DIET AND ADULT TIME BUDGETS OF AUDOUINS GULL LARUS-AUDOUINII IN RESPONSE TO CHANGES IN COMMERCIAL FISHERIES

The diet and feeding ecology of breeding Audouin's Gull Larus audouini i were examined at the Ebro Delta (NW Mediterranean) during 1993, 1994 and 1995 and found to depend on the activity of the commercial fisher ies operating within the foraging range of the birds in the colony, On e of the largest fishing fleets in the Mediterranean operates in this area, both diurnal inshore trawlers and nocturnal purse-seine boats, S ince 1991, a trawler moratorium has coincided with the breeding season of the gull, which has affected its feeding ecology in the area, Data were collected under four commercial fishing regimes: diurnal trawlin g only, diurnal trawling and purse-seine fishing at night, night fishi ng only, and no fishing. Although Audouin's Gull usually feeds mainly on epipelagic fish (65% by biomass on average), in our study they depe nded largely on trawler discards (benthonic fish represented up to 73% by biomass when only trawlers operated) because they are a food sourc e with a high energetic value and are predictable in space and time. T he active capture of clupeiform fish significantly increased when traw lers were not operating, and the gull has also broadened its foraging niche to feed in rice fields, in ecotonic habitats and occasionally on refuse tips, suggesting that the clupeiform population was not large enough to meet the food demands of the colony, The presence of some di scard prey in the diet when trawlers were not operating indicated that some breeding gulls were able to travel beyond the area affected by t he moratorium (more than 110 km from the colony), The frequency of inc ubation changeovers did not change significantly when trawlers were no t operating, but marked changes brought about by the moratorium were r ecorded during the chick-rearing stage, Chick feeding frequency signif icantly decreased during the trawler moratorium, although the number o f prey per regurgitate delivered to chicks did not vary, When trawlers did not operate, adults seemed to compensate for the lower food avail ability by carrying larger prey items to the chicks, In contrast, chic ks occasionally did not accept the regurgitated food, especially when trawler discards were available, The trawler moratorium affected not o nly the diet of Audouin's Gull but also the adult time budgets and the provisioning rates of chicks, although the species showed some abilit y to buffer against low food availability.