The Sea Otter (Enhydra lutris)

From the Exxon Valdez Oil Spill (EVOS) Trustee Council Fiscal Year '95 Project Description:

"Sea otters live 10-15 years, become reproductive at 2-5 years, and produce single pups about once per year during prime reproductive age. They rely on their pelage rather than fat to maintain body temperature . This means of thermoregulation requires a high metabolic rate that depends on a high caloric intake (up to 25% of their body mass in prey consumed per day). Constrained by diving limitations to waters <100 m in depth, sea otters in PWS and elsewhere in the Pacific are distributed along a narrow band of nearshore habitat extending offshore from the intertidal zone. In PWS, about 80% of the sea otters are observed in water depths < 40 m , with most foraging activity occurring within this depth. Home ranges of sea otters generally include from a few to> 40 km of coastline, thereby integrating environmental effects and influencing benthic community structure over large areas. Two consequences of sea otter physiology and habitat requirements are a high susceptibility to contaminants, particularly external oiling, and a large influence on prey populations. Preferred prey of sea otters include sea urchins, mussels, clams, snails and crabs.

By late 1991, results of three injury assessment studies suggested that effects from the spill were continuing: the age class distributions of sea otters dying were abnormal relative to pre-spill data, post-weaning survival was low, and surveys revealed no increase in abundance in oiled areas. By late 1993, juvenile survival had increased and mortality patterns appeared to begin returning to normal. However, surveys of abundance failed to detect increases of sea otters. Analyses of data from a new aerial-survey methodology implemented in 1992 indicate that densities of sea otters are up to an order of magnitude lower in areas of PWS where oiling was most severe and persistent and where sea otter mortality was high, and suggest that recovery had not occurred by 1994.

photos courtesy of Roy Corral

Sea otters are a good choice for investigating processes constraining recovery of the nearshore ecosystem for several reasons:

• 1) they reside nearshore and their activities (including food habits) can be quantified from shore,
• 2) their predominant prey are sedentary bivalves whose abundance and size class distribution can be easily measured,
• 3) effects of the EVOS are well documented, including both immediate and chronic injuries,
• 4) baseline data on sea otter population demographics, physiological indices and their prey are available specifically for PWS,
• 5) tools are available for unbiased and precise estimates of sea otter density, and
• 6) the role of sea otters in structuring invertebrate prey is well understood and provides an opportunity for not only a separate means of evaluating the status of the population, but also identifying potential mechanisms constraining recovery."

  

  photo courtesy of Roy Corral

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  Publications:

  Ballachey, B. E., J. L. Bodkin, and P. W. Snyder.  1999.  Cytochrome P450A as a bioindicator of exposure of sea otters to residual Exxon Valdez oil.  U.S. Geological Survey, Biological Resources Division, Anchorage, AK. poster.  [PDF file - 386 KB]

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Last Reviewed: March 26, 2003