Mitigating incidental captures of fur seals in trawl fisheries

Download the publication

MIT2006/09 Mitigating incidental captures of fur seals in trawl fisheries (PDF, 2,095K)

Background papers

• Clement report - review of worldwide practice (PDF, 162K)
• Fur seal mitigation - NIWA Review 2004 appendix 1 (PDF, 171K)

Summary

New Zealand fur seals (Arctocephalus forsterii) have a history of non fatal and more often fatal interactions by capture and subsequent drowning in trawls used in New Zealand middepth fisheries. Mitigation of such interactions is desirable for this protected species. A particular design of exclusion device (Sea Lion Exclusion Device, or SLED) is successfully used in the squid trawl fishery to improve escapement of New Zealand sea lions (Smith and Baird, 2007).

Reviews of best international practice along with characteristics of trawl gear operation were undertaken to establish a best way forward in regard mitigating fur seal interactions with particular reference to the hoki (Macruronus novaezealandiae) fishery. It was determined to trial a seal exclusion device (or SED) in the hoki fishery based on the squid fishery SLED. These devices work by physically separating the catch from the animal to be excluded using a rigid (steel) grid and providing an adjacent escape hole on the top side of the trawl. This hole has a hood designed to help prevent fish catch escapement and provide some surety that a drowned seal could not float or be washed from the trawl and thus not be reported.

The most significant construction difference in the SED used in this project compared to existing SLEDs was to reduce the bar spacing in the separator grid from 23cm to 17cm. This was done because the New Zealand fur seal is smaller in size than the New Zealand sea lion and, therefore, wider bar spacing may not act to separate fur seals from hoki catches.

Underwater cameras were used to record and make initial assessments of fur seal and fish interactions as well as SED performance. Underwater camera records were constrained by reliability and visibility issues but a useful record of events was made in a structured manner.

Review of video records remains a manual, often qualitative, and time-consuming process. Fur seals were often seen near the vessel or feeding from the trawl when it was on the surface during the two trial voyages but none were captured and none were recorded by underwater camera.

The first trial of the SED in the Cook Strait hoki fishery during the spawning season when aggregations were available it was observed that the SED components operated properly but that target fish exited from the escape hole. Hoki were observed to have a strong tendency to drift within the trawl with no consistent orientation. Other fish species were more likely to have directional mobility. This lead to hoki being particularly inclined to impact with or become trapped across the grid bars.

A second trial was carried out side of the spawning season, when catch rates were much lower than in the hoki season. It was possible to subjectively assess both rates of fish impact with the grid and grid blockage. It appears that at high catch rates, hoki behaviour leads to processes at the grid face which allow for or even encourage escapement. A large proportion of hoki impact the grid bars while passing into the codend.

This work was based on the premise that a device that is efficient in allowing New Zealand sea lions to escape from squid trawls would also work in principle for New Zealand fur seals in the hoki and like fisheries. This appears to not immediately be the case.