% c.i.: 16 to 131), with a corresponding strike rate of 3.0 (95% c.i.: 0.9 to 6.4) interactions per 100 tows. This was one of the lowest strike rates within the period of the data, but was not significantly different from the strike rate of 5.65 interactions per 100 tows assumed by the Ministry of Fisheries in setting management objectives for the squid fishery.

There was a small increase in trawl effort targeting southern blue whiting near Campbell Island, 609 tows in 2008—09 compared with 552 in 2007—08. Estimated captures in this fishery peaked at 16 (95% c.i.: 8 to 28) in the 2007 calendar year, but fell to 1 (95% c.i.: 0 to 5) in 2009.

Observed sea lion captures in other fisheries have been sporadic, and captures in these fisheries were estimated using ratio methods that assumed a constant strike rate over the period of the data. This resulted in estimates for 2008—09 of 13 (95% c.i.: 7 to 19) captures in non-squid fisheries near the Auckland Islands, and 3 (95% c.i.: 1 to 4) captures in squid fisheries on the southern Stewart-Snares shelf.

Taken together, the total estimated captures for 2008—09 were 25 (95% c.i: 16 to 39). This was the lowest number of estimated captures in the 14 year period covered by the data. If interactions in the Auckland Islands squid fishery were included, the total increased to 72 (95% c.i.: 30 to 148).