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SELENIVv[ IN RAZORBACK SLCKERS IN GREEN RIVER. UTAH [~l Selenium concentrations in eggs could result in slightly higher concentrations in hatched larvae because the chorion membrane probably would not contribute much selenium to the egg, yet would contribute mass. Selenium in fish eggs is carried as part of the yolk precursor proteins lipovitellin and phosvitin and is incorporated into egg immunoglobulin and vitellogenin and transferred to yolk molecules (Kroll and Doroshov, 1991). The selenium in larvae in the present study may also have come from waterborne and dietary uptake of selenium. Although larvae were probably less than a month old at the time of capture by light trap, they would have been exposed to selenium and other inorganic elements in water since hatching, and in food organisms since initiation of feeding at about 4-5 days old. Several reports have documented elev- ated selenium concentrations in water and biota at Brush Creek, Stewart lake Drain, Ashley Creek, Sportsman's Lake, and Sheppard Bottom, which all have contributed selenium loading to the Green River (Stephens et al., 1988, 1992; Peltz and WaddelL 1991; Waddell and Wiens, 1994a). This selenium loading has resulted in elevated selenium in adult razorback sucker and contributed to the exposure of larvae and their concomitant accumulation of selenium. Selenium Hazards to Razorback Sucker Selenium concentrations in larvae from the present study were higher than concentrations in fish from control treat- ments in laboratory studies with water, diet, or combined water and diet exposures, or reference sites in field studies (0.4-2.0Ilg/g; Table 3). The geometric inean for selenium reported in the National Contaminant Biomonitoring Pro- gram (NCBP) for 315 composite samples of whole-body adult fish (47 taxa) collected from 109 stations nationwide in late 1984 to early 1985 (Schmitt and Brumbaugh, 1990) was 1.6 Ilg/g and the 85th percentile was 2.8 Ilg/g. The 85th percentile is an arbitrary value used to identify values that are substantially above the nationwide median and possibly of concern, although the 85th percentile concentration has no toxicological significance. Selenium concentrations of about 4 Ilg/g or more in whole-body of young fish exposed through dietary or waterborne exposures have been asso- ciated with adverse effects (Table 4), Although some of the waterborne exposures may seem high in Table 4, the main point of the table is the values for whole-body residue and the resulting adverse effect. Once whole-body selenium reaches a toxic threshold concentration (i.e., 4 Ilg/g), regard- less of exposure route, adverse effects will occur. Based on the literature given in Table 4 and information from several other laboratory and field studies with a variety of fish species, lemly (1993) recommended that whole-body resi- dues of selenium in fish of 4Ilg/g, regardless of exposure route, be taken as the toxic threshold for adverse effects. This toxic threshold was equaled or exceeded in the residues in larval razorback sucker collected from the five sites in the present study (Table I). One of the cited studies in Table 4 involved feeding larval razorback sucker selenium-laden zooplankton collected from sites in Sheppard Bottom at Ouray NWR (Hamilton et al., 1996). In that study, 5-, 10-, 24-. and 28-day-old larvae tested in four experiments experienced nearly complete mortality in 20-25 days after feeding on zooplankton from three to six sites with selenium concentrations in zooplank- ton ranging from 2.3 to 96Ilg/g. Whole-body residues in these fish ranged from 3.6 to 94 Ilgjg. The range of residues in larvae demonstrates the variation that can occur in sur- viving fish, but from a toxicological standpoint, adverse effect concentrations are always linked with the lowest treat- ment or residue concentration associated with the observed adverse effect (Rand and Petrocelli, 1985). In the Ouray study, mortality occurred concurrently in larvae fed zo- oplankton from the six sites, which suggests that a toxic threshold was exceeded. The concentrations of selenium in zooplankton and larvae from the three least selenium con- taminated sites (2.3 to 4.5 Ilg/g in zooplankton and 3.6 to 14.3Ilg/g in larvae) were close to or higher than the toxic thresholds (3 Ilg/g in diet and 4 Ilg/g in whole-body) pro- posed by Lemly (1993) and supported by the results of studies summarized in Table 4. As with any normal, bell-shaped distribution of responses in a group of individuals to a stimulus, some individuals will be adversely affected at low levels of stimulus, i.e., selenium residues less than the mean response, whereas some indi- viduals will not be adversely affected until levels of stimulus are higher than the mean, i.e., selenium residues greater than the mean response. One misconception of threshold concen- trations is that once a threshold is exceeded, all organisms are adversely affected on an equal basis. Toxic thresholds are usually based on the response of the most sensitive species tested, but there may be other untested species with greater sensitivity and others with less sensitivity to a stres- sor. Likewise, within a species, sensitivity is usually greatest in very young life stages, and within a life stage, sensitivity will vary among individuals. Also, threshold values may be affected by other inorganic or organic contaminants or synergistic or antagonistic interactions between con- taminants. Consequently, threshold values should be used with caution. Some fish in the Ouray study (Hamilton et al., 1996) died with lower whole-body concentrations of selenium than selenium concentrations in live, wild larvae collected in the present study (assumes that fish drying in a treatment have the same toxicant concentration as fish still alive in that treatment). Sato et al. (1980) reported that selenium concen- trations in dead carp (e. carpio) from two experiments with waterborne selenium were lower than those in live fish sampled during the experiments, which shows the difference in sensitivity that can occur between individuals of a single