Action: Reduce seabird bycatch by releasing offal overboard when setting longlines
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Two replicated and controlled studies in the South Atlantic and sub-Antarctic Indian Ocean found significant reductions in the number of albatross and petrels attacking baits and being caught when offal was released overboard during line setting.
Many fishing boats prepare fish onboard, after catching them, in order to maximise the catch that can be stored. The offal (waste) is then normally discarded overboard. Moreno et al. 1996 describe the highest bycatch rates of a series of voyages coming from a vessel that piped offal overboard on the same side that the line was being set. However, other studies suggest that piping offal overboard from a different point to where the lines are being set will reduce bycatch, as birds spend their time eating the offal, rather than attempting to take bait.
Moreno, C.A., Rubilar, P.S., Marschoff, E. & Benzaquen, L. (1996) Factors affecting the incidental mortality of seabirds in the Dissostichus eleginoides fishery in the southwest Atlantic (subarea 48.3, 1995 season). CCAMLR Science, 3, 79–91.
Supporting evidence from individual studies
A replicated and controlled study in the South Atlantic in February 1994 (Cherel et al. 1996) found that piping offal overboard whilst setting longlines greatly reduced the number of birds caught on 69 line sets, from 33 birds to three, equivalent to a decrease from 0.49 to 0.01 birds/1000 hooks. Caught birds were white-chinned petrels Procellaria aequinoctialis, and two grey-headed albatross Thalassarche chrysostoma (formerly Diomedea chrysostoma). There was a corresponding significant decrease in the rate of bird attacks on the bait for all species except wandering albatross D. exulans (92% decrease for black-browed albatross T. melanophris (formerly D. melanophris), 96% for grey-headed albatross and 96% for white-chinned petrel). Vessels were fishing for Patagonian toothfish Dissostichus eleginoides around South Georgia and Kerguelen Islands.
A replicated, controlled study in the sub-Antarctic Indian Ocean in 1994-7 (Weimerskirch et al. 2000) found that significantly fewer white-chinned petrels Procellaria aequinoctialis were caught on longlines when offal was released over the side of the boat as lines were being set (0.46 birds/1,000 hooks caught when offal was released vs. 1.00 birds/1,000 hooks with no release, total of 524 lines studied). There were no significant changes in other species caught, possibly due to smaller sample sizes. The authors caution that significantly more birds followed fishing boats when they released offal (significant increases for six of ten species) and the practice may therefore enforce long-term associations between fishing boats and food. This study is also discussed in ‘Set longlines at night to reduce seabird bycatch’ and ‘Use streamer lines to reduce seabird bycatch on longlines’.
- Cherel Y., Weimerskirch H. & Duhamel G. (1996) Interactions between longline vessels and seabirds in Kerguelen waters and a method to reduce seabird mortality. Biological Conservation, 75, 63-70
- Weimerskirch H., Candeville D. & Duhamel G. (2000) Factors affecting the number and mortality of seabirds attending trawlers and long-liners in the Kerguelen area. Polar Biology, 23, 236-249