Biodiversity & Conservation

, Volume 13, Issue 14, pp 2741–2757 | Cite as

Density dependence and risk of extinction in a small population of sea otters

  • Leah R. Gerber
  • Kate E. Buenau
  • Glenn Vanblaricom


Sea otters (Enhydra lutris (L.)) were hunted to extinction off the coast of Washington State early in the 20th century. A new population was established by translocations from Alaska in 1969 and 1970. The population, currently numbering at least 550 animals, A major threat to the population is the ongoing risk of majour oil spills in sea otter habitat. We apply population models to census and demographic data in order to evaluate the status of the population. We fit several density dependent models to test for density dependence and determine plausible values for the carrying capacity (K) by comparing model goodness of fit to an exponential model. Model fits were compared using Akaike Information Criterion (AIC). A significant negative relationship was found between the population growth rate and population size (r2 = 0.27, F = 5.57, df = 16, p < 0.05), suggesting density dependence in Washington state sea otters. Information criterion statistics suggest that the model is the most parsimonious, followed closely by the logistic Beverton–Holt model. Values of K ranged from 612 to 759 with best-fit parameter estimates for the Beverton–Holt model including 0.26 for r and 612 for K. The latest (2001) population index count (555) puts the population at 87–92% of the estimated carrying capacity, above the suggested range for optimum sustainable population (OSP). Elasticity analysis was conducted to examine the effects of proportional changes in vital rates on the population growth rate (λ). The elasticity values indicate the population is most sensitive to changes in survival rates (particularly adult survival).

Akaike information criteria Demography Density dependence Diffusion approximation Extinction risk Oil spill Sea otter 


