Seabird mortality on longline fisheries in the western Mediterranean: factors affecting bycatch and proposed mitigating measures
Seabird mortality on longlines in the Western Mediterranean around Columbretes Islands (Spain) was studied by observers in 1998-1999, and ringing recoveries obtained from 1992 to 1998. Average numbers varied between 0.16 and 0.69 birds/1000 hooks set, and incidetal mortality affected seven different seabird species. Bycatch occurred mainly around sunrise and from mid afternoon until dusk. This was related to seabird abundance but not with fishing effort (number of hooks set). About 656-2829 birds were estimated to be killed annually, of which 66% were Cory's shearwaters (Calonectris diomedea). In this species mortality was mainly among adult birds and affected reproductive success. Taking all this into account, bycatch on longlines may be a risk for the conservation of the populations of Cory's shearwater in the area. The results show that setting of longlines at night, and from one hour after sunrise until mid afternoon may reduce seabird bycatch in the area.