Abstract
To improve the effectiveness of tori-lines it is necessary to evaluate the ability of tori-lines to mitigate seabird bycatch and determine what kind of seabird species gather during line settings, attack the bait and are incidentally caught. We conducted two experiments in the western North Pacific and examined the effectiveness for seabird mitigation of light streamer tori-lines which have no long streamers but many light (short) streamers and are mainly used in the North Pacific area. Firstly, the effectiveness of two different types of tori-line (light streamer (1 m) and long streamer (up to 7 m) tori-line) and of two different colors (yellow and red) of light streamers for seabird bycatch avoidance was evaluated using 567 sets based on data from 20 offshore surface commercial longliners. No significant difference in the bycatch number between the different tori-line types and streamer colors was found. Secondly, we investigated the characteristics of the seabird bycatch in the North Pacific and the effectiveness of three different types of streamers (light, hybrid and modified light types) by detailed observations of seabird attacks using a chartered longline vessel. Although the appearance rate of albatrosses and shearwaters were 40.9% and 27.7%, Laysan albatross was the main seabird species that followed the vessel but shearwaters seldom followed the vessel and did not aggregate during line setting. In all attacks on bait observed during line settings, 81% and 7% were by albatrosses and shearwaters, respectively. In the number of primary attacks by Laysan albatrosses which attacked most aggressively of all seabirds, there were no significant differences among the tori-line types. No individuals of shearwater were caught. The results of both experiments indicated that light streamer tori-lines were as effective as tori-lines with long streamers for mitigating seabird bycatch in the North Pacific.
Highlights
Most seabird bycatches in pelagic longline fisheries occur when seabirds attack the bait thrown into the sea before the bait sinks to the fishing depth [1]
Seabird bycatch by tuna longline fisheries has been shown to cause a negative impact on some seabird populations [1,2], and development of effective mitigation measures is needed which will have an effect of improving the efficiency of the fishery by reducing bait loss
The operations were a night soak style: most line settings were started in the afternoon and completed around a few hours after sunset, about 10% of all settings were started after sunset
Summary
Most seabird bycatches in pelagic longline fisheries occur when seabirds attack the bait thrown into the sea before the bait sinks to the fishing depth [1]. This mitigation method deters seabirds from approaching the vessels due to the motion of the streamers, and allows the bait to sink to a sufficient depth where seabirds cannot attack the bait [1] This mitigation measure has a high effectiveness to reduce the bycatch in both pelagic and demersal longline fisheries [1,2,4,5,6], and is used by many longline vessels as the most practical measure because it is cost effective and it does not require significant changes to the fishing gear or vessel to use it [6,7]
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