Trophic niche width and overlap of two sympatric gulls in the southwestern mediterranean

Abstract

The diets of two potential competitor species, Audouin's Larus audouinii 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 purse seine fisheries, and (2) diurnal trawlers only. Since the food supply for the gulls in this area was heavily reliant on the activity of purse seine fisheries, these contrasting situations allowed us to analyze short-term effects, induced by daily changes in food supply, on niche width, dietary shift, and niche overlap between the two species. Overall, both species relied mainly on fish for food, especially Clupeiforms, in the case of Audouin's gull irrespective of the fishing situation, and in the case of the yellow-legged gull, only when purse seine fishing 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, Audouin's gulls did not change their niche width, but showed a slight dietary shift away from the consumption of epipelagic fish, compensated by an increase in reliance on benthic-mesopelagic resources. Niche overlap was clearly higher on days when both fishing fleets operated, probably because a superabundant food resource facilitates high overlap without affecting coexistence between the two species. Since our study was developed on the basis of daily variations in food supply, and competition effects are to be expected on a longer-term basis, these changes can be seen as the outcome of the coexistence of two species in stable competitive equilibrium.

Impact of fisheries on activity, diet and predatory interactions between yellow-legged and Audouin's gulls breeding at the Chafarinas Islands

“…Diet composition assessed from chick regurgitates differed substantially between the two gull species on the days when purse-seine fishery did not operate (González-Solís et al, 1997a). In such days, yellow-legged gulls fed mainly on waste food from refuse tips, whereas Audouin's gulls fed chiefly on epipelagic fish.…”

“…Prey categories were established according to an ecological typology and diet composition expressed in percentage of dry weight biomass. Typological groups were based on prey characteristics, which involved different hunting behaviour, predation effort or foraging habitats, resulting in four categories: (1) epipelagic preys caught in their natural environment or in association with purse-seine fisheries; (2) benthic-mesopelagic preys obtained from trawler discards; (3) terrestrial preys obtained on land; and (4) preys obtained at refuse tips (see details in González-Solís et al, 1997a).…”

“…Fishery activities, particularly trawling, have been shown to affect diet, activity patterns and reproductive parameters of Audouin's gulls at the Ebro Delta (Oro, 1995;Oro et al, 1996a;Oro et al, 1996b;Oro et al, 1997). The yellow-legged gull has indicated that it is an opportunistic species relying heavily on human waste from refuse dumps and offal from commercial fisheries (Bosch et al, 1994;Bosch et al, 2000;González-Solís et al, 1997a;Munilla, 1997). A two-month trawling moratorium also affected diet and productivity of this species at the Ebro Delta (Oro et al, 1996b).…”

Effect of refuge supplement on Audouin's gull chick survival

“…Several studies on Audouin's gull and other Mediterranean seabirds have shown a relationship between food availability and fisheries activity near the colonies, which are usually well reflected in some breeding parameters and/or behavioural patterns (Bosch et al, 1994;Castilla and Jiménez, 1995;Oro, 1995;Oro et al, 1995;Oro et al, 1996a;Oro et al, 1996b;Arcos and Oro, 1996;González-Solís et al, 1997a;González-Solís et al, 1999). Although quantitative data on Moroccan fisheries and their catches near the Chafarinas Islands were not available, a lower activity of local fisheries due to bad weather conditions and an associated food-shortage period due to scarcity of discards were observed in 2000.…”

A comparison of two seabird communities on opposite coasts of the Alborán Sea (western Mediterranean)

“…Differences in the quantitative composition of the seabird communities in the two study areas may have been caused mainly by the asymmetrical availability of food on each side (Witt et al, 1981;González-Solís et al, 1997;Furness and Camphuysen, 1997). Important populations of seabird species essentially linked to the sea when foraging still exist on the southern coast.…”