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EXECUTIVE SUMMARY Selenium contamination of the upper and lower Colorado River basin has been documented in water, sediment, and biota in studies by u.s. Department of the Interior agencies, and academia. Concern has been raised that selenium contamination may be adversely affecting endangered fish in the upper Colorado River basin. The objective of this study was to determine if environmental exposure of adult razorback sucker (Xyrauchen texanus) to selenium in flooded bottomland sites affects their survival, growth, and reproductive success. Adult razorback sucker (3 year old) were secured from Wahweap StatePish Hatchery, UT, and stocked at three sites: hatchery ponds at Horsethief Canyon State Wildlife Area (Horsethief; reference site assumed to have little or no selenium contamination), a diked tertiary channel at Adobe Creek (assumed to have a low amount of selenium contamination), and North Pond (assumed to have an elevated amount of selenium contamination) at Walter Walker State Wildlife Area (WWSW A), all near Grand Junction, CO. The young adults were from a single mating. Older brood stock (about 15 years old) held at Horsethief were used as an additional adult reference fish. Adults at Horsethief were fed a commercial diet, whereas fish at Adobe Creek and North Pond foraged on natural food items. Fish were tagged with a passive integrated transponder and stocked in July 1995 and collected for spawning in April 1996. The water source at Horsethief was the Colorado River, at Adobe Creek it was river water and irrigation drainage, and at North Pond it was a combination of ground water and irrigation supply water. Sampling stations were established at Horsethief (4 stations), Adobe Creek (7 stations), North Pond (3 stations) at .WWSW A, and the channel area at WWSW A (7 stations). On-site water quality measurements were collected weekly. Measurements collected at monthly inteJ;Vals included fish muscle plugs and weight and total length (sometimes at 60-day intervals), water samples for water quality and inorganic element analyses, and aquatic invertebrates for inorganic element analyses. Fish were collected using nets and electrofishing equipment. Sediment samples were collected twice (October 1995 and April.1996) for inorganic element analyses, carbon content, and texture analysis. Fish were collected from the three sites after the appropriate water temperatures were reached, and moved to tanks in a building at Horsethief. Fish were induced to spawn by injection with human chorionic gonadotropin hormone. Personnel of the Colorado River Fishery Project (CRFP) office, Orand Junction, CO, used standard methods to collect fish, induce spawning, and fertilize and water harden the eggs. Samples of eggs were collected for inorganic element analyses, and the remaining eggs moved to the 24-Road Fish Hatchery (24-Road) for hatching. After spawning, adults from the three sites were held at Horsethief for a 60-day depuration period. One or two fish from each site were sacrificed and selenium concentrations measured in various tissues and muscle plugs at spawning and at 30-day intervals during depuration. A 9-day egg study was conducted in a mobile laboratory housed in the garage at the CRFP office. One-hundred-fifty eggs were collected from six spawns of adults from Horsethief and Adobe Creek and four spawns from North Pond for hatchability determinations. Each group of eggs was stocked into six groups of egg incubator cups (each containing 25 eggs) that were suspended in 2-L exposure beakers filled with 1.2 L of either reference water from 24-Road (three groups) or site water (three groups). Fifty percent of the water was renewed daily. Once daily, the egg cups were removed and placed in a petri dish containing the same Xl11 I I I I I I I I I .1 I I I I I I I I I