Age-specific birth rates of California sea lions (Zalophus californianus) in the Gulf of California, Mexico

Estimates of demographic parameters are essential for assessing the status of populations and assigning conservation priority. In light of the difficulties associated with obtaining such estimates, vital rates are rarely available even for well-studied species. We present the first estimates of age-specific birth rates for female California sea lions (Zalophus californianus) >10 yr of age. These rates were estimated from the reproductive histories of five cohorts of animals branded as pups between 1980 and 1984 at Los Islotes colony in the Gulf of California, Mexico. Age-specific birth rates varied among age classes and ranged between 0.06 and 0.80. The highest birth rates were observed for females between 10 and 15 yr of age, with decreased birth rates among older females. The effect of age, year, and resighting effort were explored using logistic regression analysis. Based on Akaike Information Criteria, birth rates were best explained by female age, while year and resighting effort did not have a significant effect. The odds ratio of producing a pup decreased with age but did not change significantly for middle-aged females. Our estimates of age-specific birth rates are consistent with general patterns observed for other large vertebrates.