Sea Otter Mortality Estimated from Carcasses Collected after the Exxon Valdez Oil Spill

Large numbers of sea otters ( Enhydra lutris) died following the 1989 Exxon Valdez oil spill. Two previous studies estimated spill‐related mortality, one from the difference between counts of otters before and after the spill, and the other from the recovery rate of tagged carcasses that had been released at sea. I used a derivative of the second approach, but revised it to account for (1) moribund otters that had hauled out on shore; (2) carcasses that were collected at sea; and (3) differences in search effort in different areas of the spill—considerations that were not addressed previously. The mortality estimation procedure presented here, and applied to Prince William Sound, Alaska, had six input parameters, two with fixed values (the number of spill‐related carcasses collected [391] and the number of otters taken alive that died in captivity [84]) and four that were assigned a range of values (the recovery rate for carcasses on the beach [60–90%], the proportion of recovered carcasses that were onshore [70–95%]), the proportion of offshore deaths recovered onshore (20–50%), and the proportion of moribund otters that hauled out (20–60%). Empirical estimates for these parameters were not available after the spill, but a range of plausible values for each was delimited using other available data. Randomly‐selected combinations of values within these ranges produced mortality estimates for the sound ( x¯= 750, 5–95% quantiles ≈ 600–1000) that were lower than indicated by previous studies. These lower estimates do not diminish the tragic nature of the mortality, but instead highlight the significance of both the process and informational basis in producing estimates of catastrophic loss. Estimates of loss based on faulty or inadequate data may mislead investigations of population recovery and may jeopardize public trust in scientific assessments of future catastrophes.