Sea Otter Population Assessment

Dan Monson examining a sea otter that has been anesthetized

With the exception of 13 small remnant populations, sea otters were extirpated from their historic range in the north Pacific Ocean during the 18th and 19th centuries as a result of the commercial harvest for their fur. During most of the 20th century, through protection and reintroduction, sea otter populations generally increased in abundance and distribution such that most of their range in Alaska, with the exception of southeast Alaska, was occupied by 2000. Although population abundance data are incomplete, there is evidence of increasing, stable and declining sea otter populations in different areas within their range. The factors that ultimately regulate sea otter population abundance are not completely understood, but can include predation, human harvest, food limitation, disease and catastrophic events such as oil spill. There is good evidence that the recent declines in sea otters in SW Alaska are related to killer whale predation and the Exxon Valdez oil spill reduced the size of the western Prince William Sound population in 1989. Human harvest of sea otters can adversely affect sea otter abundance, evidenced by the commercial fur trade leading to near extirpation. Because sea otters occupy relatively small home ranges and do not migrate, sustainable harvest requires management at appropriate spatial scales. Recently, harvest of sea otters for subsistence have been increasing, although effects of the harvest at current levels on population trend are unknown. Because sea otter populations occur over vast and remote areas and may display divergent trends in abundance over relatively small spatial scales, determining population status and trends can be challenging.

Methods to assess sea otter population status and trends are important to evaluate the recovery of populations and the potential effects of human perturbations (e.g., harvest, contaminants, and habitat modifications) on populations. This information is important to resource managers in identifying potential conflicts, identifying mechanisms of change, and improving the ability to detect and respond to change from human induced sources.

Objectives of our sea otter population assessment studies include: 1) develop and test methods to identify the degree of population structuring among north Pacific sea otter populations, 2) develop and test techniques to accurately and precisely estimate the status of sea otter populations, 3) develop and test methods to identify cause(s) of change in the status and numeric trends of sea otter populations, 4) develop and test methods to determine the role of density dependent processes in affecting change in sea otter populations, and 5) evaluate the effects of population reductions and translocations on sea otter genetic variability.

Links

Western Ecological Research Center - Sea Otter Studies

Selected References

Bowen, L., A. K. Miles, M. Murray, M. Haulena, J. Tuttle, W. Van Bonn, L. Adams, J. L. Bodkin, B. E. Ballachey, M. T. Tinker, R. Keister, and J. L. Stott. 2012. Gene Transcription in Sea Otters (Enhydra lutris); Development of a diagnostic tool for sea otter and ecosystem health. Molecular Ecology Resources 12(1):67-74. doi: 10.1111/j.1755-0998.2011.03060.x

Bodkin, J. L. and B. E. Ballachey. 2010. Modeling the effects of mortality on sea otter populations. USGS Scientific Investigation Report 2010-5096. 12p.

Bodkin, J. L., D. H. Monson, and G. G. Esslinger. 2007. Activity budgets derived from time-depth recorders in a diving mammal. J. Wildlife Management 71(6):2034-2044. doi: 10.2193/2006-258 [PDF file 450kb]

Laidre, K. L., J. A. Estes, M. T. Tinker, J. Bodkin, D. Monson, and K. Schneider. 2006. Patterns of growth and body condition in sea otters from the Aleutian archipelago before and after the recent population decline. Journal of Animal Ecology 75(4):978-989. doi: 10.1111/j.1365-2656.2006.01117.x

Bodkin, J. L. and D. H. Monson. 2002. Sea otter population structure and ecology on Alaska. Arctic Research of the United States 16:31-36. [PDF 315kb]

Monson, D. H., J. A. Estes, J. L. Bodkin, and D. B. Siniff. 2000. Life history plasticity and population regulation in sea otters. Oikos 90(3):457-468. doi: 10.1034/j.1600-0706.2000.900304.x [PDF file 262 kb]

Bodkin, J. L., B. E. Ballachey, M. A. Cronin and K.T. Scribner. 1999. Population demographics and genetic diversity in remnant and translocated populations of sea otters. Conservation Biology 13(6):1378-1385. doi: 10.1046/j.1523-1739.1999.98124.x [PDF file 548 kb]