DIFFERENCES IN DIET, POPULATION-SIZE AND REPRODUCTIVE-PERFORMANCE BETWEEN 2 COLONIES OF AUDOUINS GULL LARUS-AUDOUINII AFFECTED BY A TRAWLING MORATORIUM

Since 1991, a trawling moratorium has been enforced in the provinces o f Tarragona and Castello, NE Spain, where two Audouin's Gull Larus aud ouinii colonies are located 80 km apart: the Ebro Delta (on the mainla nd) and the Columbretes Is. (60 km off the coast). While the Columbret es Is. colony was established in 1973 and has grown only slightly (ca. 600 pairs in 1995), the Ebro Delta colony was established in 1981 and today holds 70% of total world population (ca. 10300 pairs in 1995). The trawling moratorium has coincided with the breeding season of the species, though the particular breeding stage affected was different e ach year. The lack of trawl discards caused by the moratorium and the likely decrease of clupeiform populations have affected both the feedi ng and breeding biology of the species. However, the effects of a redu ced 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 morato rium started, significant differences were found at both taxonomic and typologic levels. Prey from rice fields acted as a buffer for the Ebr o Delta colony, whereas in the Columbretes Is. the gulls were forced t o fish actively, probably at greater energy costs. In the moratorium y ears, breeding success was always significantly lower in the Columbret es Is. than in the Ebro Delta; extremely, no young fledged in the Colu mbretes Is. in 1993. Clutch size was also significantly lower in the C olumbretes Is. colony, even when the moratorium did not coincide with the laying stage, suggesting that predation by Yellow-legged Gull Lane s 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-dependen t effects may not operate in the Ebro Delta colony because of the larg e 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 facto rs until the establishment of the trawling moratorium, the size of the Columbretes Is. colony indicated a breeding site limitation, and prob ably competition with the Yellow-legged Gull.