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Summary
Although the white sharks (Carcharodon carcharias) have been protected in New Zealand since 2007, but they continue to be caught as bycath in commercial and recreational fisheries. This study was aimed to review and characterise commercial set net bycatch in order to assess fishery impacts on white sharks, and develop mitigation solutions.
Due to the limited number of bycatch events in the Observer data, the analyses were done on 53 white sharks reported by fishers on Non Fish and Protected Species (NFPS) forms since 2008, 36 of those caught in set nets.
Three regions, Great Exhibition Bay (GEB), Taranaki (TAR) and Foveaux Strait (FOV), accounted for 89% of the 36 white shark set net captures, but only 20% of the length of net set.
The main target set net fisheries responsible for catching white sharks were different in all three regions, indicating that target-specific features of the fishing operations were unlikely to have been responsible for high catches. Similarly, seasonality of the fisheries was not an important factor in GEB and TAR, although all FOV sharks were caught in summer-autumn.
Comparisons were made of set net gear parameters among regions, target species, vessels and shark- or non-shark sets. Higher nets tended to catch more sharks in all regions, but sharks were caught across a range of mesh sizes, net lengths and set durations. Spatial factors were important in GEB and FOV, indicating that fishing location may be an important factor driving white shark bycatch.
White sharks occur throughout New Zealand's coastal waters, and they are susceptible to capture by set nets. Bycatch could be reduced by identifying important hotspots of abundance and reducing or ceasing set net fishing in those areas at appropriate times of year.
Restrictions on set netting in the Foveaux Strait–Stewart Island region during summer–autumn would greatly reduce white shark bycatch, as would closure to set netting of some other key white shark habitats. Reduction of set net height in key fisheries could also reduce bycatch.
Overall, 69% of sharks reported on NFPS forms were judged by fishers to be alive and in good condition, but the survival of live sharks after release is unknown. A post-release mortality (PRM) experiment using electronic Survival Popup Archival Tags (sPATs) would be necessary to determine the mortality rate of white sharks released alive from set nets.
Publication information
Francis, M. 2017. Bycatch of white sharks in commercial set nets. Report prepared by NIWA for the New Zealand Department of Conservation, Wellington. 27p.