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<br />4 <br />equilibrium irregularities. Never-the-less, temperature control methods <br />were changed in the 1985 experiment when hot and cold water was first mixed <br />to the desired temperature by temperature control valves, then trickled <br />down the degassing column before entry in to test aquaria. <br />Fish were subjected to abrupt and gradual temperature changes. Water <br />temperature was decreased 5, 10, and 15 C in 5 minutes in some aquaria and <br />in 240-300 minutes in others. There was no change in control aquaria (22 <br />C). Fish were held at the shock temperatures for 19-24 hrs depending on <br />rate of change, afterwhich all aquaria were returned to 22 C over a 96 hr <br />period. The test regime roughly simulated what a fish might experience <br />when drifting from a warm river into one receiving a cold dam release, and <br />then drifting downstream as water warmed with distance from the dam. <br />Temperatures were recorded manually in all aquaria on a routine basis; <br />recording thermographs were employed in one aquaria for each treatment <br />(Figure 1). <br />The number of dead fish were counted daily and removed. Visual <br />observations of behavior were made hourly during shock, and then daily <br />afterward. At the end of the experiment, a random sample of survivors were <br />measured for total length. <br />RESULTS <br />Mortality was significant only in the cases where 14-day-old fish were <br />subjected to 15 C decreases in 5 minutes (Table 1). When shocks were less <br />or occurred at the slower rate, mortality was no different than expected. <br />The 16-34% mortality in control tanks probably occurred because of the <br />difficulty of artificially rearing these young, undomesticated fish.