Preliminary Report of 2010 Weighted Branchline Trials in the Tuna Joint Venture Fishery in the South African EEZ
The lack of comprehensive research developing and comparing seabird bycatch mitigation technologies appropriate to pelagic longline fisheries has led to considerable debate regarding best-practice mitigation to prevent seabird mortality among tuna commission and their member countries. Our research in the South African tuna joint venture fishery in 2009 obviated the need to shrink the area astern of the vessel that birds have access to baited hooks via weighted branchlines to force seabird interactions into an area that can be successfully defended with streamer lines - shrink and defend.
Taking this philosophy further, in 2010 we compared the performance of a revised "hybrid" streamer lines deployed with weighted (W) and un-weighted (UW) branchlines on two Japanese vessels participating in the 2010 tuna joint venture fishery in the South Africa EEZ. Seventeen birds species attended the vessel during line setting, but only four made primary attacks on baits and were killed. White-chinned petrels were the most abundant bird; they were present during all sets, attacked at the highest rate and were the bird most killed. Albatross attack rates were near two orders of magnitude lower than that of white-chinned petrels, but eight were killed suggesting strongly that secondary attacks - birds stealing baits from birds having made a primary attack - drove albatross mortality. Twenty-four of the 27 bird mortalities occurred after nautical dawn. All three birds caught at night were on UW lines. Weighting branchlines with hybrid streamer lines dramatically reduced seabird attacks, secondary attacks and seabird mortalities with little effect on fish catch. Four of 27 bird mortalities (2 white-chinned petrels, 1 shy albatross, and 1 cape gannet) were on W branchlines - a reduction in seabird bycatch rate of 86 % compared to UW (UW = 0.290 and W = 0.040 birds/1,000 hook). Mean tuna catch was near equal on the two branchline types, but W branchlines tangled on themselves three times more often than UW branchlines. No crew injuries occurred from either branchline type.
These preliminary results indicate that the shrink and defend conceptual framework of seabird bycatch mitigation is effective at reducing seabird interactions with pelagic longline fishing gear. Specifically, these results strongly suggest that two hybrid streamer lines together with weighted branchlines and night setting constitute best-practice seabird bycatch mitigation for the joint venture fleet operating in the South Africa EEZ and other white-chinned petrel dominated fishing areas. These results also suggest that the Column A and Column B mitigation approach adopted by WCPFC (CMM 2007-04) and IOTC (Resolution 10/06), as currently written, would not prompt the simultaneous use of two hybrid streamer lines, branchline weighting and night setting, and therefore, falls short of the best-practice mitigation identified in this study.