Estimating the Juvenile Survival Rate of Male Northern Fur Seals (Callorhinus ursinus)

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Abstract
Three methods for estimating the survival rate of juvenile northern fur seals (Callorhinus ursinus) are developed from the earlier works of Chapman, Smith and Polacheck and Lander. Each of the methods I propose divides the estimated number of males alive at 2 yr of age by the estimated number of pups born in their year class. The number of surviving juveniles are reconstructed by back calculation using the number of males killed during the commercial harvest and the subsequent counts of bulls. The three methods differ in their assumptions concerning subadult survival and escapement from the harvest, although all produce similar estimates when applied to the St. Paul Island fur seals. These new estimates of juvenile survival (1950–80) are strongly correlated with the ratio of cohort kill to pup production and with estimates from the currently-used Lander procedure. This is because the harvest morality of males is large compared with natural morality. The new methods perform acceptably over a wider class of data than Lander's. Their greatest advantage over current procedures is that they provide a better insight into the reliability of the survival estimates they produce.

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