Demersal trawling waste as a food source for Western Mediterranean seabirds during the summer

We Studied the use of demersal trawler discards by scavenging seabirds at one location in the western Mediterranean (Benidorm, SE Spain) from the end of June to the beginning of October 2000. Yellow-legged gull and Cory's shearwater were the most common species in the study area (52.35%, 35.49%) and behind boats (82.4%, 7.07%). Eight other seabird species were observed in much lower numbers following boats (Audouin's gull 2.6%, black-headed gull 2.3%, sandwich 1.5%, black 0.82% and common terns 0.97%, Balearic shearwaters 1.4%(, storm-petrel 0.52%) and shags 0.36%,). Yellow-legged gulls were present behind trawlers in higher numbers than might have been expected by its abundance at sea, whereas Cory's shearwaters were less so. The discarded fish (discard) comprised mainly of sardine (22.4%), flatfish ( 19.1%) and horse-mackerel (17.3%) and included fish of a small size (median 10.5 cm) in the main. Yellow-legged gull made the largest use of discards, albeit lower than expected by its Presence Index. Average percentage consumption was about 54% of the edible discards, suggesting that yellow-legged gulls were not highly efficient at catching discard. The average ratio of fish discarded over fish landed was ca. 65%, although the range was very variable (23-175%). Estimates of the energy requirements of yellow-legged gulls and energy availability from the ca. 8 tonnes of discard produced every fishing day, suggest that trawling waste was probably enough to support a local gull Population four times larger than that present during the study period. (C) 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.