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Show habitats would be determined. The use of shoreline vegetation as fish habitat would be investigated, and this use correlated with water levels. Finally, cooperative programs to share information and coordinate aquatic habitat management would be established ( or expanded) with other agencies. 111. J. Heavy Metals in Fish Flesh III. J. 1. Statement of the problem. Because of its position on the Colorado River, Lake Powell is trapping sediments from upstream watersheds. Certain heavy metal ions attached to these sediments are also moving into the lake ( Graf 1985). With the exception of a small amount remaining in suspension, the metals are being deposited with the sediments in the upper reaches of the lake near the inflow of the Colorado and San Juan Rivers. Among the trace metals found in the lake environment, mercury and selenium are of the greatest concern because of their persistence and toxicity in small amounts. Mercury is presumed to be from natural sources because of the geochemistry of certain rock formations found upstream in Utah and Colorado. Graf ( 1985) believes that Lake Powell is a regional sink for heavy metals, especially mercury, and that almost all of the mercury is derived from weathering and erosion of rock formations of the Colorado Plateau. While data are limited, studies by Potter et al. ( 1975) indicate that mercury concentrations are being bioamplified in fish. ( Bioamplification is the process in which trace element concentrations in animal tissue increase with successively higher levels of the food chain. In reservoirs, larger individuals of predatory fish are more likely to have the highest trace element content.) In certain species, concentrations approaching Food and Drug Administration maximum standards for human consumption are sometimes reached. A significant problem in the management of the Lake Powell fishery would occur if mercury content above 500 parts per billion ( ppb) is confirmed as common in the muscle tissues of large game fish. Potter et al. ( 1975) analyzed samples from Lake Powell and reported yields of mean mercury levels of 0.01 parts per billion ( ppb) in water, 30 ppb in bottom sediments, 10 ppb in shoreline substrates, 34 ppb in plant leaves, 145 ppb in plant debris, 28 ppb in algae, 10 ppb in crayfish, and 232 ppb in fish muscle. Mercury concentrations increased with increased body weight and higher levels in the food chain. Muscle tissue of some large fish ( over 2 kg whole body weight) exceeded 500 According to analyses of whole fish conducted by the U. S. Fish and Wildlife Service from 1970 to 1984, mercury levels in largemouth bass and rainbow trout taken from Lake Powell range from 50 to 490 ppb. It should be noted that in 1975 the reservoir was filling, and the above data reflect " new water" conditions. 46 |