The importance of both potency and mechanism in dose–response analysis: An example from exposure of Pacific herring (Clupea pallasi) embryos to low concentrations of weathered crude oil

► We reviewed a study of petroleum toxicity to Pacific herring embryos and larvae. ► The study did not show monotonic dose–response relationships and proof of causality. ► Reported effects at 0.4μg/L aqueous PAH levels were not supported by the data. ► A conclusion that weathering makes oil more toxic was not supported by the data. ► Confounding factors that could have influenced results are identified. This paper reanalyzes data from an earlier study that used effluents from oiled-gravel columns to assess the toxicity of aqueous fractions of weathered crude oil to Pacific herring embryos and larvae. This reanalysis has implications for future similar investigations, including the observance of two distinct dose–response curves for lethal and sublethal endpoints for different exposures in the same experiment, and the need to consider both potency and slope of dose–response curves for components of a toxicant mixture that shows potentially different toxicity mechanisms/causation. Contrary to conclusions of the original study, the aqueous concentration data cannot support the hypothesis that polycyclic aromatic hydrocarbons (PAHs) were the sole cause of toxicity and that oil toxicity increased with weathering. Confounding issues associated with the oiled gravel columns include changes in the concentration and composition of chemicals in exposure water, which interfere with the production of reliable and reproducible results relevant to the field.