The trawling fishery off the Ebro Delta is regarded as unsustainable and its collapse would probably result in a decline in the breeding population due to the increase in density-dependence15,25. A similar outcome would arise if waste from the trawlers were used industrially to produce food for domestic animals, as occurs in other areas, rather than being dumped near the Ebro colony.More important could be the reduction of small-pelagic fish stocks, the main natural prey for Audouin's Gull, due to increasingly high fishing pressure around the breeding grounds, owing partly to high demand by tuna-farming28. Other important threats include coastal tourism developments, regulation of the river Ebro, mortality due to entanglement in fishing gear (mainly longlines and sporting lines)17,18, and predation by terrestrial predators, such as red fox Vulpes vulpes, badger Meles meles and domestic dogs15,19. Predation by sympatric Yellow-legged Gull L. michahellis can be high at some breeding colonies, especially when densities of Audouin's Gulls are low6,11,16. Predation on chicks by black rats Rattus rattus has a negative impact at some breeding colonies35. Nevertheless Audouin's Gull shows a very nomadic breeding site selection and high dispersal rates from year to year, probably avoiding large densities of Yellow-legged Gulls20,21,22. Peregrines Falco peregrinus, other raptors, some herons and snakes can also prey on adults and nests but only accidentally and locally23,24. Very high levels of mercury and other pollutants are found in this species13, partly due to the consumption of discards 29, thus posing a potential threat, although no negative effects have been demonstrated. Current marine wind-farm projects, particularly around the main breeding colony (Ebro Delta), could also represent a serious threat. Overgrazing of some islets by goats in the east of its breeding range may reduce breeding success. Natal and breeding dispersal are extremely high ensuring genetic mixing and buffering against bad local environmental conditions through emigration and colonisation9,25.
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