Pacific salmon species including the U.S. federally endangered coho salmon (Oncorhynchus kisutch) and the U.S. federally threatened steelhead trout (Oncorhynchus mykiss) have declined at an alarming rate in the last 40 years. Two of the main causes for the decline in coastal coho populations include increases in temperature and contaminant loads in coastal watersheds. Zinc, in particular, is one of the most common contaminants in aquatic systems. Using an experimental mesocosm design, we examined physiological, biochemical, and behavioral responses of coho salmon to excess dietary zinc and increased temperatures, with the ultimate goal of relating results to wild populations of coho salmon and steelhead in the Navarro River, California. Fish were obtained from a hatchery and divided into four treatments: low water temperature-no dietary zinc, high temperature-no zinc, low temperature-zinc, and high temperature-zinc. Each treatment had four replicate tanks. Zinc concentrations in liver increased during exposure to a high zinc diet. Iron concentrations in liver increased during simultaneous exposure to high zinc diet and increased temperature, and growth was reduced in this experimental treatment. Expression of hsp-70 was not significantly different between treatments, but showed decreasing trends with high dietary zinc and high temperature. Feeding rate increased with exposure to a high zinc diet. Comparison with steelhead trout samples from the Navarro River, California, showed levels of zinc, iron, and hsp-70 greater than those found in the experimental Coho salmon. All comparisons between the hatchery coho salmon and wild steelhead should be viewed with caution due to the differences between species, the laboratory and natural environment, and the genetic differences between wild and hatchery fish.
