Set longlines at night to reduce seabird bycatch
Overall effectiveness category Trade-off between benefit and harms
Number of studies: 8
Background information and definitions
Most seabirds feed during the day and so longlines set at night may catch fewer birds. Some species, however, do hunt at night and so night-setting is unlikely to be an effective mitigation measure. An adequate knowledge of bycatch species’ ecology is therefore important in determining whether to promote night-setting.
Many of the studies in this section do not explicitly say whether night-setting was done for conservation purposes or not. However, given the valuable information they contain, we have included them below.
Supporting evidence from individual studies
A study using data from bluefin tuna Thunnus thynnus boats operating off New Zealand between 1988 and 1992 (Murray et al. 1993) found that the effects of night-setting on seabird bycatch rates varied between fishing areas, probably due to different species making up the majority of bycatch. In southern areas, 87% of 47 identified birds caught were albatrosses and 73% of 88 birds were caught between 0600 and 1400 (when 41% of 1,009 lines were set). In contrast, in northern fishing grounds, where 59% of 75 identified birds were petrels, 44% of the 181 birds caught were hooked within 90 minutes of dawn or dusk and 42% were caught at night (when 54% of 1,180 lines were set).Study and other actions tested
A replicated and controlled study (Cherel et al. 1996) in the Patagonian toothfish Dissostichus eleginoides fishery in the South Atlantic found that 38 birds (two grey-headed albatross Thalassarche chrysostoma, formerly Diomedea chrysostoma, and 36 white-chinned petrels Procellaria aequinoctialis) caught on 72 longlines (174,000 hooks) set in February 1994, were caught at a much higher rate on lines set at night, than during the day (1.00 vs. 0.38 birds/1,000 hooks). The study took place around South Georgia (UK) and Kerguelen Islands (France) (sectors 332 and 333). This study is also discussed in ‘Turn decklights off during night-time setting of longlines to reduce bycatch’.Study and other actions tested
A replicated, controlled study using data from 86 longlining vessels operating in Australian waters, between April 1992 and March 1995 (Klaer & Polacheck 1998) found that longlines set at night caught approximately five times fewer seabirds than those set during the day (1.0 birds/1,000 hooks for 924 line-sets set at night vs. 4.8 birds/1,000 hooks for 1,372 line-sets set during the day). The difference was greatest on nights close to a new moon (with 7% of the bycatch rates of day sets), but there were always significant reductions (sets on full moon nights had approximately one-third the bycatch rates of day sets). This study does not discuss which birds were caught, but previous studies have shown that this fishery catches mainly albatross. This study is also discussed in ‘Use bait throwers to reduce seabird bycatch’ and ‘Thaw bait to reduce seabird bycatch’.Study and other actions tested
A replicated, controlled study in the sub-Antarctic Indian Ocean in 1994-7 (Weimerskirch et al. 2000) found that longlines set at night had significantly lower seabird bycatch, compared to those set during the day (0.91 birds caught/1,000 hooks for day-set lines vs. 0.17 birds/1,000 hooks for night-set lines, total of 524 lines studied). This result was consistent across all species, except for wandering albatross Diomedea exulans, which was only caught on 12 occasions. The authors note that whilst the number of white-chinned petrels Procellaria aequinoctialis caught was half that caught during the day, these levels may still be unsustainably high: they quote an unpublished figure of 340 petrels caught during a single line set off Kerguelen Island. The study took place on three Ukranian and one Japanese longliners fishing for Patagonian toothfish Dissostichus eleginoides off Kerguelen Island (France). Deck lights were not switched on during night setting. This study is also discussed in ‘Use streamer lines to reduce seabird bycatch on longlines’ and ‘Reduce seabird bycatch by releasing offal overboard when setting longlines’.Study and other actions tested
A replicated and controlled study (Melvin et al. 2001) in the North Pacific in late summer 1999 and 2000, found that lines set at night or during sunrise had higher rates of seabird bycatch (0.13 and 0.07 birds/1,000 hooks respectively) than those set during the day or at sunset (0.02 and 0.01 birds/1,000 hooks respectively). Differences were due to bycatch rates of northern fulmars Fulmarus glacialis (the most numerous species caught and the only species caught at night). Shearwaters Puffinus spp. constituted 67% of species caught during the day, but were never caught during the night. A total of 490 line sets were studied from the Pacific cod Gadus macrocephalus and walleye pollock Theragra chalcogramma fishery southeast of the Pribilof Islands, USA. This study is also discussed in ‘Weight baits or lines to reduce longline bycatch of seabirds’, ‘Use streamer lines to reduce seabird bycatch on longlines‘, ‘Use a line shooter to reduce seabird bycatch’ and ‘Set lines underwater to reduce seabird bycatch’.Study and other actions tested