Improving tori line performance in small-vessel longline fisheries

Tori lines are one of the most thoroughly tested seabird bycatch mitigation measures available, and have been proven effective in reducing seabird bycatch. The objective of this project was to develop improved tori lines which are specifically optimised for safe and effective use on small longline vessels. We conducted trials on land and on four different smaller longline vessels at sea. Tori line designs were tested at a range of vessel speeds to emulate the setting speeds used across the smaller-vessel longline fisheries of interest. Our primary measure of effectiveness was aerial extent, and tests confirmed that a number of different tori line designs delivered satisfactory aerial extents (of 70 m or more). Predictably, increasing tori line deployment height and vessel speed increased the aerial extents delivered. The most challenging component of the tori line design to refine was the in-water section, required to provide drag. However, as a broad rule-of-thumb, where the in-water section of a tori line delivers 15 kg of drag, satisfactory aerial extent should be achievable. This has important implications for small-vessel tori line specifications, such as those provided in CMM 2015-03 to mitigate the impact of fishing for highly migratory fish stocks on seabirds.