Abstract
The Mediterranean shag (Phalacrocorax aristotelis desmarestii) and the great cormorant (P. carbo sinensis) are syntopic birds in an area encompassing Grosa Island and the Mar Menor coastal lagoon (SE Spain, western Mediterranean) during the breeding season of the former and the wintering period of the latter. Diet composition of both birds was studied through pellet analysis and otolith identification. Competition for fish resources between these two seabirds and with local artisanal fisheries was assessed. Shags preyed preferentially on small pelagic fish, and great cormorants mainly consumed demersal fish. Shag diet consisted of marine fish strictly, but great cormorant fed in all the available environments in the study area, including marine, transitional (coastal lagoon) and freshwater reservoirs. The great dissimilarity observed between shag and great cormorant diet composition showed no competition between them. The low shag population density and fishing effort in their foraging area suggested no competition with fisheries. Conversely, great cormorant population density in the study area was very high, and they foraged on fish of high commercial value. However, competition between great cormorant and artisanal fisheries only affected some of the less abundant species fished.
Highlights
Conflicts between cormorant species and fishery activities are a worldwide concern (Carss et al 1997)
Summary: The Mediterranean shag (Phalacrocorax aristotelis desmarestii) and the great cormorant (P. carbo sinensis) are syntopic birds in an area encompassing Grosa Island and the Mar Menor coastal lagoon (SE Spain, western Mediterranean) during the breeding season of the former and the wintering period of the latter. Diet composition of both birds was studied through pellet analysis and otolith identification
Shag diet consisted of marine fish strictly, but great cormorant fed in all the available environments in the study area, including marine, transitional and freshwater reservoirs
Summary
Conflicts between cormorant species and fishery activities are a worldwide concern (Carss et al 1997). Cormorant mobility for feeding is greater during wintering, including both coastal and inland areas (Farinós and Robledano 2010) Both bird species are opportunistic piscivorous predators (Carss 2002, Morat et al 2014), we do not expect competition between them during syntopy, in line with the findings of Lilliendahl and Solmundsson (2006). The large difference in their population sizes could suggest some interaction Foraging of both species could increase pressure on prey populations (Lilliendahl and Solmundsson 2006), interacting with the local artisanal fisheries occurring near the breeding-wintering area. Local fishermen are concerned with the increasing abundance of cormorants, though this interaction has not yet been evaluated
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