830 CHERRY et al. 



second 10-min timing was then conducted to ensure preference for 

 the warmer water, and then chlorine was added to the heated side in 

 the same dosing increments as in the previous chlorine trials. In all 

 avoidance trials the residence time in the control or reference period 

 was compared with that during each chlorinated treatment with 

 Duncan's new multiple range test (Steel and Torrie, 1960). The first 

 significant (P < 0.05) reduction in residence time after each chlori- 

 nated increment, as compared with the reference period, represented 

 the avoidance threshold. 



RESULTS 



Chlorinated Trials 



The first significant avoidance to TRC varied from 0.10 mg/liter 

 when the fish were acclimated and tested at 18°C to 0.20 mg/liter at 

 12, 24, and 30°C (Fig. 1). Residence times declined steadily when 

 chlorine exposures surpassed 0.10 mg/liter and were generally lowest 

 at the highest residual encountered (0.40 mg/liter). At 12 and 24°C, 

 the threshold avoidance to chloramine residuals (0.20 mg/liter) was 

 the same as that for TRC (Fig. 1), but, unlike those in TRC trials, 

 residence times declined steadily after the first exposure to chlora- 

 mine treatments (0.025 mg/liter). 



In the TRC avoidance trials, the range of variation at the first 

 significant avoidance between acclimation temperatures was highest 

 for TRC concentrations (0.112 to 0.209 mg/hter), intermediate for 

 CRC (0.077 to 0.159 mg/liter) and FRC (0.035 to 0.122 mg/liter) 

 levels, aiid least variable for HOCl (0.010 to 0.028 mg/liter) 

 determinations (Table 2). Avoidance thresholds for the chloramine 

 trials (0.205 to 0.207 mg/liter at 12 and 24°C, respectively) were 

 considerably higher than for CRC fractions in the TRC trials (0.087 

 to 0.106 mg/liter). 



Chlorinated -Preferred-Temperature Trials 



The TRC avoidance thresholds for common shiners at preferred 

 temperatures of 17.6 or 26.8°C (when acclimated at 12 and 24°C, 

 respectively) were 0.20 mg/liter (Fig. 2). Avoidance thresholds for 

 chloramine trials, however, were 0.40 mg/liter at 17.6°C and 

 0.20 mg/liter at 26.8°C. Thus the threshold concentration for 

 chloramine trials at 12°C was approximately three times as great as 

 the CRC threshold level in the TRC trials (0.399 to 0.138 mg/liter). 

 At 24°C, avoidance thresholds in chloramine trials were also higher 



