The unexpected diet of breeding Imperial Shags (Phalacrocorax atriceps) at the Nahuel Huapi Lake, Patagonia: implications on population trends?

Abstract

The Imperial Shag Phalacrocorax atriceps is widely distributed along the coast of South America and in Malvinas/Falkland Islands(Orta 1992). This shag is marine in most its range and forages predominantly on fish, although invertebrates were frequently reported as present in the diet(e.g., Punta etal. 2003, Ferrari et al. 2004, Bulgarella et al.2008)Reynolds (1934) reported the presence of the Imperial Shag breeding at Yehuin Lake,Tierra del Fuego, and later on Pereyra (1945, quoted in Navas 1970), Chebez & Gómez (1988) and Rasmussen et al. (1992) reported the existence of the only other three populations of this shag breeding in freshwater environments,the Nahuel Huapi (NHL),Fagnano, and Vintter lakes, respectively. Despite the potential interest in studying these populations, few studies provided information on the biology of this shag species. Although not globally threatened, the Imperial Shag at NHL is locally endangered and between the summers of 1990 (Rasmussen et.al. 1993) and 2005 (Pastore et al. 2005)the total number of individuals decreased in 68%. Thus,the aim of this study is provide the first information on the diet of the Imperial Shag breeding at NHL to improve the knowledge of the foraging behavior of this species as well as to explore the linkage between diet composition and the reported declining trend of this population.