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Survival rates of Steller sea lions from Oregon and California

Due to significant population declines in the 1970s and 1980s, Steller sea lions (Eumetopias jubatus) were listed as threatened under the U.S. Endangered Species Act in 1990, and subsequently partitioned in 1997 into an endangered western stock and a threatened eastern stock. We estimated survival rates from a mark-recapture study of 7 eastern stock cohorts marked as pups in California and Oregon from 2001 to 2009 (n = 1,154 pups) and resighted range-wide from 2002 to 2013. First-year survival rates were among the lowest found for Steller sea lions thus far, averaging 0.46 (range 0.21–0.72) for females and 0.44 (0.21–0.68) for males; yearling survival rates, however, were among the highest, averaging 0.85 for females and 0.81 for males. Low pup and high yearling rates offset each other, however, so that cumulative survival rates to age 4, averaging 0.33 for females and 0.27 for males, were similar to those found in studies from Alaska and Russia. While range-limit effects and environmental variation may be related to the low and variable pup survival rates we found, populations in Oregon and California nonetheless continued to grow, which contributed to delisting of the eastern stock in 2013. Continued monitoring and incorporation of new information on vital rates into regional population models will help inform post-delisting monitoring for the eastern stock of Steller sea lions.