Effects of the Exxon Valdez Oil Spill on Sea Otters

George Esslinger examining a dead sea otter on the beach

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.