EXPLOITATION OF TRAWLER DISCARDS BY BREEDING SEABIRDS IN THE NORTH-WESTERN MEDITERRANEAN - DIFFERENCES BETWEEN THE EBRO DELTA AND THE BALEARIC-ISLANDS AREAS

Local trawler fisheries operate around the seabird colonies of the Ebr o Delta and Majorca. The use made of the discards produced by this fis hery by scavenging breeding seabirds was examined from 1992 to 1996. A ll breeding species at each site (except little terns at the Ebro Delt a) were observed following trawlers while fishermen were discarding fi sh: Audouin's, yellow-legged, lesser black-backed, black-headed and sl ender-billed gulls, common and Sandwich terns in the Ebro Delta area; and Audouin's and yellow-legged gulls, Gory's and Balearic shearwaters , storm-petrel and common shags in the Majorca area. Some non-breeding seabird species, such as gannets, skuas, and Chlydonias spp. terns we re also recorded in small numbers. Nevertheless, species diversity of the seabird community associated with trawlers was significantly highe r at the Ebro than at Majorca, because Procellariiformes, which breed only in the latter area, were displaced by large numbers of gulls. In Majorca, Audouin's gull was significantly more abundant than expected from the size of its breeding population, and in the Ebro area the sam e was true for the lesser black-backed gull, the black-headed gull, an d the common tern. However, the success rates of feeding on trawler di scards were not related to species' Presence Indices. In the Ebro area , Audouin's gull took greatest advantage of discards, whereas in Major ca only the yellow-legged gull consumed significantly higher amounts o f discards than expected from its Presence Index. The consumption rate of fish discards was significantly higher in the Ebro area (72%) than in the Majorca area (64%). Although the size of fish consumed was ass ociated with seabird body mass, most of the discard items were of a su itably small size. Crude estimates of the weight of fish discarded thr oughout a breeding season suggest that seabirds, especially Audouin's gulls at the Ebro Delta, may obtain a substantial part of their energy demands from this fishery, while discard availability at the Balearic Archipelago does not support the energy requirements of seabirds bree ding there. (C) 1997 International Council for the Exploration of the Sea.