Abstract
We analysed and compared the diet of Audouin´s gulls Larus audouinii between their two largest breeding sites in the world: the Ebro Delta and the Chafarinas Islands (western Mediterranean). These two localities showed marked differences in the features of the commercial fishing fleet: in the Ebro Delta area a large fishing fleet produced large amounts of discards, while in the Chafarinas the fleet discarded smaller amounts of fish and marine invertebrates, due to the smaller number of vessels. It is also likely that the percentage of discards from total catches is also lower around the Chafarinas than at the Ebro Delta. We distinguished two types of fishing to compare diet compositions: diurnal (only trawling activity) and diurnal and nocturnal (trawling and purse-seine activity, respectively). We also differentiated regurgitates from young nestlings (up to 20 days old) and from older nestlings or adult birds. At the two localities, fish was the main food of Audouin´s gulls, with epipelagic prey (mainly clupeoids) being more important when both diurnal and nocturnal fisheries were operating. This confirms that epipelagic prey either caught actively by the gulls or linked to fisheries was particularly important in the feeding habits of Audouin´s gulls. Nevertheless, differences between the two colonies appear mainly when only trawlers operated: while at the Ebro Delta gulls showed higher consumption of benthic-mesopelagic prey (probably linked to a higher trawler discard availability), gulls from the Chafarinas Islands consumed higher biomass of epipelagic prey probably caught actively at night. When both fleets operated around the two colonies, the average biomass of prey in a regurgitate of younger chicks was significantly higher at the Ebro Delta than at Chafarinas, and the opposite trend was recorded for older nestlings and adults. Niche width was broader in Chafarinas than in the Ebro Delta for both age classes and for any fishing fleet schedule, suggesting again that the exploitation of discards was higher at the Ebro Delta than at the Chafarinas, where gulls showed a more varied diet. Despite the fact that availability of discards was probably higher at the Ebro Delta than at Chafarinas, the per capita availability was not so different at both localities due to the increasing seabird community population at the Ebro Delta, which ca. doubled that at Chafarinas in the last decade.
Highlights
Audouin’s gull Larus audouinii is considered a specialist in the capture of clupeiforms during the night, though it is well known that the species scavenges at nocturnal and diurnal commercial fisheries (e.g.González-Solís et al, 1997a; Oro et al, 1997; Oro and Ruiz, 1997)
Niche width was broader in Chafarinas than in the Ebro Delta for both age classes and for any fishing fleet schedule, suggesting again that the exploitation of discards was higher at the Ebro Delta than at the Chafarinas, where gulls showed a more varied diet
Taxonomic and typologic diet composition according to fishing situations for both localities is described in Tables 1 and 2
Summary
Audouin’s gull Larus audouinii is considered a specialist in the capture of clupeiforms during the night (see Oro 1998 and references therein), though it is well known that the species scavenges at nocturnal and diurnal commercial fisheries (e.g.González-Solís et al, 1997a; Oro et al, 1997; Oro and Ruiz, 1997). The increase of the world population of Audouin’s gull has been dramatic in the last 15 years, and its growth rate is one of the largest ever recorded for a larid species. Most of this increase occurred at a new colony established in the Ebro. Delta in 1981, which currently holds 60% of the total world population (Oro, 1998). The high availability of discards from trawler fishing boats has probably enhanced the growth of this colony (Oro and Ruxton, 2001), as suggested in other studies of scavenging seabird species The second largest colony of Audouin’s gulls in the world is at the Chafarinas Islands, which holds ca. 25% of total population
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