Foraging Ecology of Seabirds at Sea: Significance of Commercial Fisheries in the NW Mediterranean.

Abstract

Seabirds feeding ecology was studied at sea to assess the significance of two commercial fisheries, trawlers (demersal fishery with diurnal activity) and purse seiners (pelagic fishery with nocturnal activity), in the NW Mediterranean. Trawlers offered high amounts of discards (mostly fish of small size) on a predictable basis, and consequently attracted large numbers of seabirds. At the Ebro Delta, discards provided in average 65% of the energy requirements of the seabird breeding community, being most important for large gulls and of little significance for terns. Purse seiners produced few discards, on an irregular basis, and attracted lower numbers of seabirds during discarding (after dawn). During the fishing operation, at night, only Audouin's Gulls Larus audouinii attended these vessels, capturing live fish concentrated near the sea surface, attracted by powerful lamps. During the breeding season, Audouin's Gulls preferentially attended trawlers in spite of potential competition with the more aggressive and opportunistic Yellow-legged Gulls Larus cachinnans, since the former showed to be more efficient at capturing discards and reduced competition by showing a wider range of feeding techniques. Only during trawler closures, Audouin's Gulls occurred in important numbers at purse seiners (thus acting as a secondary food resource for most individuals). During the non-breeding season Audouin's Gulls shifted to preferentially attend purse seiners, presumably because competition at trawlers strengthened at this period. The rare and threatened Balearic Shearwater Puffinus mauretanicus also made significant use of discards, which represented more than 40% of the energy requirements of the whole population during the breeding season. Seabirds feeding upon discards showed higher levels of mercury, as discards make accessible to them demersal fish that is more polluted than epipelagic fish (natural prey for most seabirds). Thus, the risks of toxicity by this metal are higher for those species capturing discards. Given the significance of commercial fisheries for seabirds in the NW Mediterranean, and considering the increasing ecosystem-based approach to fisheries management, fisheries managers should regard these birds when designing fishing policies.