Historic diet change of the South American sea lion in Patagonia as revealed by isotopic analysis

Abstract

Carbon and nitrogen isotopic analyses of skull bone were used to investigate how sealing and the development of industrial fishing have affected the diet of the South American sea lion Otaria flavescens in northern Patagonia. Males from Tierra del Fuego were used as a control, as the species there was decimated by sealing, but industrial fishing is only poorly developed. The δ13C of both males and females from northern Patagonia increased from the 1940s to the 1970s, and then declined steadily. The decline in the slope was similar in both sexes, although females were more depleted in 13C than were males. The δ15N remained unaffected in males throughout the period, whereas that of females decreased from the 1940s to the 1970s and then stabilized. Conversely, no change was found in either the δ13C or δ15N in the skulls from Tierra del Fuego animals. As benthic prey off northern Patagonia are more enriched in 13C than are pelagic prey, the above results indicate increased consumption of benthic coastal prey in this region from the 1940s to the 1970s, when sea lions were decimated by commercial hunting, and increased consumption of pelagic prey since the 1970s, simultaneous with sea lion population recovery. Reinforced intraspecific competition and massive discard of pelagic fish likely contributed to the observed dietary shift, while the poorly developed industrial fishing off Tierra del Fuego did not facilitate a similar change there. Nevertheless, physical forcing related to regime changes observed in the Pacific Ocean since the 1970s may also have played a role.