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NOAA Technical Memorandum NMFS-AFSC-312

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California sea lion sex- and age-specific morphometry


Pinnipeds are primarily aged by marks (e.g., flipper tags, brands) applied to individuals when they are of known age (e.g., pups). Age determination is useful for researchers and managers for various reasons including estimating survival rates and assessing mortality of individuals that interact with fisheries. Age can be determined from unmarked animals by extracting a tooth and counting the number of dentine layers. For most populations, however, relatively few individuals are marked and it is often impractical to extract a tooth. Here we developed sex-specific length-age and weight-age curves for the California sea lion (Zalophus californianus). Additionally, we fit a power law curve for an allometric relationship between length and weight. We obtained length and weight measurements from known-age California sea lions captured at various haulout or rookery sites on islands off California and locations in Oregon and Washington. Gompertz and Von Bertalanffy growth models were compared after fitting them to length-at-age (cm) and weight-at-age (kg) data for male and female sea lions from 1 month to age 16 years and 19 years, respectively. The length-at-age data included initial capture and recapture records of female (n = 8,035) and male (n = 5,813) sea lions. The Von Bertalanffy growth curve was a better model than the Gompertz for length- and weight-at-age for both males and females. The weight-at-age relationship was much more variable than for length due to the smaller sample size and the vagaries of seasonal and annual weight loss and gain. However, a linear relationship for weight for ages 3.5-8 months is very reasonable based on the fitted growth curve.

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