COPPER, CADMIUM, AND ZINC CONCENTRATIONS IN JUVENILE CHINOOK SALMON AND SELECTED FISH-FORAGE ORGANISMS (AQUATIC INSECTS) IN THE UPPER SACRAMENTO RIVER, CALIFORNIA MICHAEL K. SAIKI 1∗ , BARBARA A. MARTIN 1 , LARRY D. THOMPSON 2 and DANIEL WELSH 2 1 U.S. Geological Survey, Biological Resources Division, Western Fisheries Research Center, Dixon Duty Station, 6924 Tremont Road, Dixon, California 95620, U.S.A.; 2 U.S. Fish and Wildlife Service, Sacramento Fish and Wildlife Office, 2800 Cottage Way, Suite W-2605, Sacramento, California 95825, U.S.A. ( ∗ author for correspondence, e-mail: michael_saiki@usgs.gov) (Received 11 April 2000; accepted 11 November 2000) Abstract. This study assessed the downstream extent and severity of copper (Cu), cadmium (Cd), and zinc (Zn) contamination from acid mine drainage on juvenile chinook salmon (Oncorhynchus tshawytscha) and aquatic insects over a roughly 270-km reach of the Sacramento River below Keswick Reservoir. During April–May 1998, salmon were collected from four sites in the river and from a fish hatchery that receives water from Battle Creek. Salmon from river sites were examined for gut contents to document their consumption of various invertebrate taxa, whereas salmon from river sites and the hatchery were used for metal determinations. Midge (Chironomidae) and caddisfly (Trichoptera) larvae and mayfly (Ephemeroptera) nymphs were collected for metal determinations during April–June from river sites and from Battle and Butte creeks. The fish hatchery and Battle and Butte creeks served as reference sites because they had no history of receiving mine drainage. Salmon consumed mostly midge larvae and pupae (44.0%, damp-dry biomass), caddisfly larvae (18.9%), Cladocera (5.8%), and mayfly nymphs (5.7%). These results demonstrated that insects selected for metal determinations were important as fish forage. Dry-weight concentrations of Cu, Cd, and Zn were generally far higher in salmon and insects from the river than from reference sites. Within the river, high metal concentrations persisted as far downstream as South Meridian (the lowermost sampling site). Maximum concentrations of Cd (30.7 μgg -1 ) and Zn (1230 μgg -1 ), but not Cu (87.4 μgg -1 ), in insects exceeded amounts that other investigators reported as toxic when fed for prolonged periods to juvenile salmonids. Keywords: Chironomidae, Ephemeroptera, fish gut contents, heavy metals, Oncorhynchus tshaw- ytscha, Trichoptera 1. Introduction For over a century, fish kills from metal pollution have occurred in the Sacra- mento River at or below the mouth of Spring Creek, a small tributary that re- ceives runoff from Iron Mountain Mine in Shasta County, California (Finlayson and Wilson, 1979). In 1963, the Spring Creek Debris Dam (which forms Spring Water, Air, and Soil Pollution 132: 127–139, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.