Differences in Diet, Population Size and Reproductive Performance between Two Colonies of Audouin's Gull Larus audouinii Affected by a Trawling Moratorium

Abstract

discards caused by the moratorium and the likely decrease of clupeiform populations have affected both the feeding and breeding biology of the species. However, the effects of a reduced food supply were greater in the Columbretes Is., probably because of the lack of alternative food resources. When trawlers operated, the gulls' diet was very similar in the two colonies, but when the moratorium started, significant differences were found at both taxonomic and typologic levels. Prey from rice fields acted as a buffer for the Ebro Delta colony, whereas in the Columbretes Is. the gulls were forced to fish actively, probably at greater energy costs. In the moratorium years, breeding success was always significantly lower in the Columbretes Is. than in the Ebro Delta; extremely, no young fledged in the Columbretes Is. in 1993. Clutch size was also significantly lower in the Columbretes Is. colony, even when the moratorium did not coincide with the laying stage, suggesting that predation by Yellow-legged Gull Larus cachinnans could be an additional limiting factor at this locality, where the ratio of breeding pairs of Yellow-legged to Audouin's Gulls is much higher than in the Ebro Delta. In contrast to previous studies, the larger colony (Ebro Delta) had better breeding performance than the smaller colony (Columbretes Is.), suggesting that density-dependent effects may not operate in the Ebro Delta colony because of the large variety of foraging habitats (rice fields, lagoons, dunes, beaches) and the high productivity of the sea off this colony. While population size in the Ebro Delta colony suggested the absence of limiting factors until the establishment of the trawling moratorium, the size of the Columbretes Is. colony indicated a breeding site limitation, and probably competition with the Yellow-legged Gull.