2.4 mg/liter NH3 in ammonium sulfate solution. Mean survival time of 

 adapted minnows was 65 minutes, and of the unadapted control group was 45 

 minutes. 



Fromm (1970) has measured urea excretion rates of rainbow trout initial- 

 ly subjected to either 5 or 0.5 mg/liter NH3, and then subjected to 3 

 mg/liter NH3. The trout previously exposed to 5 mg/liter NH3 excreted 

 slightly less urea than those previously exposed to the lower concentration. 

 Lloyd and Orr (1969) measured urine flow rates of rainbow trout exposed for 

 24 hours to 0.27 mg/liter NH3, and then exposed for another 15 hours to 0.53 

 mg/liter NH3. Pretest urine flow rates of 2.8 ml/kg/hr increased first to 



6.4 and then to 8.0. One fish died during the lower ammonia level exposure 

 and none during the higher exposure. A control batch of fish with a pretest 

 urine flow rate of 0.75 ml/kg/hr was subjected directly to the higher (0.53 

 mg/liter NH3) armionia concentration. The urine flow rate jumped to 11 

 ml/kg/hr, and all fish died within 3 hours. 



In a second experiment by Lloyd and Orr (1969), rainbow trout were sub- 

 jected to 0.32 mg/liter NH3 for successive 22-hour time periods, separated 

 by a 24-hour non-exposure period. Although urine flow rates were higher 

 during exposure periods than during pre-exposure, they were less during the 

 second exposure period than during the first. This suggests that some ac- 

 climation was developed and subsequently retained, at least for a 1-day rest 

 period. A third experiment indicated that this acclimation was not retained 

 during a 3-day rest period between two similar ammonia exposures. 



Schulze-Wiehenbrauck (1976) conducted a study on the effect of sublethal 

 ammonia exposures on young rainbow trout growth, food consumption, and food 

 conversion. In one experiment, trout were acclimated for 3 weeks at 0.007 

 (control), 0.131, and 0.167 mg/liter NH3; the fish from these three tanks 

 were then subjected to concentrations of approximately 0.45 mg/liter NH3 for 



8.5 hr. Fish from the two ammonia acclimation concentrations had 100 per- 

 cent survival, whereas only 50 percent of the control group survived the 

 test period. In the second experiment, the acclimation concentrations were 

 0.004 (control) and 0.16 mg/liter NH3; these fish were placed in NH3 concen- 

 trations of approximately 0.5 mg/liter for 10 hours. There was 100 percent 

 survival of the ammonia acclimated fish, and 85 percent survival of the con- 

 trol fish. The results of these experiments thus showed an increase in re- 

 sistance of rainbow trout to acutely toxic concentrations of ammonia after 

 prior exposure to sublethal ammonia concentrations. 



At Fisheries Bioassay Laboratory we have conducted experiments to inves- 

 tigate the effect of acclimation of rainbow trout to sublethal ammonia con- 

 centrations on the fish's response to acutely lethal ammonia concentrations. 

 Seven 96-hour flow-through bioassays (using NH4CI) were conducted, six of 

 these on fish that had been acclimated for 29 days to concentrations ranging 

 from 0.018 to 0.078 mg/liter NH3, and the seventh on a control group accli- 

 mated at 0.001 mg/liter NH3. For each bioassay there were 5 test tanks and 

 1 control tank containing 10 fish each; mean fish sizes for the tests were 

 12 to 15 g. Additional details of these tests and data treatment will be 

 reported elsewhere (Thurston and Russo, in preparation). 



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