It has taken nearly 25 years, but scientists tracking the health of sea otters in western Prince William Sound have found enough evidence to consider the population recovered from the Exxon Valdez oil spill.
Numbers of sea otters counted on surveys of northern Knight Island, one of the most heavily oiled sites and important sea otter habitat, reached pre-spill levels as of 2012. In addition, normal mortality patterns have resumed. The third factor used to evaluate sea otter recovery, gene transcription rate, appears to be returning to normal but the data from 2012 make a final evaluation of this aspect difficult.
Knight Island’s sea otter population has been consistently monitored using aerial surveys since 1993, with numbers increasing at approximately 3 percent per year. The 2011-2013 abundance estimates indicate an increase of 850-2,200 sea otters since 1993 and Knight Islands counts are similar to the pre-spill population estimate.
In the years after the spill, high death rates in prime-age (2-8 year old) otters were a serious concern. The normal pattern for this species is to have the highest death rates among the very young and the very old. In the first few years after the spill, juvenile sea otter mortality was high. Then prime age mortality increased. As these are the animals that would be reproducing under normal circumstances, that abnormal mortality rate was considered an important factor in the population recovering so slowly.
Because there is a consistent link between certain types of gene transcription and oil-induced pathology, gene transcription has also been monitored. For much of the time following the oil spill, samples taken from otters found in oiled areas showed gene transcription patterns consistent with chronic, low-grade exposure to organic compounds. Given the persistence of oil in some areas of Prince William Sound, this is not surprising.
While results in 2012 showed transcription to be decreasing, the decrease in 2012 was found in all otters, whether from the oiled areas or not. This confounding factor will require further study but reduced gene transcription between 2008 and 2012 coincided with improvements in survival rate, making it likely that there is some other explanation for the 2012 results.
A copy of the complete report can be downloaded at US Geological Survey website.