Average Age at First Reproduction of the Northern Fur Seal (Callorhinus ursinus)

Three methods of estimating average age at first reproduction of the northern fur seal, Callorhinus ursinus, are presented. For the first, the percentage of females pregnant for the first time is estimated from the percentage of primiparous pregnant females in the population. For the second, the percentage of females pregnant for the first time is estimated from the difference in the percentages of females pregnant at least once at successive ages. The jackknife variances of both estimates are presented. The estimates are calculated for the 1954–64 year-classes of northern fur seals using data collected at sea by the United States and Canada during 1958–74. By fitting parametric curves to the fraction pregnant at least once as a function of age and applying the second method to that resulting curve, the average age at first reproduction is estimated in a third way; this method is applicable to year-classes for which data are incomplete and estimates are given for the 1952, 1953, 1965, and 1966 year-classes. Age at first reproduction was lowest for the 1952–55 year-classes, increased sharply beginning with the 1956 year-class, and then gradually decreased almost to the level of the 1952–55 year-classes. Average age at first reproduction is negatively correlated with juvenile survival (birth to age 2 yr).Key words: fur seal, northern fur seal, Callorhinus ursinus; average age at first reproduction, cohort analysis, population dynamics