Ecological risk assessment is a systematic way to evaluate the likelihood that an environmental accident has caused significant ecological consequences. I apply this framework retrospectively to evaluate a scenario linking the Exxon Valdez oil spill to population effects on harlequin ducks (Histrionicus histrionicus) through hydrocarbon contamination of mussels in spill-affected shorelines of Prince William Sound, Alaska. By evaluating the plausibility of each step of this scenario in turn, it becomes apparent that it is highly unlikely the oil spill is having continuing effects on harlequins through this pathway. This case study shows how ecological risk assessment can help clarify potential cause–effect relationships in an emotionally and socially charged situation.