Assessing oil-spill effects requires rigorous definitions of “impact” and “recovery.” Impact is defined as a statistically significant difference between samples exposed to oil and reference samples. Recovery is then the disappearance through time of such a statistical difference. Both impact and recovery must be assessed in relation to the background of natural variation that characterizes marine environments. There are three primary avenues of potential spill impacts on seabirds: on population size and structure, on reproduction, and on habitat occupancy and use. Detecting oil-spill effects involves comparisons of (1) observations taken following the spill with prespill data; (2) data gathered following the spill from oiled areas (“treatments”) and unoiled areas (“controls”) surveyed at the same time; or (3) measurements taken from sites along a gradient of oiling magnitude. The strengths and weaknesses of these approaches are discussed. In many situations, the third approach may be most useful.
Following the Exxon Valdez oil spill in March 1989, over 35 000 dead birds were retrieved. Model analyses suggested that actual seabird mortality could have been in the hundreds of thousands, prompting concerns about severe and persistent impacts on populations of several species, especially murres (Uria spp.). Recovery for some populations was projected to take decades. The findings of several studies conducted following the oil spill, however, indicate that these concerns may not be justified. These studies examined colony attendance and reproduction of murres as well as habitat utilization for the prevalent species in Prince William Sound and along the Kenai Peninsula. Surveys of attendance by birds at murre breeding colonies in 1991 indicated no overall differences from prespill attendance levels when colonies were grouped by the degree of oiling in the vicinity. At a large colony in the Barren Islands, where damage was described as especially severe, counts of murres were generally similar to historical estimates made in the late 1970s. In 1990 and 1991, murres breeding at the Barren Islands colony also produced young at levels that were within the range of natural (prespill) variation for this site. Incidental observations indicated that several other species reproduced successfully in oiled areas in Prince William Sound and along the Kenai Peninsula following the spill.
Investigations of habitat occupancy indicated that the majority of species analyzed showed no initial oiling impacts on their use of habitats. Of the species that did exhibit initial oiling impacts, many had apparently recovered by late 1991, when the study ended. In Prince William Sound, there were no consistent differences in ecological or life-history attributes between the species that suffered impacts and those that did not. Although most of the species that did not show clear evidence of recovery in habitat use by the end of this study were wintering and resident forms, other ecologically similar species were not affected or recovered rapidly. Consequently, the prognosis for recovery of the species that continued to show evidence of oiling impacts on habitat use in late 1991 would seem to be good.
Overall, these studies indicate that recovery in use of habitats by many seabird species, and in colony attendance and reproduction by murres, appeared to be well advanced by late 1991.