The demographic decline of a sea lion population followed multi-decadal sea surface warming

Abstract

AbstractBackgroundThe population growth of top predators depends largely on environmental conditions suitable for aggregating sufficient and high-quality prey. We reconstructed numerically the population size dynamics of a resident population of California sea lions in the Gulf of California during 1978 – 2019, and its relation with the gulf’s multi-decadal sea surface temperature trend. This are the first long-term insights to the oceanic environment of the Gulf of California and to the population trend of one of its major predators.ResultsOur results indicate that a three-decade sustained warming explains statistically the population’s trend, including a ∼65% decline between 1991 – 2019, accounting for 92% of the variance. Long-term warming conditions started in the late 80’s, followed by the population’s decline, from a peak of 43,834 animals (range: 34,080 - 58,274) in 1991 to only 15,291 (range: range: 11,861 - 20,316) in 2019. The models suggested a century-scale optimum sea surface habitat for the population occurring in mildly temperate waters, from 0.18 to 0.39°C above the 100- year mean of 22.24°C.ConclusionsThe negative relation of the population size with warming sea surface conditions was evident, and the predictability of the former from the sea surface temperature 100-year anomalies was high. The mechanistic links of this relation are still untested, but an apparent diversification of pelagic fish catches suggests a reduction of high quality prey. We propose that this sea lion population should be considered as vulnerable to any disturbance that could add to the negative effects of the current sea surface warming conditions in the Gulf of California.