Tori-lines with weighted branch lines reduce seabird bycatch in eastern South Pacific longline fishery
The effectiveness of tori-lines combined with double-weighted branch lines in reducing seabird bycatch was evaluated in a pelagic longline fishery. Seabird bait attack behaviour, bycatch number, and sinking rate of baited hooks were examined on double-weighted and unweighted branch lines deployed on the same longline with a single tori-line. Comparisons were conducted from July to October in 2011 during two cruises on a chartered longline vessel in the eastern South Pacific Ocean outside the Chilean and Peruvian exclusive economic zones. Cape petrels (Daption capense), white-chinned petrels (Procellaria aequinoctialis) and Westland petrels (Procellaria westlandica) were abundant during line setting. There were 275 primary bait-taking attacks by these species, but there were only eight primary attacks by albatrosses. Nevertheless, six albatrosses and six diving seabirds were incidentally caught as bycatch. Of the primary attacks by Cape petrels, white-chinned petrels and Westland petrels, 153 led to secondary attacks. These results suggest that off Chile and Peru there is frequent secondary bycatch of albatrosses as a result of their stealing bait from Cape petrels and diving seabirds. Best-fit models for the number of primary attacks and of bycatch included the weighted branch line; the use of weighted branch lines resulted in a lower number of primary attacks. Hooks on unweighted branch lines did not reach any of the benchmark depths (3, 5, and 10 m) within the aerial extent of the tori-lines (the tori-line remaining above the water surface), hooks on weighted branch lines reached 5 m depth within the aerial extent. These results suggest that, for the pelagic longline fishery off Chile and Peru, combining double-weighted branch lines and tori-lines reduces the bycatch more effectively than tori-lines with unweighted branch lines.