Polar bears in the Beaufort Sea: A 30-year mark-recapture case history

  • S. C. AmstrupEmail author
  • T. L. McDonald
  • I. Stirling


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 ≈ 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.

Key Words

Capture-recapture Covariates Logistic modeling Population estimation 


  1. Akaike, H. (1973), “Information Theory as an Extension of the Maximum Likelihood Principle,” in Second International Symposium on Information Theory, eds. B. N. Petrov and F. Csaki, Budapest: Akademiai Kiado, pp. 267–281.Google Scholar
  2. Amstrup, S. C. (2000), “Polar Bear,” in The Natural History of an Arctic Oil Field: Development and Biota, eds. J. C. Truett and S. R. Johnson. New York: Academic, pp. 133–157.CrossRefGoogle Scholar
  3. Amstrup, S. C., Durner, G., Stirling, I., Lunn, N. J., and Messier, F. (2000), “Movements and Distribution of Polar Bears in the Beaufort Sea,” Canadian Journal of Zoology, 78, 948–966.CrossRefGoogle Scholar
  4. Amstrup, S. C., Gardner, C., Myers, K. C., and Oehme, F. W. (1989), “Ethylene Glycol (Antifreeze) Poisoning of a Free-Ranging Polar Bear,” Veterinary and Human Toxicology, 31, 317–319.Google Scholar
  5. Amstrup, S. C., Stirling, I., and Lentfer, J. W. (1986), “Past and Present Status of Polar Bears in Alaska,” Wildlife Society Bulletin, 14, 241–254.Google Scholar
  6. Anderson, D. R., Burnham, K. P., and White, G. C. (1994), “AIC Model Selection in Overdispersed Capture-Recapture Data,” Ecology, 75, 1780–1793.CrossRefGoogle Scholar
  7. Burnham, K. P., and Anderson, D. R. (1998), Model Selection and Inference: A Practical Information-Theoretic Approach, New York: Springer-Verlag.zbMATHGoogle Scholar
  8. Burnham, K. P., Anderson, D. R., White, G. C., Brownie, C., and Pollock, K. H. (1987), “Design and Analysis Methods for Fish Survival Experiments Based on Release-Recapture,” American Fisheries Society, Monograph5.Google Scholar
  9. Burnham, K. P., Anderson, D. R., White, G. C., Brownie, C., and Pollock, K. H. (1987), “Design and Analysis Methods for Fish Survival Experiments Based on Release-Recapture,” American Fisheries Society, Monograph5.Google Scholar
  10. Calvert, W., and Ramsay, M. A. (1998), “Evaluation of Age Determination of Polar Bears by Counts of Cemetum Growth Layer Groups,” Ursus, 10, 449–453.Google Scholar
  11. DeMaster, D. P., and Stirling, I. (1981), “Ursus maritimus,” Mammalian Species, 145, 1–7.CrossRefGoogle Scholar
  12. Durner, G. M., and Amstrup, S. C. (1996), “Mass and Body-Dimension Relationships of Polar Bears in Northern Alaska,” Wildlife Society Bulletin, 24, 480–484.Google Scholar
  13. Fretwell, S. D. (1972), “Populations in a Seasonal Environment,” in Monographs in Population Biology (Vol. 5), ed. R. H. MacArthur, Princeton, NJ: Princeton University Press.Google Scholar
  14. Hwang, W.-D., and Chao, A. (1995), “Quantifying the Effects of Unequal Catchabilities on Jolly-Seber Estimators Via Sample Coverage,” Biometrics, 51, 128–141.zbMATHCrossRefMathSciNetGoogle Scholar
  15. Lebreton, J. D., Burnham, K. P., Colbert, J., and Anderson, D. R. (1992), “Modeling Survival and Testing Biological Hypotheses Using Marked Animals: A Unified Approach With Case Studies,” Ecological Monographs, 62, 67–118.CrossRefGoogle Scholar
  16. Lentfer, J. W., Hensel, R. J., Gilbert, J. R., and Sorensen, F. E. (1980), “Population Characteristics of Alaskan Polar Bears,” International Conference on Bear Research and Management, 3, 109–115.Google Scholar
  17. Lunn, N. J., Stirling, I., Andriashek, D., and Kolenosky, G. B. (1997), “Re-estimating the Size of the Polar Bear Population in Western Hudson Bay,” Arctic, 50, 234–240.Google Scholar
  18. Manly, B. F. J. (1994), The Design and Analysis of Research Studies, Cambridge: Cambridge University Press.Google Scholar
  19. McCullagh, P., and Nelder, J. A. (1989), Generalized Linear Models (2nd ed.), New York: Chapman and Hall.zbMATHGoogle Scholar
  20. McDonald, T. L., and Amstrup, S. C. (2001), “Estimation of Population Size Using Open Capture-Recapture Models,” Journal of Agricultural, Biological, and Environmental Statistics, 6, 206–220.CrossRefGoogle Scholar
  21. Press, W. H., Flannery, B. P., Teukolsky, S. A., and Vetterling, W. T. (1988), Numerical Recipes in Fortran: The Art of Scientific Computing. Cambridge: Cambridge University Press.zbMATHGoogle Scholar
  22. Pollock, K. H., Nichols, J. D., Brownie, C., and Hines, J. E. (1990), “Statistical Inference for Capture-Recapture Experiments,” Wildlife Monographs, 107, 1–97.Google Scholar
  23. Seber, G. A. F. (1982), The Estimation of Animal Abundance and Related Parameters (2nd ed.), London: Charles Griffin.Google Scholar
  24. Stirling, I. (1990), “Polar Bears and Oil: Ecological Perspectives,” in Sea Mamnals and Oil: Confronting the Risks, eds. J. R. Geraci and D. J. St. Aubin, San Diego: Academic, pp. 223–234.Google Scholar
  25. Stirling, I., and Andriashek, D. (1992), “Terrestrial Maternity Denning of Polar Bears in the Eastern Beaufort Sea Area,” Arctic, 45, 363–366.Google Scholar
  26. Stirling, I., Spencer, C., and Andriashek, D. (1989), “Immobilization of Polar Bears (Ursus maritimus) With Telazol in the Canadian Arctic,” Journal of Wildlife Disease, 25, 159–168.Google Scholar
  27. Taylor, M. K., DeMaster, D. P., Bunnell, F. L., and Schweinsburg, R. E. (1987), “Modeling the Sustainable Harvest of Female Polar Bears,” Journal of Wildlife Management, 51, 811–820.CrossRefGoogle Scholar
  28. Treseder, L., and Carpenter, A. (1989), “Polar Bear Management in the Southern Beaufort Sea,” Information North, 15, 2–4.Google Scholar

Copyright information

© International Biometric Society 2001

Authors and Affiliations

  1. 1.U.S. Geological Services, Biological Resources DivisionAlaska Science CenterAnchorage
  2. 2.Western Ecosystems Technology Inc.Cheyenne
  3. 3.Canadian Wildlife ServiceEdmontonCanada

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