Neonatal survival of Steller sea lions (Eumetopias jubatus)



Neonatal survival of Steller sea lions (Eumetopias jubatus) are often considered inconsequential to their population dynamics. However, observations of dead animals on rookeries and in surrounding waters suggest that early mortality is not uncommon. This study used the natural markings of adult females in a mark and resighting framework to estimate the apparent survival (φ) of pups with the Cormack–Jolly–Seber model at two sites on Lowrie Island, Alaska from birth to 3 wk old. Estimates varied greatly by site and year; 2002 Area 5: inline image (95% CI: 0.199, 0.684; n= 21), 2002 Area 1: inline image (0.437, 0.916; n= 21), 2003 Area 5: inline image (0.414, 0.738; n= 56), and 2003 Area 1: inline image (0.695, 0.997; n= 32). The mean estimate across the four area × year combinations was inline image (0.569, 0.772). Survival was lowest on the first day of life and then leveled off at a higher rate. None of the four environmental covariates we considered (swell height, interaction of tide and swell heights, density, or birth date) were significantly related to neonatal survival. Our results suggest that estimates of first-year survival that do not account for mortality prior to dispersal from the natal rookery may significantly overestimate survival rate.