Resolving allegations of oil damage to incubating pink salmon eggs in Prince William Sound
The Exxon Valdez Oil Spill Trustee Council concluded that oil caused mortality of pink salmon (Oncorhynchus gorbuscha) eggs in Prince William Sound streams. Their conclusion was based primarily on Alaska Department of Fish and Game (ADF&G) studies which reported that mean mortality of embryos in eggs was higher in oiled than non-oiled streams when sampled shortly after spawning completion. However, developing embryos are vulnerable to shock mortality for a period of 20 days after fertilization, and the embryos in eggs from the latest spawners were still in the sensitive period at the time sampling took place. We argue that the original ADF&G analysis should have included sample timing in statistical comparisons of mortality between streams. Analysis of a subset of the ADF&G data showed that sampling shock was a major source of embryo mortality in these samples, and that source of mortality in the original survey would likely have been mistakenly interpreted as an oiling effect. Compensating for sample timing removed all statistical evidence for an oiling effect in the data subset. We conclude that the ADF&G study design confounded the ability to assess for the effect of oil exposure on pink salmon eggs.