Alaska Science Center
Effects of the Exxon Valdez Oil Spill on Sea Otters
Sea otters were severely impacted by the 1989 Exxon Valdez oil spill. Estimates of acute spill related mortality range from about 1,000 to 5,500 in the first months after the spill. Scientists with the Alaska Science Center were among the first responders to the1989 spill and continue work today to documentthe process of recovery form this spill and to better understand the effects future contamination events on sea otters and the nearshore ecosystems they occupy.
One of the factors limiting our ability to clearly understand and document the spill effects was a lack of accurate estimates of sea otter abundance. This was true for nearly all species in the Gulf of Alaska and remains an impediment in assessing injury from such catastrophes across most landscapes today. Initial research efforts following the spill focused on damage assessment, including developing methods to accurately estimate the abundance of affected populations and studies of reproduction and survival.
Large scale ecosystem level studies of nearshore species and habitats most affected by the spill completed in 1999, found evidence of long-term spill effects among nearshore species dependent on a nearshore food web where benthic invertebrates transfer primary production to upper level consumers such as sea otters and sea ducks. Biochemical and gene techniques suggested that lingering oil may have contributed to a protracted recovery period for nearshore species. Subsequently, surveys of beaches where oil was deposited nearly a decade earlier found unanticipated volumes of oil sequestered in nearly 20 acres of widely distributed soft sediment intertidal beaches in Prince William Sound.
Our most recent surveys of sea otter abundance indicate significant progress toward recovery, when we consider the entire spill affected area in the Sound. By 2009 our estimate of sea otter abundance in the western Sound was nearly 2,000 animals more than our first post spill estimate in 1993 of about 2,000 individuals. However, when we look only at those areas that were most severely affected by the spill, where sea otter mortality approached 90% and where much of the lingering oil has been located, evidence of recovery remains incomplete. Our most recent research, based on the diving behavior of sea otters in the intertidal and published oil encounter rates, indicates that all sea otters in those heavily oiled areas are likely to encounter Exxon Valdez oil at least annually and some as often as weekly. Long term continuation of studies investigating mortality from the annual collections of beach cast sea otter carcasses implicates elevated mortality as the factor most likely contributing to delayed recovery, and suggests that chronic mortality after the spill may meet or exceed the acute mortality experienced after the spill.
Lingering Oil Studies
Our work on the effects and recovery of sea otters from the Exxon Valdez spill continues. Criteria used to evaluate sea otter recovery include a return to pre-spill abundance and a condition where exposure to oil is no longer evident. Determining a return to pre-spill abundance is difficult, as pre-pill data are incomplete. However, in those locations in the Sound where mortality approached 90% we can use the number of carcasses recovered as a baseline to approximate abundance and gauge recovery. Recent survey data suggest that if recent trends continue recovery in these most affected areas could occur within a few years. We are now using gene expression techniques to evaluate the potential response of sea otters (and other species) to exposure to lingering oil. Recent work indicates that responses to oil at the cellular level may be diminishing in some species but that evidence of exposure to oil continues in some areas. As long as oil exists in the environment, there remains a probability that nearshore occupants will be exposed.
Determining the status of recovery of sea otters is now being integrated into the long term monitoring of the nearshore communities in Prince William Sound and elsewhere in the Gulf of Alaska. The Alaska Science Center, in cooperation with the National Park Service, the National Marine Fisheries Service, the University of Alaska and other partners, and with support from the Exxon Valdez Oil Spill Trustee Council, has established a nearshore ecosystem monitoring program that will identify recovery from the spill, provide improved preparation for future events, and will also provide a better understanding of the factors that influence the nearshore system more generally and may allow inference as to the cause of changes that will likely be observed over time.
Bodkin, J. L., B. E. Ballachey, H. A. Coletti, G. G. Esslinger, K. A. Kloecker, S. D. Rice, J. A. Reed, and D. H. Monson. 2012. Long-term effects of the Exxon Valdez oil spill: Sea otter foraging in the intertidal as a pathway of exposure to lingering oil. Marine Ecology Progress Series 447:273-287. doi: 10.3354/meps09523 [PDF file 443 kb]
Miles , A. K., L Bowen, B. E. Ballachey, J. L. Bodkin, M. Murray, J. A. Estes, R. A. Keister and J. L. Stott. 2012. Variations of transcript profiles between sea otters Enhydra lutris from Prince William Sound, Alaska, and clinically normal reference otters. Marine Ecology Progress Series. doi: 10.3354/meps09572
Monson, D. H., D. F. Doak, B. E. Ballachey, and J. L. Bodkin. 2011. Could residual oil from the Exxon Valdez spill create a long-term population “sink” for sea otters in Alaska? Ecological Applications 21:2917–2932. doi: 10.1890/11-0152.1
Bodkin, J. L., B. E. Ballachey, and G. G. Esslinger. 2011. Trends in sea otter population abundance in western Prince William Sound, Alaska. Progress toward recovery following the 1989 Exxon Valdez oil spill: U.S. Geological Survey Scientific Investigations Report 2011–5213, 14 p.
Peterson C. H., S. D. Rice, J. W. Short, D. Esler, J. L. Bodkin, B. E. Ballachey, and D. B. Irons. 2003. Long-term ecosystem response to the Exxon Valdez oil spill. Science 302:2082-2086. doi: 10.1126/science.1084282