Specific growth and mortality rates of juvenile rainbow trout (Salmo gairdneri) were determined for 50 days at seven constant temperatures between 8 and 22 °C and six diel temperature fluctuations (sine curve of amplitude ±3.8 deg C about mean temperatures from 12 to 22 °C). For constant temperature treatments the maximum specific growth rate of trout fed excess rations was 5.12%/day at 17.2 °C. An average specific mortality rate of 0.35%/day was observed at the optimum temperature and lower. At temperatures in excess of the growth optimum, mortality rates were significantly higher during the first 20 days of this experiment than the last 30 days. The highest constant temperature at which specific growth and mortality rates became equal (initial biomass remained constant over 40 days) was 23 °C. The upper incipient lethal temperature was 25.6 °C for trout acclimated to 16 °C. A yield model was developed to describe the effects of temperature on the living biomass over time and to facilitate comparison of treatment responses. When yield was plotted against mean temperature, the curve of response to fluctuating temperatures was shifted horizontally an average 1.5 deg C towards colder temperatures than the curve of response to constant temperature treatments. This response pattern to fluctuating treatments indicates that rainbow trout do not respond to mean temperature, but they acclimate to some value between the mean and maximum daily temperatures. These data are discussed in relation to establishment of criteria for summer maximum temperatures for fish. Key words: constant temperature, fluctuating temperature, specific growth rate, specific mortality rate, yield, lethal temperature, zero net biomass, rainbow trout, thermal criteria

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The acute (96-h) toxicity of copper sulfate to fathead minnows was tested in a 2 ϫ 2 factorial experiment, with factors being low (ca. 1 meq/L) and high (ca. 4 meq/L) water hardness during an acclimation period and low and high hardness during the exposure period. Acclimation hardness was found to have no significant effects on copper lethality, except that the 24-h LC50 at high exposure hardness was 44% higher for fish subject to low acclimation hardness than fish subject to high acclimation hardness. In contrast, exposure hardness was found to have major effects, with LC50s being two to three times greater at high exposure hardness than at low, regardless of the acclimation hardness. These results suggest that the extent of the acclimation period is not a general, severely confounding factor for previous reports of the effects of hardness on copper toxicity to fish, although it may assume some importance for some species and test conditions. Keywords-CopperToxicity Fathead minnow Hardness AcclimationEnviron. Toxicol. Chem. 16, 1997 R.J. Erickson et al.

Abstract-The acute (96-h) toxicity of copper sulfate to fathead minnows was tested in a 2 ϫ 2 factorial experiment, with factors being low (ca. 1 meq/L) and high (ca. 4 meq/L) water hardness during an acclimation period and low and high hardness during the exposure period. Acclimation hardness was found to have no significant effects on copper lethality, except that the 24-h LC50 at high exposure hardness was 44% higher for fish subject to low acclimation hardness than fish subject to high acclimation hardness. In contrast, exposure hardness was found to have major effects, with LC50s being two to three times greater at high exposure hardness than at low, regardless of the acclimation hardness. These results suggest that the extent of the acclimation period is not a general, severely confounding factor for previous reports of the effects of hardness on copper toxicity to fish, although it may assume some importance for some species and test conditions.