Changes in foraging range of Audouin's Gulls Larus audouinii in relation to a trawler moratorium in the western Mediterranean

The effect of a trawler moratorium on the foraging range of Audouin's Gull Larus audouinii was studied in the colony of the Ebro Delta (northwest Mediterranean). Trawlers were excluded from an area within a 110 km radius from the colony. In 1990, the year before the establishment of the moratorium, few gulls were observed from a land-based observation point outside the moratorium zone, while in 1991 and 1992 numbers of gulls significantly increased. Although most of the gulls probably were nonbreeders, breeding gulls were also able to travel long distances to seek food from trawler discharge. Increased foraging range may result in an increased cost of reproduction.