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<br />Copper concentrations in control fish in the present <br />study were 11-13 p,g/g in razorback sucker and 4-6 <br />p,g/g in bonytail. These concentrations were equal to <br />or higher than in control fish from other studies; Marr <br />et al. (1996) reported 3-4 p,g/g and Shearer (1984) <br />reported 2-7 p,g/g, both in rainbow trout. However, <br />background or normal concentrations of copper can <br />vary considerably by fish species (W. Brumbaugh, per- <br />sonal communication, 1998). The higher copper con- <br />centration in control fish in our study may have been <br />due in part to the higher copper concentration in diet <br />(11- 28 p,g/ g) than in the diet used by Marr et al. <br />(1996) (9 p,g/ g). Selenium concentrations in control <br />fish in the present study were similar to several other <br />studies with a variety of fish species, which ranged from <br />0.4 to 2.0 p,g/g (summarized in Hamilton et al., 1998). <br />Zinc concentrations in control fish in the present study <br />were also slightly higher than those reported by Pierson <br />(1980, which ranged from 25 to 110 p,g/g, but similar <br />to those reported by Spehar (1976), which ranged from <br />75 to 120 p,g/g. <br />Inorganic accumulation in the present study seemed <br />to reach a steady state by 60 days of exposure. Others <br />have reported a steady state of copper accumulation in <br />rainbow trout in 60 days or less (Buckley et al., 1982; <br />Marr et al., 1996), and zinc accumulation in guppy, <br />Poecilia reticulata (Pierson, 1981). Lemly (1982) re- <br />ported that selenium uptake in bluegill and largemouth <br />bass (Micropterus salmoides) during a waterborne expo- <br />sure reached an equilibrium in 60 days in a soft water <br />(hardness 25 mg/L as CaC03), and 90 days in the hard <br />water (hardness 200 mg/L as CaC03). However, in the <br />natural environment where selenium concentrations in <br />water and food organisms could fluctuate, defining an <br />equilibrium point could be difficult as was observed by <br />Woock (1984) for golden shiners (Notemigonus cryso- <br />leucas) held in cages in a selenium-contaminated reser- <br />voir. Metabolic energy must be diverted from other <br />demands such as growth to a detoxification mechanism <br />if a fish is to reach a steady state of residue accumula- <br />tion (Marr et al., 1996). If the detoxification mechanism <br />can not handle the inorganic exposure, either the con- <br />centration or the duration of the exposure, a steady <br />state will not be reached and residues will accumulate <br />until the end result is death of the organism. In the <br />present study, reduced regulatory control of residues <br />was observed in the 8X and 16X treatments for razor- <br />back sucker and the 16X treatment for bony tail. <br /> <br />Hazard Assessment <br /> <br />Fish in our study were continuously exposed to test <br />concentrations, which might have underestimated the <br />adverse effects observed. Pulsed, episodic, or intermit- <br />tent exposures to contaminants can have greater effects <br /> <br />SIMULATING IRRIGATION WATER TOXICITY 61 <br /> <br />on fish than continuous exposures (Seim et aL, 1984; <br />Pascoe and Shazili, 1986; Siddens et aL, 1986). In <br />intermittent exposures, fish usually do not have time to <br />acclimate to the exposure and may accumulate suffi- <br />cient residue such that an irreversible toxic effect oc- <br />curs even if they move to clean water and depurate a <br />portion of the body burden. The magnitude, duration, <br />and frequency of pulsed or episodic events are impor- <br />tant in affecting the response of fish to toxicants, as is <br />fish size and water quality (reviewed by Marr et aL, <br />1995; Gunn and Noakes, 1987). At the mouth of Ashley <br />Creek during 1986-1988, concentrations of copper <br />ranged from < 10 to 10 p,g/L, selenium 25 to 78 <br />p,g/L, and zinc < 10 to 20 p,g/L (Stephens et al., <br />1988; 1992). Consequently, adverse effects of pulsed or <br />episodic exposures of inorganic mixtures at Ashley <br />Creek, in addition to changes in water quality, might <br />cause greater adverse effects that those observed. <br />Although this study involved only waterborne expo- <br />sure, exposure to inorganics in the diet are an impor- <br />tant route of exposure that would probably have in- <br />creased the adverse effects observed in this study. <br />Farag et al. (1994) and Woodward et aL (1994; 1995) <br />showed that inorganics in the diet simulating condi- <br />tions in the inorganic-contaminated Clark Fork River <br />of Montana was an important route of exposure to <br />rainbow trout. Several reviews of the ecotoxicology of <br />selenium have shown that low waterborne concentra- <br />tions of selenium can biomagnify substantially in the <br />aquatic food web and that dietary selenium exposure <br />causes adverse effects at lower concentrations than <br />direct waterborne exposure (Lemly, 1993b; Maier and <br />Knight, 1994; Presser et al., 1994). Adverse effects on <br />endangered larval fish at Ashley Creek might be greater <br />due to dietary exposure of the inorganic mixture. <br />The validity of extrapolating laboratory results to <br />field situations has been questioned because of the lack <br />of environmental realism with single-species tests <br />(Cairns, 1983; Kimball and Levin, 1985). Yet, most <br />investigators acknowledge that ecosystem studies re- <br />quire more subjective scientific judgements than labo- <br />ratory tests, are prohibitively expensive to conduct, lack <br />theoretical basis to explain and relate the confounding <br />morass of data collected, and absence of standardized <br />protocols for conducting ecosystem studies. <br />However, several investigators have independently <br />reviewed numerous laboratory and field studies of a <br />wide variety of chemical contaminants and concluded <br />that laboratory-derived information are good predictors <br />of ecosystem effects (Slooff et aL, 1986; Arthur, 1988; <br />Emans et aL, 1993; Sprague and Rodrigue, 1996). <br />Achieving comparability between laboratory and field <br />studies is enhanced by using the species of concern, <br />similar conditions such as water quality, photoperiod, <br />temperature, and relevant biological endpoints that can <br />