Quantifying long-term risks to sea otters from the 1989 ‘Exxon Valdez’ oil spill: Comment on Bodkin et al. (2012)

Abstract

Bodkin et al. (2012; Mar Ecol Prog Ser 447:273−287) assessed the frequency at which sea otters Enhydra lutris might encounter subsurface oil residues from the 'Exxon Valdez' oil spill. They concluded that a pathway exists for exposures of sea otters to residual oil in the intertidal zone, and imply that this pathway has delayed recovery of sea otters. We agree that the potential exposure pathway exists, and the Bodkin et al. (2012) estimates of the frequency of encountering subsurface oil residues (4 to 10 times per year) comport with our previously published studies (2 to 7 times per year). However, we disagree that this pathway constitutes a significant risk to sea otters. We discuss results from our quantitative ecological risk assessment using an individualbased model that specifically simulated this pathway of exposures to a population of 500 000 sea otters. This conservative model predicted that assimilated doses of polycyclic aromatic hydrocarbons in subsurface oil residues to the 1-in-1000 th most-exposed sea otters would be 1 to 2 orders of magnitude below the chronic effects thresholds that we established using USEPA data and methodology. When we artificially increased the rate of encountering subsurface oil residues, it required 4 to 10 encounters per day to reach effects levels. We conclude that the subsurface oil residues from the oil spill could not plausibly be responsible for any individual-or populationlevel effect on the sea otters at northern Knight Island.

Quantifying long-term risks to sea otters from the 1989 ‘Exxon Valdez’ oil spill: Reply to Harwell & Gentile (2013)

“…It was not possible to quantify the exposure, and biological and ecological effects of long-term exposure are difficult to re solve and remain largely unknown (Bodkin et al 2012). Harwell & Gentile (2013) agree with our conclusion that a pathway of exposure to lingering 'Exxon Valdez' oil exists and generally agree with our estimates of rates of encountering oil. However, they disagree with our conclusion that this exposure constitutes a significant risk to sea otters.…”

“…Harwell et al (2010) concluded that there is no plausible risk of toxicological effects from oil exposure, but we find no plausible alternative to the conclusion that presence of lingering oil has been a constraint to recovery of species inhabiting nearshore areas that were heavily oiled in 1989 (Monson et al 2000, Bodkin et al 2002, Ballachey et al 2003, Miles et al 2012; see also Esler et al 2002, Golet et al 2002, Thomas et al 2007, Springman et al 2008. Other explanations for population depression described by Harwell & Gentile (2013) do not adequately explain the spatial distribution of the observed effects.…”

“…However, they disagree with our conclusion that this exposure constitutes a significant risk to sea otters. Harwell & Gentile (2013) reiterate results of their risk assessment model (Harwell et al 2010), which accounted for exposure through ingestion of PAHs, and conclude that a toxicological response by sea otters to chronic oiling is implausible and therefore lack of recovery of sea otters at Knight Island cannot be attributed to lingering oil.…”

“…These additional risk factors are not included in the laboratory-based exposure models considered in Harwell et al (2010) or Harwell & Gentile (2013).…”