Effectiveness of tori line use to reduce seabird bycatch in the Uruguayan pelagic longline fleet and modifications to improve its performance
Industrial longline fisheries cause the death of large numbers of seabirds annually, with consequent serious impacts on many populations. Various mitigation measures have been proposed, including the use of tori lines. This measure has been successfully applied in demersal fisheries; however, the effectiveness of this measure has been poorly demonstrated in pelagic longline fisheries. In this work the efficiency of a single tori line to reduce seabird bycatch was tested in the Uruguayan pelagic longline fleet. Thirteen trips were carried out on longline vessels in the area and season of high bycatch rates recorded in the SW Atlantic during 2009-2011. Based on a randomized order we employed two different longline set treatments: sets with a mix tori line (with long and short streamers) and sets without tori line (control treatment). The tori line was set on the leeward side of the mainline and towed from a height of 6m from sea level and a horizontal distance of 5m (range 4-6m) from the setting station.
Forty three birds were captured in the control treatment (n=49 sets; 50,613 hooks), while seven captures were recorded in the tori line treatment (n=51 sets; 52,371 hooks). However, in a high proportion (47%) of these later sets, the tori line broke either because entanglement with the longline gear or by tension. Five of the seven birds occurring in the tori line treatment were caught in those sets. The mean aerial coverage of the tori line was 72 m, and after ruptures caused by entanglements was 47 m. The use of tori line showed a significant decrease in the seabird bycatch.
In a second phase of the experiment, four additional trips were conducted during 2012 and only sets (n=26) with a tori line with few modifications on design and setting operation were conducted. Only two entanglements were recorded, reducing the rupture rate of the tori line to 7.7%. This work shows that tori line use reduce seabird bycatch in pelagic longline fisheries. Further studies should focusing on improve its performance and test its combined effect with other mitigation measures.
Recommendations 1. Since the Uruguayan waters are renowned for high seabird-longline interactions, the tori line presented here should be useful to reduce seabird mortality in other regions of the southern hemisphere.
2. Based on data presented here, tori lines and night setting should be used in combination to reduce seabird mortality. These measures should be complemented by other measures, particularly during the first quarter and full moon phases, and line weighting.