Significance of fisheries discards for a threatened Mediterranean seabird, the Balearic shearwater Puffinus mauretanicus

The Balearic shearwater Puffinus mauretanicus is the rarest and most threatened Mediterranean seabird, The biology of this shearwater is poorly understood, and its study is important to design conservation strategies. We studied the feeding ecology of the Balearic shearwater at sea in the western Mediterranean (1996 to 2000), focusing on the importance of fisheries discards for this species. Fieldwork was conducted on board commercial bottom trawlers (demersal fishery with diurnal activity) and purse seiners (pelagic fishery with nocturnal activity), as well as during experimental trawling surveys. The shearwaters made extensive use of discards, mostly those from trawlers, This was especially so during the late breeding season, which could be related to the general impoverishment of Mediterranean surface waters. At this time of year, most birds foraged along the eastern Iberian coast, with the largest concentrations occurring off the Ebro Delta, This distribution seems determined by favourable local hydrographic conditions and by the presence of important trawling fleets. Balearic shearwaters captured discards by diving at some distance behind fishing vessels, thus reducing interactions with other seabirds. A bioenergetic model estimated that 40.8% of the energy obtained by the Balearic shearwater population comes from trawler discards during the breeding season (March to June), although this value was subject to strong variability (+/-36.2% SD). In addition to the capture of discards (38% of the feeding instances observed), Balearic shearwaters also obtained food by capturing fish under floating drifting objects (33%), associating with sub-surface predators (10%), capturing small shoaling fish (10%), and feeding upon plankton (10%), The latter behaviour was observed in crepuscular hours, but the shearwaters did not appear to feed at night. In winter, Balearic shearwaters attended fishing vessels to a lesser extent. Upcoming fishing policies could affect Balearic shearwaters in the short term through reduction of discards, although good design of management strategies (such as trawling moratoria) could help to reduce their negative effects.