At-sea experiment to evaluate the effectiveness of multiple mitigation measures on pelagic longline operations in western north Pacific
There is wide consensus that experimental research is essential to the introduction of effective mitigation measures that reduce the incidental mortality (bycatch) of seabirds in commercial fisheries. Recent research in the South African EEZ revealed that the simultaneous deployment of multiple bycatch mitigation measures was highly effective at reducing seabird bycatch. In order to address the lack of similar studies in the North Pacific, we compared the effectiveness of hybrid tori lines (with short and long streamers) with and without weighted branch lines to a control of no mitigation. Weighted branch lines design was based on the Yamazaki double-weight configuration reported effective and safe in the tuna longline of the South African EEZ. We carried out 62 longline operations (59,520 hooks) in the Western North Pacific from December 2011 to June 2012. Each operation (set) of 960 hooks was divided into six experimental treatments: single, double or no tori lines coupled with weighted or unweighted branch lines. Metrics of evaluation were the attack rates of seabirds on baited hooks and the bycatch rates of the dominant and most vulnerable seabirds to longline mortality in the North Pacific – Laysan and black-footed albatrosses. Three albatross and four shearwater species attended the vessel during line setting and 107 Laysan and 21 black-footed albatrosses were recorded as catch. Both single and double tori lines eliminated most bird attacks within 75 m of the stern, resulting in a dramatic reduction in seabird mortality rates (96-100%, for single and double tori lines, respectively) compared to non-tori line treatments, irrespective of branch line weighting. Weighted branch lines without tori lines also reduced mortality rates, but less effectively than tori line treatments (60-68% reduction). These results suggest that, unlike the Southern Hemisphere, deployment of a single mitigation measure – well-designed tori lines – dramatically reduce albatross bycatch in the pelagic longline fisheries in the Western North Pacific, and therefore are recommended as best-practice seabird mitigation for these fisheries.