The toxic properties of naturally weathered Exxon Valdez crude oil (WEVC) to avian and mammalian wildlife species were evaluated using the surrogate species, mallard duck, Anas platyrhynchos, and European ferret, Mustela putorius. This study was conducted to evaluate the potential for toxic (rather than physical) injury to wildlife species that may have been exposed to WEVC, either through external contact or through dietary uptake. Previous studies have assessed the toxicity of unweathered crude oils, including Alaskan North Slope Crude, but little information exists regarding the toxicity of a naturally weathered crude oil, typical of that encountered following a spill.

A battery of laboratory toxicity tests was conducted, in compliance with standard and published test procedures, to evaluate acute and subchronic toxicity of WEVC. These included tests of food avoidance, reproductive effects, and direct eggshell application toxicity. Naturally weathered EVC, recovered postspill from Prince William Sound, was used as the test material. No treatment-related mortalities or gross signs of toxicity were noted in ducks at oral doses of 5 000 mg/kg body weight or at dietary concentrations up to 100 000 ppm (mg WEVC/kg diet), or in ferrets at oral doses up to 5 000 mg/kg body weight. Test animals did not avoid oil-contaminated food. No significant adverse effects were noted in birds fed diets containing up to 2 000 mg WEVC/kg diet in the reproductive toxicity test. However, significant reductions in mean eggshell thickness and strength were observed in birds fed 20 000 mg WEVC/kg diets. Applications of up to 92 mg/egg of WEVC to developing eggs produced no toxic effects.

Feeding weathered EVC to ducks and ferrets at oral doses or dietary concentrations exceeding those representing maximum likely field exposures from heavily affected spill areas did not significantly affect survival, growth, or reproduction. These results suggest that naturally weathered EVC posed little toxic risk to wildlife consuming oil or oiled food items following the Exxon Valdez spill, particularly considering the environmental exposure conditions that existed in the spill-affected area after 1989.