Long-term impacts of the Exxon Valdez oil spill on sea otters, assessed through age-dependent mortality patterns

1. Daniel H. Monson * , †, 
2. Daniel F. Doak ‡, 
3. Brenda E. Ballachey *,
4. Ancel Johnson § , ¶, and 
5. James L. Bodkin *

+Author Affiliations

1. ^*United States Geological Survey, Alaska Biological Science Center, 1011 East Tudor Road, Anchorage, AK 99503; ^‡Department of Biology, University of California, Santa Cruz, CA 95064; and ^§United States Fish and Wildlife Service, Alaska Fish and Wildlife Research Center, 1011 East Tudor Road, Anchorage, AK 99503

1. Communicated by Robert T. Paine, University of Washington, Seattle, WA (received for review October 17, 1999)

Abstract

We use age distributions of sea otters (Enhydra lutris) found dead on beaches of western Prince William Sound, Alaska, between 1976 and 1998 in conjunction with time-varying demographic models to test for lingering effects from the 1989Exxon Valdez oil spill. Our results show that sea otters in this area had decreased survival rates in the years following the spill and that the effects of the spill on annual survival increased rather than dissipated for older animals. Otters born after the 1989 spill were affected less than those alive in March 1989, but do show continuing negative effects through 1998. Population-wide effects of the spill appear to have slowly dissipated through time, due largely to the loss of cohorts alive during the spill. Our results demonstrate that the difficult-to-detect long-term impacts of environmental disasters may still be highly significant and can be rigorously analyzed by using a combination of population data, modeling techniques, and statistical analyses.