California sea lion population decline at the southern limit of its distribution during warm regimes in the Pacific Ocean

We evaluated the effect of regime shifts on the population dynamics of the California sea lion colony of Santa Margarita Island from 1979–2020. This colony is found at the southern extreme of the California sea lion's range in the North Pacific Ocean in the temperate–tropical​ transition zone characterized by high primary productivity where oceanic and lagoon environments converge. It is largely isolated from other colonies. We assessed the influence of sea surface temperature and net primary productivity on sea lion population dynamics using historical abundance data taken from the literature and collected in the field (1979–2015). We also performed a Population Viability Analysis to determine the population trend and predict the probability that the colony could go extinct. Anomalies of sea surface temperature (1981–2020) and net primary productivity (2002–2020) were analyzed. Warm and cold periods were determined using the Regime Shift Detector. Colony size decreased at a rate of 4% per year, a decrease of ∼75% over 36 years. Four regimes (two cold and two warm) were identified in relation to the sea surface temperature and two regimes (one cold and one warm) in relation to the net primary productivity. The number of births decreased during strong El Niño events under both warm and cold regimes. During warm regimes, a lower abundance of sea lions was observed compared to the cold regimes that preceded them. Population declines due to regime shifts have also been observed in several species of pinnipeds that inhabit the North Pacific (e.g., the Steller sea lion). The Santa Margarita Island breeding colony meets the IUCN's quantitative criteria for “Vulnerable” or “Endangered”. Our results highlight the importance of a colony-level approach in understanding how populations respond to changes in environmental conditions.