Shrink and defend: A comparison of two streamer line designs in the 2009 South Africa Tuna Fishery

Although pelagic longline tuna fisheries managed by international agreements constitute one of the greatest conservation threats to seabirds of the southern oceans, best mitigation practices for these fisheries, including the best streamer lines design, are the subject of considerable debate. We compared the performance of a "light" streamer line with short streamers to a "hybrid" streamer line that mixes long streamers with short streamers using seabird attack rates in the Japanese joint venture tuna fishery in the South Africa EEZ. We also determine the sink rates of weighted (60 g placed 70 cm from the hook) and unweighted branchlines (status quo) to inform the distance astern that birds have access to baited hooks. Most primary attacks were made by white-chinned petrels and most of those occurred beyond 100 m astern, the target aerial extent of tori lines. A third of primary attacks led to secondary attacks by albatrosses. Virtually all albatross attacks occurred within 100 m. Fewer birds attacked baits inside 100 m when hybrid lines were used, but mean rates were not statistically significant for divers or surface foragers. Unweighted branchlines sank beyond the reach of diving seabirds (10 m) more than 3 times (307 m) further from the stern than did weighted branchlines (~ 100 m). These data strongly suggest that in order to defend baited hooks from bird depredation in a white-chinned petrel dominated system with streamer lines, the distance at which baits sink to 10 m must be reduced to within an achievable streamer line aerial extent.

Note: also found among the meeting documents of the Third Meeting of the Seabird Bycatch Working Group (ACAP). SBWG-3 Doc 13