SEA OTTER MORTALITY ESTIMATED FROM CARCASSES COLLECTED AFTER THE EXXON-VALDEZ OIL-SPILL

Large numbers of sea otters (Enhydra lutris) dies following the 1989 E xxon Valdez oil spill. Two previous studies estimated spill-related mo rtality, one from the difference between counts of otters before and a fter the spill, and the other from the recovery rate of tagged carcass es that had been released at sea. I used a derivative of the second ap proach, but revised it to account for (1) moribund offers that had hau led out on shore; (2) carcasses that were collect at sea; and (3) diff erences in search effort in different areas of the spill-consideration s that were not addressed previously. The mortality estimation procedu re presented here, and applied to Prince William Sound, Alaska, had si x input parameters, two with fixed values (the number of spill-related carcasses collected [391] and the number of otters taken alive that d ied 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 mori bund otters that hauled out (20-60%). Empirical estimates for these pa rameters were not available after the spill, but a range of plausible values for each was delimited using other available data. Randomly-sel ected combinations of values within these ranges produce mortality est imates for the sound ((x) over bar = 750, 5-95% quantiles approximate to 600-1000) that were lower than indicated by previous studies. These lower estimates do not diminish the tragic nature of the mortality, b ut instead highlight the significance of both the process and informat ional 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 asse ssments of future catastrophes.