No signs of recovery: The long-term consequences of harvesting on a South American sea lion population in Uruguay

Abstract

Understanding long-term population trajectories is essential for effective wildlife management, particularly in exploited species. In Uruguay, the South American sea lion was subjected to intense commercial harvesting throughout much of the 20th century; however, post-exploitation population trajectories remain poorly documented in the country. We assessed long-term demographic trends of the two main Uruguayan rookeries, Isla de Lobos and Cabo Polonio, both Marine Protected Areas, by compiling historical pup counts (1953–2000) and conducting standardized aerial photographic surveys in 2011, 2013, and 2017. Pup abundance was estimated from aerial mosaics, corrected for detection bias, and analysed using log-linear regression to estimate annual rates of change. Between 1956 and 2017, pup production in Uruguay declined significantly at a mean annual rate of 1.9%. Cabo Polonio remained relatively stable, exhibiting only a weak and non-significant decline (–0.7% yr⁻¹), whereas Isla de Lobos showed a strong and significant annual decline of approximately 5.0%. In 2017, estimated pup abundance reached 2006 at Cabo Polonio and only 299 at Isla de Lobos. These contrasting trajectories likely reflect the legacy of intensive commercial exploitation at Isla de Lobos, where approximately 47,000 pups were removed over 24 years, followed by the continued extraction of live non-breeding individuals for zoos and aquariums. Limited demographic rescue between colonies, ongoing anthropogenic pressures, and recent stochastic events (including the 2023 outbreak of highly pathogenic avian influenza (HPAI H5N1)), further increase vulnerability of the Uruguayan colonies. Our findings underscore the need for sustained monitoring and colony-specific management to promote long-term population resilience in Uruguay.