- The 1989 Exxon Valdez oil spill was a major perturbation of nearshore habitats of Prince William Sound, a wintering area for harlequin ducks (Histrionicus histrionicus). This research was designed to evaluate harlequin duck population recovery from the oil spill from 1995 to 1998, using a demographic approach to assess both the "product" of population changes (e.g., density differences; Chapter 4) and the demographic "processes" affecting population dynamics (e.g., survival; Chapter 3).
Wintering aggregations of harlequin ducks are core population units from a population structure perspective and are largely demographically independent, due to high rates of winter site fidelity, pair formation during winter, and evidence that juveniles follow hens to wintering areas. Therefore, evaluation of population-level effects of the oil spill is appropriately directed at these wintering aggregations. We selected adult female survival as a primary demographic attribute to measure, as population dynamics of animals with life history characteristics like harlequin ducks are particularly sensitive to variation in adult female survival rates. Also, we speculated that oil spill effects during winter would be manifested as survival reductions.
We found that winter densities of harlequin ducks were lower on oiled study areas than unoiled areas, after accounting for effects of habitat (Chapter 4), indicating that population recovery had not occurred. Also, we determined that adult female survival was lower on oiled areas than unoiled areas (Chapter 3), and speculated that this was related to documented oil exposure of harlequin duck populations in oiled areas coincident with our study. We also confirmed that our survival estimates were not biased by assumptions about effects of radio transmitters or fate of missing radios (Chapter 2). In Chapter 5, we reviewed all data relevant to harlequin duck recovery from the oil spill, including our demographic data, and concluded that population recovery had not occurred by 1998, deleterious effects persisted, and continued oil exposure likely was the primary constraint to full recovery. Although populations of many species may have recovered quickly from the Exxon Valdez spill, characteristics of harlequin ducks make their winter populations particularly susceptible, including their habitat associations, diet, life history. and energetics.