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Se Concentrations in Colorado River R lucius was performed to determine how change in flow affected water selenium concentrations at 18 different flows in the Colorado River during 1994 and 1995. Results and Discussion Quality Control-Selenium by Neutron Activation Accuracy and precision of the irradiation method were esti- mated in two ways (Table 1). For an independent blind accuracy check, two samples of tissue reference or research material (-30 mg each) were prepared, sealed, and submitted with the 1994 samples to MURR. One sample of a tissue research material (whole-body striped bass, 92 mg) was submitted with the 1995 samples. Secondly, from 1994-1996, MURR con- ducted its own accuracy and method precision check by the irradiation of samples of National Institute of Standards and Technology (NIST) bovine liver SRM 1577. Results from 1994-1996 for these analyses are given in Table 1. Recoveries of selenium from all of the tissue materials were deemed successful, and results indicated that accuracy and precision were within the recommended or certified range. Method precision, determined from NIST bovine liver tissues analyzed by MURR was 3.1% relative standard deviation (% RSD) for 1994 samples, 2.7% RSD for 1995 samples, and 3.6% RSD for 1996 samples. The limit of detection for the Se77m method, based on a 30-mg sample weight, was 0.015 pg/g selenium. Difference Between Grand Valley Locations During 1994 Lemly (1996) recommended the following tissue residues of selenium to be used as levels for concern (i.e., toxic threshold) for the health and reproductive vigor of freshwater fish: whole body = 4 µg/g dry weight (DW); muscle tissue = 8 µg/g DW. Fifteen of 39 total muscle plug samples taken from Colorado pikeminnow captured in the Grand Valley during 1994 con- tained selenium concentrations that exceeded the toxic thres- hold of 8 µg/g DW for muscle tissue (Table 2). Of these 15 samples, 12 were taken from fish captured from the backwater area at WWSWA at rkm 263.1 (Table 2; Figure 1). The 16 samples collected from fish at WWSWA during 1994 contained a mean selenium concentration of 16.6 µg/g DW, over twice the recommended guideline concentration (Table 2). The mean selenium concentration of 16.6 µg/g DW at WWSWA was over twice as high as the mean selenium concentration of the other plugs collected in the Gunnison River-to-Loma stretch of the Colorado River, and over three times as high as the mean selenium concentration of those collected in the Colorado River above the Gunnison River confluence (Table 2; Figure 1). Stephens and Waddell (1998) reported a similar situation in the Green River, Utah. Carp collected near Ashley Creek, a high-selenium site with water concentrations ranging from 25 to 73 pg/L, contained higher selenium concentrations than carp collected both upstream and downstream. The riverine back- water at WWSWA comprise a primary staging area for Colo- rado pikeminnow during spring runoff, 1-2 months prior to their summer spawning season, and may be used as spawning sites by a remnant population of razorback sucker while flooded during spring (McAda 1977; Valdez and Wick 1983). Compari- 481 sons of these muscle tissue concentrations with toxicological effects from selenium on other fish species (Lemly 1996) suggested that endangered fish staging at WWSWA probably deposit enough selenium in their eggs prior to spawning to adversely affect reproductive success by reducing survival of larvae. Colorado pikeminnow containing high selenium concen- trations in muscle tissue were also collected from other Colorado River locations. These fish may have accumulated selenium from WWSWA earlier or accumulated selenium from other high-selenium locations along the Colorado River. Other Selenium Studies with Endangered Colorado River Fish Eggs collected from three female razorback suckers and milt from five male razorback suckers captured in the Green River of northeastern Utah contained selenium concentrations elevated above control fish from other laboratory studies, but were below those of selenium-exposed fish demonstrating reproductive problems (Hamilton and Waddell 1994). Selenium concentra- tions of 3.7-10.6 µg/g DW in the eggs were believed by Hamilton and Waddell (1994) to be adversely affecting razor- back sucker reproduction. Concentrations of selenium in razor- back sucker eggs and milt were significantly correlated with selenium concentrations in muscle plugs taken from the same fish. The highest selenium concentrations in muscle plugs of these razorback suckers were within the range of those we found in Colorado pikeminnow at WWSWA in Colorado: ?20 gg/g selenium DW. Hamilton and Waddell (1994) cited a personal communication from Bruce Haines (U.S. Fish and Wildlife Service, Vernal, UT) who collected a small number of fertilized eggs from three pairings of the wild razorback suckers and attempted to hatch these eggs using techniques that had proved successful in previous years. All three spawns failed to hatch. Although a cause-and-effect conclusion cannot be made, the elevated selenium concentrations in eggs may have played a role in the failure of eggs to hatch. Another ongoing study (S. Hamilton, USGS, personal com- munication) is investigating the effects of selenium on razor- back sucker reproductive success at WWSWA. Razorback sucker adults were held prior to spawning at three different sites along the Colorado River, each with different water selenium concentrations. Preliminary results indicated that concentra- tions of 12-17 µg/g DW in muscle of female razorback suckers was associated with adverse effects on larvae produced by that female, even when larvae were maintained in clean water and fed clean diets. Colorado pikeminnow, captured from WWSWA contained similar selenium residues, high enough that adverse affects on their offspring might be expected. Annual Variation in Selenium Concentrations in Colorado Pikeminnow from WWSWA The elevated selenium concentrations in Colorado pikeminnow, particularly at WWSWA during 1994, were cause for concern. Therefore, more muscle plugs were taken from Colorado pikeminnow staging at WWSWA during 1995. Of 52 total plugs taken from 45 fish at WWSWA during 1995, 11 were from fish recaptured from 1994 (Tables 3 and 4). Selenium concentra-