The toxicity of two organoselenium diets was evaluated in 90‐ to 120‐d partial life cycle tests with two life stages of chinook salmon (Oncorhynchus tshawytscha Walbaum). One of the diets contained fish meal made from high‐selenium mosquitofish (Gambusia affinis Baird and Girard) collected from the selenium‐laden San Luis Drain, California (here termed SLD diet) and the other contained meal made from low‐selenium mosquitofish (collected from a reference site) fortified with selenomethionine. A 90‐d study was conducted with swim‐up larvae in a water‐simulating dilution of San Luis Drain water in a standardized fresh water; and a 120‐d study was conducted with fingerlings 70‐mm long in a water of similar quality but prepared with a standardized brackish water. After 90 d of exposure in the freshwater study, survival was reduced in fish fed ≥9.6 μg Se/g of either diet, and growth was reduced in fish fed ≥5.3 μg Se/g of SLD diet or ≥18.2 μg Se/g of selenomethionine diet. Reduced fish growth, whole‐body concentrations of selenium and survival were strongly correlated to concentrations of selenium in both diets. After 120 d of exposure in the brackish‐water study, survival was unaffected but growth was reduced in fish fed ≥18.2 μg Se/g of SLD diet or 35.4 μg Se/g of selenomethionine diet. After 120 d of dietary exposure, survival during a 10‐d seawater challenge test was reduced in fish fed 35.4 μg Se/g of either diet. In this second dietary study, concentration—response relations were observed in both dietary treatments between the dietary concentrations of selenium and all three characteristics — fish growth, whole‐body concentrations of selenium and survival in seawater.

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