There is a paucity of studies focusing on how total mercury (THg) and total selenium (TSe) concentrations vary in walleye tissues and prey organisms from a reservoir that is downstream from a major mining district and whose basin consists of Cretaceous sedimentary Se-bearing outcroppings. Pueblo Reservoir in Colorado is fed by water from the upper Arkansas River, which has been polluted by trace metals and metalloids since mining began in 1859. Pueblo Reservoir is the only mainstem impoundment on the Upper Arkansas River and is located about 251 km downstream from the historic Leadville Mining District. Nearly all the contaminated colloids and sediment eventually reach Pueblo Reservoir and its walleye fishery. We compared THg and TSe concentrations among 15 walleye tissues clustered within "functional tissue groups,"and with walleye stomach contents and the walleye's primary forage fish (gizzard shad). We found that THg concentration in epaxial muscle and cardiac muscle ranked first and second, respectively, and results indicated a significant difference in THg between the 2 muscle types. Unlike THg, TSe levels were highest for liver, spleen, kidney, ovary/egg, and gill tissues, and TSe levels in all but 2 tissue pairings were significantly different. Significant differences occurred when THg and TSe in gizzard shad and stomach contents were compared to levels in each tissue. Our THg concentrations at the Pueblo Reservoir were generally lower than at other sites in western North America; however, TSe concentrations were higher and markedly different. The data indicate that THg tissue levels appear to be lower than established fish health benchmarks, yet many TSe tissue concentrations were above critical thresholds.
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