Feathers of Audouin's Gull chicks as indicators of Hg availability

摘要

The Audouin's Gull is an endemic species of the Mediterranean. The main breeding colonies are located in the Ebro Delta (NE Spain) and Chafarinas Islands (North coast of Morocco, SW Mediterranean). Feeding habits of Audouin's gulls differ between both colonies. At Chafarinas consists mainly of epipelagic fish (clupeiformes) and, on an opportunistic basis, they also consume fish discards. However, at the Ebro Delta they exploit discards from trawler fisheries (mesopelagic preys), as the main food resource. Levels of mercury have been shown to be significantly lower in epipelagic than in demersal fish as a consequence of both, their different trophic level (biomagnification) and of Hg availability, which is higher in deeper waters. Thus, the exposition to Hg compounds must be higher in gulls from the Ebro Delta. Previous analyses in primary feathers of adults from both colonies have shown that, while N and C isotopic signatures reflect the differences reported in diet, no Hg differences between colonies were detected. This lack of differences concerning Hg was attributed to the fact that p1, being the first feather moulted after breeding, is strongly influenced by the body pool of Hg accumulated during the breeding season.