Spatial distribution, movements, and geographic range of Steller sea lions (Eumetopias jubatus) in Alaska

Abstract

The two stocks of Steller sea lions (Eumetopias jubatus) in Alaska include an endangered western stock, recently recovering in parts of its range following decades of decline, and an eastern stock which was removed from the U.S. Endangered Species List in 2013 following increasing numbers since the 1970s. Information on overlapping distributions of eastern and western sea lions is needed for management considerations. We analyzed >30,000 sightings collected from 2000–2014 of 2,385 sea lions that were branded as pups at 10 Alaskan rookeries to examine mesoscale (mostly <500km) spatial distribution, geographic range, and geographic population structure based on natal rookery, sex, and age during breeding and non-breeding seasons. Analyses of summary movement measures (e.g., natal rookery, sex, and age-class differences in spatial distribution and geographic range) indicate wide variation in rookery-specific movement patterns. Correlations between movement measures and population dynamics suggested movement patterns could be a function of density dependence. Animals from larger rookeries, and rookeries with slower population growth and lower survival, had wider dispersion than animals from smaller rookeries, or rookeries with high growth and survival. Sea lions from the largest rookery, Forrester Island, where survival and population trends are lowest, were the most widely distributed. Analysis of geographic population structure indicated that animals born in the eastern Aleutian Islands had the most distinct movements and had little overlap with other western sea lions. Northern Southeast Alaska, within the eastern stock, is the area of greatest overlap between stocks, and is important to western animals, especially those born in Prince William Sound. Detailed knowledge of distribution and movements of western sea lions is useful for defining recovery and population trend analysis regions that better reflect dispersion and population structure and provides valuable information to managers as critical habitat is re-evaluated and the location of the stock boundary reconsidered.