Effects of trawler discard availability on egg laying and breeding success in the lesser black-backed gull Larus fuscus in the western Mediterranean

The availabil~ty of trawler discards is hkely to have a pronounced effect on the reproductlon of seabirds, but this importance IS normally difficult to quant~fy.A comparison between 2 breeding seasons of the lesser black-backed gull, one of them affected by a trawl~ng moratorium, has allowed an assessment of the effects of discard availability on its e g g 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 nce flelds.Although neither b r e e d ~n g phenology was different nor was clutch size s~gnif~cantly lower in the year of the trawling moratorium, the volume of the eqgs decreased significantly, showing the ~mportance of trawler discard availabil~ty as a proximate determinant of egg production of scavenging seabirds Since the moratonum overlapped with the c h ~c k rearing stages in both years, breeding success \\.as probably lotver than before the estabhshment of the trawling mol-at o r ~u m In 1991 However, the breeding success was higher in 1994 than In 1995, when the moratorium overlapped i v ~t h the entire period of chlck growth.Population dynamics of the species is probably affected by changes in trawler discard availab~lity, especially if a trawling moratorium continues for many years KEY : Trawler discards Food availabil~ty -Moratorium Lesser black-backed gull Egg-laying Population dynamics .