Diet and adult time budgets of Audouin's Gull Larus audouinii in response to changes in commercial fisheries

Abstract

The diet and feeding ecology of breeding Audouin's Gull Larus audouinii were examined at the Ebro Delta (NW Mediterranean) during 1993, 1994 and 1995 and found to depend on the activity of the commercial fisheries operating within the foraging range of the birds in the colony. One of the largest fishing fleets in the Mediterranean operates in this afea, both diurnal inshore trawlers and nocturnal purse-seine boats. Since 1991, a trawler moratorium has coincided with the breeding season of the gull, which has affected its feeding ecology in the afea. Data were collected under four commercial fishing regimes: diurnal trawling only, diurnal trawling and purse-seine fishing at night, night fishing only, and no fishing. Although Audouin's Gull usually feeds mainly on epipelagic fish (65% by biomass on average), in our study they depended largely on trawler discards (benthonic fish represented up to 73% by biomass when only trawlers operated) because they are a food source with a high energetic value and are predictable in space and time. The active capture of clupeiform fish significantly increased when trawlers were not operating, and the gull has also broadened its foraging niche to feed in rice fields, in ecotonic habitats and occasionally on reCuse tips, suggesting that the clupeiform population was not large enough to meet the food demands of the colony. The presence of some discard prey in the diet when trawlers were not operating indicated that some breeding gulls were able to travel beyond the afea affected by the moratorium (more than 110 km from the colony). The frequency of incubation changeovers did not change significantly when trawlers were not operating, but marked changes brought about by the moratorium were recorded during the chick-rearing stage. Chick feeding frequency significantly decreased during the trawler moratorium, although the number of prey per regurgitate delivered to chicks did not vary. When trawlers did not operate, adults seemed to compensate for the lower food availability by carrying larger prey items to the chicks. In contrast, chicks occasionally did not accept !' the regurgitated food, especially when trawler discards were available. The trawler moratorium affected not only the diet of Audouin's Gull but also the adult time budgets and the provisioning rates of chicks, although the species showed some ability to buffer against low food availabiUty.