AbstractA one‐generation reproductive toxicity study and a direct eggshell application toxicity study were conducted using the mallard (Anas platyrhynchos) to assess the toxicity of naturally weathered North Slope crude oil (WEVC), obtained following theExxon Valdezspill. In the reproductive study, birds were fed diets containing 0, 200, 2,000, and 20,000 mg of WEVC/kg diet. No significant differences (p≤ 0.05) in mortality, body weight, food consumption, reproductive parameters, or hatchling parameters were observed. Significant decreases in mean serum phosphorous, serum total protein, albumin, bilirubin, and calcium concentrations were observed in high‐dose‐group females; no differences were noted among males. Eggshell strength and thickness in the high‐dose group were significantly reduced compared to controls. Trends toward increased liver weights and decreased spleen weights were observed in WEVC‐treated birds. Applications of WEVC to developing eggs showed it to be less toxic than unweathered North Slope crude oil. Doses covering up to one‐third of the shell area of developing mallard eggs (92 mg) resulted in no effects on developing embryos. Eggs treated with the control material, petrolatum, were adversely affected by applications covering approximately one‐sixth of the eggshell (24 mg), suggesting inhibition of gas exchange. Neither material affected hatchling sur
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