Warp strike in New Zealand trawl fisheries, 2004–05 to 2008–09

Ministry of Fisheries observers have been making warp strike observations since the 2004—05 fishing year. During these observations, they record the numbers of birds that are struck by trawl warps during 15 minute periods. In this report, a summary is presented of all the strike data collected from 2004—05 to 2008—09. Warp strike observations largely ceased in 2008—09, with the only observations being made on a trip that began during the 2007—08 fishing year. The warp strike dataset contains 7266 observations made on the trawl warps until the 2008—09 fishing year, Over all years, 67.4% of warp observations were made during squid trawls. The next most frequently sampled fishery was hoki, with 12.1% of warp observations being made during hoki target trawls. In 2007—08 and 2008—09, 120 warp strike observations were also made on smaller trawlers targeting inshore species.

The use of mitigation devices, which deter seabirds from entering the regions between the stern of a trawler and the trawl warps, was made mandatory in January 2006 for all trawlers over 28 m in length fishing in New Zealand waters. Warp strike rates were highest during the 2004—05 fishing year, before the legislated requirement to use mitigation devices. In that year, in the squid fishery, average strike rates of 1.53 heavy contacts per hour were recorded for small birds (all birds other than albatrosses and giant petrels), with the strike rate of large birds (albatrosses and giant petrels) being 3.41 heavy contacts per hour. By 2007—08, the squid fishery warp strike rates for small and large birds had reduced to 0.36 and 0.52 heavy contacts per hour, respectively. There is strong evidence that this reduction was due to an increased use of mitigation devices, particularly tori lines. The results from statistical modelling are that using tori lines reduces the warp strike rate to less than 25% of the strike rate that occurs when no mitigation is used.

In 2007—08, the warp strike rates recorded in inshore fisheries were higher than the rates recorded in the squid fishery. Although the number of observations was small, these data confirm that warp strikes do occur on smaller inshore vessels. Moreover, there were 3 large bird and 1 small bird warp captures during tows with targeting inshore species that had warp strike observations.

The analysis confirmed the importance of offal management, with few warp strikes or captures being recorded in the absence of offal discharge. Across all the data, the average large bird warp strike rate was 0.03 strikes per hour when there is no discharge, compared with 3.30 strikes per hour when there was intermittent or continuous discharge of factory waste. Corresponding to the reduction in warp strikes when there was reduced discharge, there was also a reduction in warp captures when discharge was low. The large bird warp capture rate when no discharge was recorded during the warp observations was 0.2 birds per 100 tows, compared with an average capture rate of over 6.7 birds per 100 tows when offal or discards were discharged.

For every large bird that was reported by the observers as being captured on the warps, there were an estimated 244 (95% c.i.: 190 to 330) large bird warp strikes. For every small bird reported by the observers as being captured on the warps, there were an estimated 6440 (95% bootstrap c.i.: 3400 to 20000) small bird warp strikes. It is likely that some birds are killed by warp interactions, but not brought on board the vessels. Currently, estimates of seabird mortality in New Zealand fisheries are based solely on landed captures and the true mortality from trawl fishing is likely to be underestimated. An understanding of the fate of birds that have struck the warps is needed before the true seabird mortality in trawl fisheries can be assessed.