Assessing Risks to Sea Otters and the Exxon Valdez Oil Spill: New Scenarios, Attributable Risk, and Recovery

Abstract

The Exxon Valdez oil spill occurred more than two decades ago, and the Prince William Sound ecosystem has essentially recovered. Nevertheless, discussion continues on whether or not localized effects persist on sea otters (Enhydra lutris) at northern Knight Island (NKI) and, if so, what are the associated attributable risks. A recent study estimated new rates of sea otter encounters with subsurface oil residues (SSOR) from the oil spill. We previously demonstrated that a potential pathway existed for exposures to polycyclic aromatic hydrocarbons (PAHs) and conducted a quantitative ecological risk assessment using an individual-based model that simulated this and other plausible exposure pathways. Here we quantitatively update the potential for this exposure pathway to constitute an ongoing risk to sea otters using the new estimates of SSOR encounters. Our conservative model predicted that the assimilated doses of PAHs to the 1-in-1000th most-exposed sea otters would remain 1-2 orders of magnitude below the chronic effects thresholds. We re-examine the baseline estimates, post-spill surveys, recovery status, and attributable risks for this subpopulation. We conclude that the new estimated frequencies of encountering SSOR do not constitute a plausible risk for sea otters at NKI and these sea otters have fully recovered from the oil spill.

Keywords: Enhydra lutris; Exxon Valdez oil spill; attributable risk; ecological recovery; ecological risk assessment; individual-based models; sea otter.

Figure 1.

Map of Prince William Sound, Alaska. The location of the Exxon Valdez oil spill is indicated by the red star; the location of the 1964 Great Alaska Earthquake (NRC 1971) is indicated by the black star. Shoreline oiling locations and initial oiling levels were derived from the Shoreline Cleanup & Assessment Technique (SCAT) II surveys conducted by Exxon (1989) following the oil spill. GIS data and map produced by Allison Zusi-Cobb, ABR Inc., Environmental Research and Services, Fairbanks, Alaska. (Color figure available online.)

Figure 2.

Sea otter exposure pathways conceptual model. Exposure pathways conceptual model for sea otters at NKI from PAHs in SSOR, showing: (a) the release mechanism of direct exposure to PAHs by intersection of SSOR by a sea otter pit, including ingestion of both oil-phase and particulate-bound PAHs; (b) the release mechanisms of indirect exposure to PAHs through ingestion of PAHs in the diet. Reprinted from Harwell et al. (2010a) by permission of the publisher (Taylor & Francis Group, LLC). (Color figure available online.)

Figure 3.

Hazard quotients versus number of SSOR encounters per day. HQ values are for simulations of 500,000 adult female sea otters exposed to 4–6-ring PAHs through encounters with SSOR. (HQ values are not shown here but were lower for both 2–3-ring PAHs and TPAH.) The horizontal red line shows the chronic effects threshold (i.e., HQ = 1), above which effects would be expected and below which no effects would be expected. NOAEL and LOAEL HQ values are shown for three selected quantiles (99.9%, 99%, and 95%); actual effects would be expected to occur between the NOAEL and LOAEL TRV values. Selected rates of encountering SSOR are shown, including the minimum, mean, and maximum rates predicted by Bodkin et al. (2012) and the probabilities of encountering SSOR predicted by the Harwell et al. (2010a) base model. Also shown is the range of SSOR encounters per day that would be required in order to cause effects for 4% of the population (i.e., the 96% quantile). (Color figure available online.)

Figure 4.

Pre-oil spill survey of sea otters at NKI, with the survey subsets and data reported by Pitcher (1975). The thick lines delineate the segments totaling a count of 237 sea otters, the value reported in Dean et al. (2000) as a pre-spill baseline count for NKI. The dotted lines delineate the area reported for all post-spill surveys by Dean et al. (2002), Bodkin et al. (2011), and Garshelis and Johnson (2013b). The Pitcher (1975) survey within the dotted lines totaled 105 sea otters, one of the best-available pre-spill estimates of NKI abundance.

Figure 5.

Sea otter abundances at (A) northern Knight Island and (B) western Prince William Sound (WPWS) following EVOS. Data are from Bodkin et al. (2011) based on the aerial surveys that they conducted for the two areas, showing all of the years for which their surveys were reported. Shown are sea otter abundance and standard error (s.e.) values. (No s.e. was reported for NKI in 1993 and 1994 as only a single survey was made during those years; no data were reported for NKI in 2006 or for WPWS in 2001 and 2006.) For comparison, the red stars indicate the two best-available estimates of the pre-oil spill sea otter abundance at NKI: 105 derived from the helicopter-based survey in 1973 by Pitcher (1975); and 58 derived by Garshelis and Johnson (2013b) from the data collected in the 1983–1984 boat-based surveys by Irons et al. (1988). (Color figure available online.)