A reassessment of the impact of the Exxon Valdez oil spill on harbor seals(Phoca vitulina richardsi) in Prince William Sound, Alaska

Analyses of population trends and movements of harbor seals in Prince Willi am Sound (PWS) casts doubt on published findings that 302 seals were killed by the Exxon Valdez oil spill in 1989. Assumptions that seals have 100% fi delity to a haul-out, that they were not displaced by the spill and associa ted disturbances, and that population trends throughout PWS varied similarl y, except for oil spill effects, are not supported. Survey efforts to accou nt for missing seals in 1989 were incomplete, too late in the year, and geo graphically limited. Basic assumptions required for statistical comparisons of oiled and unoiled haul-outs were violated. Fourteen dead seals, mostly pups, were recovered in PWS. Cause of death in most instances could not be determined, nor could the proportion that would have died naturally. Eviden ce does not support high unsubstantiated mortality, but is more consistent with seals avoiding or moving away from some oiled haul-outs. Interpretatio n of survey results requires consideration of temporal and regional variati on. "Route A" surveys of central and eastern PWS do not represent populatio n trends in western PWS or at glacial haul-outs. To adequately monitor popu lation trends of PWS as a whole, broader sampling must be conducted on a ro utine basis.