TROPHIC NICHE WIDTH AND OVERLAP OF 2 SYMPATRIC GULLS IN THE SOUTHWESTERN MEDITERRANEAN

The diets of two potential competitor species, Audouin's Larus audouin ii and yellow-legged gulls Larus cachinnans, were examined while they bred at the Chafarinas Islands during 1993, 1994, and 1995. Data were collected during two commercial fishing regimes: (1) trawling and purs e seine fisheries, and (2) diurnal trawlers only. Since the food suppl y for the gulls in this area was heavily reliant on the activity of pu rse seine fisheries, these contrasting situations allowed us to analyz e short-term effects, induced by daily changes in food supply, on nich e width, dietary shift, and niche overlap between the two species. Ove rall, both species relied mainly on fish for food, especially Clupeifo rms, in the case of Audouin's gull irrespective of the fishing situati on, and in the case of the yellow-legged gull, only when purse seine f ishing was in operation. When purse seine boats did not operate (food shortage), yellow-legged gulls broadened their niche, consuming equal amounts of all the feeding resources, and they showed a dietary shift toward a greater consumption of prey from refuse tips. In contrast, Au douin's gulls did not change their niche width, but showed a slight di etary shift away from the consumption of epipelagic fish, compensated by an increase in reliance on benthic-mesopelagic resources. Niche ove rlap was clearly higher on days when both fishing fleets operated, pro bably because a superabundant food resource facilitates high overlap w ithout affecting coexistence between the two species. Since our study was developed on the basis of daily variations in food supply, and com petition effects are to be expected on a longer-term basis, these chan ges can be seen as the outcome of the coexistence of two species in st able competitive equilibrium.