Tori line 8 showing the large road cone comprising the in-water section and the splash it created at 4 knots. The rope used to measure aerial extent is visible on the left.
Tori lines are one of the most thoroughly tested seabird bycatch reduction measures available, and have been proven effective in reducing seabird bycatch in both trawl and longline fisheries.
Despite the efficacy of this mitigation measure, there is ongoing controversy around the benefits of tori line usage in some New Zealand fisheries, particularly amongst the operators of smaller longline vessels. 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, to explore tori line designs and materials appropriate for use during bottom and surface longline fishing methods. Tori line designs were tested at a range of vessel speeds (from 2.7 – 7 knots) to emulate the setting speeds used across the smaller-vessel longline fisheries of interest.
Tests confirmed that a number of different tori line designs delivered aerial extents of 70 m (our key performance criterion). 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, an aerial extent of 70 m should be achievable.
We provide recommendations on tori line designs to be tested in a broader range of weather conditions than was encountered during our trials and under real fishing conditions. We also recommend tori line use as part of mitigation strategies for small-vessel longliners that include effective line-weighting.