Acute toxicity of copper to three species of Pacific salmon fry in water with low hardness and low dissolved organic carbon.

Proposed development of a mine within Alaska's Bristol Bay watershed has raised concerns about the potential impact of copper (Cu) on Pacific salmon (Oncorhynchus spp.). We conducted 96-hour flow-through bioassays using low-hardness and low-dissolved organic carbon water to determine the acute lethal toxicity of Cu to sockeye (O. nerka), Chinook (O. tshawytscha), and coho salmon (O. kisutch) fry. We aimed to determine Cu toxicity under field-relevant water quality conditions and to assess three methods of calculating ambient Cu criteria: the USEPA-endorsed biotic ligand model (BLM), a multiple linear regression model, and the hardness-based model currently used by the State of Alaska. The criteria generated by all models were below 20% lethal Cu concentrations by factors ranging from 2.2 to 54.3, indicating that all criteria would be protective against mortality. The multiple linear regression-based criteria were the most conservative and comparable to BLM-based criteria. The median lethal concentrations (LC50s) for sockeye, Chinook, and coho, were 35.2, 23.9, and 6.3 µg Cu/L, respectively. We also used the BLM to predict LC50s for each species. Model-predictions differed from empirical LC50s by factors of 0.7 for sockeye and Chinook salmon, and 1.1 for coho salmon. These differences fell within the acceptable range of ± 2, indicating the model's accuracy. We calculated critical lethal Cu accumulation values for each species to account for differing water chemistry in each bioassay, revealing that coho salmon were most sensitive to Cu, followed by sockeye, and Chinook salmon. Our findings underscore the importance of considering site- and species-specific factors when modeling Cu toxicity. The empirical data presented in this study may enhance Cu risk assessments for Pacific salmon.