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Ballachey B.E., Bodkin J.L. and DeGange A.R. 1994. An overview of sea otter studies. In: Loughlin T.R. (ed.), Marine Mammals and the Exxon Valdez. Academic Press, San Diego, California, pp. 47–59.Google Scholar
  2. Barabash-Nikiforov I.I., Marakov S.V. and Nikolaev A.M. 1968. Sea Otters. Idz-vo Nauka, Leningrad, U.S.S.R., 194 pp. (in Russian).Google Scholar
  3. Benz C. 1996. The second southern sea otter recovery plan. Endangered Species Update 13(12): 36–37.Google Scholar
  4. Beverton R.J.H. and Holt S.J. 1957. On the dynamics of exploited fish populations. Fisheries In-vestigations, London (Series II) 19: 1–533.Google Scholar
  5. Bonnell M.L., Ford R.G. and Brody A.J. 1996. Assessing the threat of oil spills to southern sea otters. Endangered Species Update 13(12): 38–42.Google Scholar
  6. Bowlby C.E., Troutman B.J. and Jeffries S.J. 1988. Sea otters in Washington: distribution, abundance and activity patterns. Final report. National Coastal Resources Research and Development Institute, Newport, Oregon.Google Scholar
  7. Caswell H. 2001. Matrix Population Models: Construction, Analysis, and Interpretation, 2nd edn. Sinauer Associates, Sunderland, Massachusetts.Google Scholar
  8. Clark R.C. Jr. and Finley J.S. 1973. Interagency investigations of a persistent oil spill on the Washington coast. In: Proceedings of a Joint Conference on Prevention and Control of Oil Spills. American Petroleum Institute, Washington, DC, pp. 793–808.Google Scholar
  9. Costa D.P. and Kooyman G.L. 1982. Oxygen consumption, thermoregulation, and the effect of fur oiling and washing on the sea otter, Enhydra lutris. Canadian Journal of Zoology 60: 2761–2767.Google Scholar
  10. Demaster D.P., Marzin C. and Jameson R.J. 1996. Estimating the historical abundance of sea otters in California. Endangered Species Update 13: 79–81.Google Scholar
  11. Eberhardt L.L. and Schneider K.B. 1994. Estimating sea otter reproductive rates. Marine Mammal Science 10: 31–37.Google Scholar
  12. Garrott R.A., Eberhardt L.L. and Burn D.M. 1993. Mortality of sea otters in Prince William Sound following the Exxon Valdez oil spill. Marine Mammal Science 9: 343–359.Google Scholar
  13. Geraci J.R. and Williams T.D. 1990. Physiologic and toxic effects on sea otters. In: Geraci J.R. and St. Aubin D.J. (eds), Sea Mammals and Oil: Confronting the Risks. Academic Press, San Diego, California, pp. 211–221.Google Scholar
  14. French D. 2000. Review of Draft Southern Sea Otter Recovery Plan (Revised) Sections on Oil Spill Risks and Impacts. Applied Science Associates, Narragansett, Rhode Island, ASA 99–94.Google Scholar
  15. Gerber L.R. and VanBlaricom G.R. 1999. Potential fishery conflicts involving sea otters (Enhydra lutris) in Washington State waters. Final report under contract T30917202 to the U.S. Marine Mammal Commission, 107 pp.Google Scholar
  16. Gerber L.R., Tinker M.T., Doak D. and Estes J. 2004. Mortality sensitivity in life-stage simulation analysis: a case study of Southern sea otters. Ecological Applications 14: 1554–1565.Google Scholar
  17. Gilpin M.E., Case T.J. and Ayala F.J. 1976. θ Selection. Mathematical Biosciences 32: 131–139.Google Scholar
  18. James Dobbins Associates, Inc. 1984. Southern sea otter: compilation and mapping of available biological, ecological and socio-economic information bearing on the protection, management and restoration of the southern sea otter. Final report, contract 14–16–009–81–050. U.S. Fish and Wildlife Service, Sacramento, California.Google Scholar
  19. Jameson R.J. 1995. Some preliminary observations on the foraging of sea otters off the outer coast of Washington State, USA. IUCN Otter Specialist Group Bulletin 11: 8–11.Google Scholar
  20. Jameson R.J. 1998. Translocated sea otter populations off the Oregon and Washington coasts. In: Mac M.J., Opler P.A., Puckett Haecker C.E. and Doran P.D. (eds) Status and Trends of the Nation's Biological Resources. Vol. 2. U.S. Geological Survey, Washington, DC, pp. 684–686.Google Scholar
  21. Jameson R.J. and Johnson A.M. 1993. Reproductive characteristics of female sea otters. Marine Mammal Science 9: 156–157.Google Scholar
  22. Jameson R.J., Kenyon K.W., Johnson A.M. and Wight H.N. 1982. History and status of translocated sea otter populations in North America. Wildlife Society Bulletin 10: 100–107.Google Scholar
  23. Jameson R.J., Kenyon K.W., Jeffries S.J. and VanBlaricom G.R. 1986. Status of a translocated sea otter population and its habitat in Washington. Murrelet 67: 84–87.Google Scholar
  24. Kenyon K.W. 1969. The sea otter in the eastern Pacific Ocean. North American Fauna 68: 1–352.Google Scholar
  25. Laidre K.L., Jameson R.J., Jeffries S.J., Hobbs R.C., Bowlby C.E. and VanBlaricom G.R. 2002. Estimates of carrying capacity for sea otters in Washington State. Wildlife Society Bulletin 30: 1172–1181.Google Scholar
  26. Loughlin T.R. (ed) 1994. Marine Mammals and the Exxon Valdez. Academic Press, San Diego, California.Google Scholar
  27. Morris W.F. and Doak D.F. 2002. Quantitative Conservation Biology: Theory and Practice of Population Viability Analysis. Sinauer Associates, Sunderland, Massachusetts.Google Scholar
  28. Read A.J. and Wade P.R. 2000. Status of marine mammals in the United States. Conservation Biology 14: 929–940.Google Scholar
  29. Riedman M.L., Estes J.A. and Staedler M.M. 1994. Breeding patterns and reproductive success of California Sea Otters. Journal of Wildlife Management 58: 391–399.Google Scholar
  30. Rogne T. and MacDonald I. 1993. Multispectral remote sensing and truth data from the Tenyo Maru oil spill. Photogrammatic Engineering and Remote Sensing 59: 391–396.Google Scholar
  31. Rotterman L.M. and Simon-Jackson T. 1988. The sea otter, Enydra lutris. In: Lentfer J.W. (ed) Selected Marine Mammals of Alaska: Species Accounts with Research and Management Recommendations Marine Mammal Commission, Washington, DC, pp. 237–275.Google Scholar
  32. Saunders R.T. 1996. Does ‘sanctuary’ mean secure? Endangered Species Update 13(12): 43–46.Google Scholar
  33. Siniff D.B. and Ralls K. 1991. Reproduction, survival and tag loss in California sea otters. Marine Mammal Science 7: 211–229.Google Scholar
  34. Siniff D.B., Williams T.D., Johnson A.M. and Garshelis D.L. 1982. Experiments on the response of sea otters, Enhydra lutris, to oil contamination. Biological Conservation 23: 261–272.Google Scholar
  35. Taylor B.L. and Demaster D.P. 1995. Implications of nonlinear density-dependence. Marine Mammal Science 9: 360–371.Google Scholar
  36. Townsend R. and Glazer M. 1994. Safe Passage: Preventing il Spills in our Marine Sanctuaries. Center for Marine Conservation, San Francisco, California.Google Scholar
  37. VanBlaricom G.R. 1996. Saving the sea otter population in California: Contemporary problems and future pitfalls. Endangered Species Update 13(12): 85–91.Google Scholar
  38. Washington (State) Department of Ecology 1997. Oil Spills in Washington State: A Historical Analysis. Publication No. 97–252. Washington (State) Department of Ecology, Olympia, Washington.Google Scholar
  39. Williams T.M., Kastelein R.A., Davis R.W. and Thomas J.A. 1988. The effects of oil contamination and cleaning on sea otters (Enhydra lutris). I. Thermoregulatory implications based on pelt studies. Canadian Journal of Zoology 66: 2776–2781.Google Scholar
  40. Yaroch G.N. 1991. The Nestucca oil spill. Chemtech 21: 722–727.Google Scholar

Copyright information

© Kluwer Academic Publishers 2004

Authors and Affiliations

  • Leah R. Gerber
    • 1
  • Kate E. Buenau
    • 1
  • Glenn Vanblaricom
    • 2
  1. 1.School of Life SciencesArizona State UniversityTempeUSA
  2. 2.U.S. Geological Survey, Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fishery SciencesUniversity of WashingtonSeattleUSA

Personalised recommendations