Polar Bears in the Beaufort Sea: A 30-Year Mark-Recapture Case History

Knowledge of population size and trend is necessary to manage anthropogenic risks to polar bears (Ursus maritimus). Despite capturing over 1,025 females between 1967 and 1998, previously calculated estimates of the size of the southern Beaufort Sea (SBS) population have been unreliable. We improved estimates of numbers of polar bears by modeling heterogeneity in capture probability with covariates. Important covariates referred to the year of the study, age of the bear, capture effort, and geographic location. Our choice of best approximating model was based on the inverse relationship between variance in parameter estimates and likelihood of the fit and suggested a growth from approximately 500 to over 1,000 females during this study. The mean coefficient of variation on estimates for the last decade of the study was 0.16--the smallest yet derived. A similar model selection approach is recommended for other projects where a best model is not identified by likelihood criteria alone.