ABSTRACT: The availability of trawler discards is likely to have a pronounced effect on the reproduction of seabirds, but this importance is normally difficult to quantify. A comparison between 2 breeding seasons of the lesser black-backed gull, one of them affected by a trawling moratorium, has allowed an assessment of the effects of discard availability on its egg laying. When trawlers operated, diet was dominated by fish from discards, while during the trawling moratorium gulls fed from refuse tips, olive tree fields and especially rice fields. Although neither breeding phenology was different nor was clutch size significantly lower in the year of the trawling moratorium, the volume of the eggs decreased significantly, showing the importance of trawler discard availability as a proximate determinant of egg production of scavenging seabirds. Since the moratorium overlapped with the chick rearing stages in both years, breeding success was probably lower than before the establishment of the trawling moratorium in 1991. However, the breeding success was higher in 1994 than in 1995, when the moratorium overlapped with the entire period of chick growth. Population dynamics of the species is probably affected by changes in trawler discard availability, especially if a trawling moratorium continues for many years.

KEY WORDS: Trawler discards . Food availability . Moratorium . Lesser black-backed gull . Egg-laying . Population dynamics . Reproduction

Published in MEPS Vol. 132 (1996) on February 29
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996