• Exxon Valdez oil spill;
  • harbor seal;
  • Phoca vitulina richardsi;
  • population monitoring;
  • mortality;
  • Prince William Sound


Analyses of population trends and movements of harbor seals in Prince William 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% fidelity to a haul-out, that they were not displaced by the spill and associated disturbances, and that population trends throughout PWS varied similarly, except for oil spill effects, are not supported. Survey efforts to account for missing seals in 1989 were incomplete, too late in the year, and geographically 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. Evidence does not support high unsubstantiated mortality, but is more consistent with seals avoiding or moving away from some oiled haul-outs. Interpretation of survey results requires consideration of temporal and regional variation. “Route A” surveys of central and eastern PWS do not represent population trends in western PWS or at glacial haul-outs. To adequately monitor population trends of PWS as a whole, broader sampling must be conducted on a routine basis.