Examining the effect of environmental variability on the viability of endangered Steller sea lions using an integrated population model

Abstract

Understanding spatio-temporal variability in demography and the influence of environmental conditions offers insight into the factors underlying population dynamics. This is particularly true for species with divergent demographic patterns across large geographic areas. The contrasting abundance trends observed across the range of Steller sea lions Eumetopias jubatus have been studied extensively, with research suggesting that the primary drivers of localized population dynamics vary over time and space. We developed a Bayesian integrated population model for the endangered western distinct population segment of Steller sea lions that combines mark-recapture and count data from 2000 to 2021 to estimate demographic rates, abundance trends, and the effects of environmental variability on population growth. Our results highlight subregional demographic differences, including reduced pup survival in the central Aleutian Islands and reduced yearling survival west of Samalga Pass. Range-wide abundance increased by 1.7% yr-1 (95% credible interval: 0.14 to 3.4%) over the study period, with a positive annual growth rate of 3.0% (1.1 to 5.1%) yr-1 east of Samalga Pass, a negative growth rate of -2.1% (-4.6 to 0.5%) yr-1 west of Samalga Pass, and an overall low probability of local extirpation (<2%) in 100 yr even in subregions experiencing continued decline. The effect of environmental variability on population growth varied depending on subpopulation size and vital rates and was strongest in the area of greatest decline. Our model improves upon existing approaches for estimating abundance, accounts for environmental variability within the viability analysis, and can facilitate evaluating the efficacy of conservation actions and progress toward recovery goals.