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Open-File Report 2014–1030
2013 Update on Sea Otter Studies to Assess Recovery from the 1989 Exxon Valdez Oil Spill, Prince William Sound, Alaska
By B.E. Ballachey, D.H. Monson, G.G. Esslinger, K. Kloecker, J. Bodkin, L. Bowen, and A.K. Miles
Executive Summary
On March 24, 1989, the tanker vessel Exxon Valdez ran aground in Prince William
Sound, Alaska, spilling an estimated 42 million liters of Prudhoe Bay crude oil. Oil spread in a
southwesterly direction and was deposited on shores and waters in western Prince William
Sound (WPWS). The sea otter (Enhydra lutris) was one of more than 20 nearshore species
considered to have been injured by the spill. Since 1989, the U.S. Geological Survey has led a
research program to evaluate effects of the spill on sea otters and assess progress toward
recovery, as defined by demographic and biochemical indicators. Here, we provide an update on
the status of sea otter populations in WPWS, presenting findings through 2013. To assess
recovery based on demographic indicators, we used aerial surveys to estimate abundance and
annual collections of sea otter carcasses to evaluate patterns in ages-at-death. To assess recovery
based on biochemical indicators, we quantified transcription rates for a suite of genes selected as
potential indicators of oil exposure in sea otters based on laboratory studies of a related species,
the mink (Mustela vison). In our most recent assessment of sea otter recovery, which
incorporated results from a subset of studies through 2009, we concluded that recovery of sea
otters in WPWS was underway. This conclusion was based on increasing abundance throughout
WPWS, including increasing numbers at northern Knight Island, an area that was heavily oiled
in 1989 and where the local sea otter population had previously shown protracted injury and lack
of recovery. However, we did not conclude that the WPWS sea otter population had fully
recovered, due to indications of continuing reduced survival and exposure to lingering oil in sea
otters at Knight Island, at least through 2009. Based on data available through 2013, we now
conclude that the status of sea otters—at all spatial scales within WPWS—is consistent with the
designation of recovery from the spill as defined by the Exxon Valdez Oil Spill Trustee Council.
The support for this conclusion is based primarily on demographic data, including (1) a return to
estimated pre-spill abundance of sea otters at northern Knight Island, and (2) a return to pre-spill
mortality patterns. Gene transcription rates in 2012 were similar in sea otters from oiled,
moderately oiled and unoiled areas, suggesting abatement of exposure effects in 2012. However,
because 2012 gene transcription rates generally were low for sea otters from all areas relative to
2008, we cannot fully interpret these observations without data from a wider panel of genes. This
slight uncertainty with respect to the data from the biochemical indicator is outweighed by the
strength of the data for the demographic indicators. The return to pre-spill numbers and mortality
patterns suggests a gradual dissipation of lingering oil over the past two decades, to the point
where continuing exposure is no longer of biological significance to the WPWS sea otter
population.
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First posted February 28, 2014
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Suggested citation:
Ballachey, B.E., Monson, D.H., Esslinger, G.G., Kloecker, K., Bodkin, J., Bowen, L., and Miles, A.K.,
2014, 2013 update on sea otter studies to assess recovery from the 1989 Exxon Valdez oil spill, Prince
William Sound, Alaska: U.S. Geological Survey Open-File Report 2014-1030, 40 p.,
https://dx.doi.org/10.3133/ofr20141030.
ISSN 2331-1258 (online)
Contents
Executive Summary
Introduction
Chapter 1. Sea Otter Abundance in Western Prince William Sound through 2013
Chapter 2. Age Distributions of Sea Otters Found Dead in Western Prince William Sound, Alaska,
through 2013
Chapter 3. Gene Transcript Profiles in Sea Otters from Prince William Sound, Alaska, 2008 and
2012, and Clinically Normal and Wild Reference Sea Otters
Synthesis and Conclusions
References Cited
Appendix 1. Publications and Reports on Sea Otter Oil Spill Studies, Authored or Co-authored by
USGS Scientists, 1989–2013