Résumé
The population genetic structure of a species can be an important conservation tool informing us about the potential for genetic loss and the capacity for species recovery. Depending on the nature of population subdivision, it can also provide a means for assessing the source population of dead or injured individuals of unknown geographic origin. This type of information can be particularly useful in instances of large-scale environmental accidents, such as oil spills. Following the wreck of the ‘Erika’ oil tanker in the Bay of Biscay in December 1999, more than 80 000 seabirds were washed ashore along the west coast of France. The most heavily affected species (80% of all birds) was the common guillemotUria aalge, a widespread long-lived colonial seabird. In an attempt to evaluate the ‘true’ geographic extent of this accident, we carried out population genetic analyses using 6 microsatellite markers on samples from 22 breeding colonies in the North Atlantic and on individuals collected after the oil spill. A pattern of isolation by distance was detected among common guillemot populations, but populations were only weakly structured, even at large spatial scales. The low level of genetic differentiation between colonies prevented clear assignments of oiled birds to their population of origin using only the genetic information. The weak genetic structure suggests that little genetic variability was lost during the oil spill and implies a high potential for population recovery via dispersal. However, current gene flow among extant colonies may overestimate the capacity of locally extinct colonies to recover due to the behavioural processes involved in recruitment. Clearly, the mortality of tens of thousands of high-latitude seabirds due to oil pollution warrants ongoing scientific scrutiny and conservation effort.