Long Term Analysis of the Diet of South American Sea Lions and the South American Fur Seals on the Base of Instantaneous Supply Over Time in a Context of Ecosystem Change

The instantaneous diet of South American sea lions ( Otaria byronia ) and South American fur seals ( Arctocephalus australis ) was analyzed using scats collected from shared haul‐out islands in the northern region of San Jorge Gulf, Patagonia, Argentina. Prey species were identified using otoliths, squid beaks, and crustacean exoskeletons during a long‐term project from 1999 to 2024. Sea lions and fur seals share several prey species, as well as with the fishing fleet, since most of them are commercial species. The diet of fur seals was predominantly pelagic, with Argentine anchovy ( Engraulis anchoita ) as a key prey, whereas sea lions showed a more demersal feeding pattern, with Argentine hake ( Merluccius hubbsi ) as the main fish prey. Argentine shortfin squid was also a major dietary component in both species, particularly during the early years of the study, when it dominated several sampling periods. Over time, both species exhibited a consistent shift in diet composition, characterized by a decrease in fish and cephalopods and an increasing contribution of crustaceans. The changes observed over the 26‐year period generally include a decrease in fish and an increase in crustaceans in the diet of both pinnipeds. The most important crustaceans were Argentine red shrimp and squat lobster, although their relative contribution differed between species. These changes, observed over a multi‐decadal period, are consistent with a broader reorganization of the Patagonian marine ecosystem. The observed patterns likely reflect changes in prey availability associated with sustained fishing pressure, including the extraction of key fish stocks and the input of fishery discards, acting in combination with environmental variability. Overall, our results support the use of pinnipeds as sentinel species for detecting long‐term trophic changes, as their diet integrates ecosystem‐level responses across multiple trophic levels.