FISHERY BULLETIN: VOL. 72, NO. 4 



fications reported by Beitinger et al. (in press). 

 The design (Neill, Magnuson, and Chipman, 1972) 

 substitutes a temperature gradient over time 

 for the spatial gradient typical of most tempera- 

 ture preference studies and allows an individual 

 fish to serve as its own tank thermostat. Each 

 50-liter test tank was divided into halves with 

 a molded fiber glass partition. A tunnel in the 

 partition allowed the fish to choose between halves 

 differing by a fixed 2°C temperature interval. 

 When a fish selected the higher temperature, the 

 temperature of the tank increased at a constant 

 rate of 3°C/h while the 2°C differential between 

 halves remained constant. When the fish moved to 

 the cooler tank half, the temperature decreased at 

 the same rate (3°C/h) until the fish again moved to 

 the warmer tank half. By moving from one side to 

 the other, a fish was able to control the tempera- 

 ture to which it was exposed. For this study, a 

 potential temperature range of 4° to 55°C was 

 available. 



Temperatures of each tank half were monitored 

 by a thermistor-wheatstone bridge circuit con- 

 nected to a multichannel analog recorder. Avoid- 

 ance temperatures (i.e., turnaround tempera- 

 tures), preferred temperature range and midpoint 

 of the preferred range (midpoint temperature) 

 were the same as defined by Neill and Magnuson 

 (in press). During the experiment, tunnel passes, 

 recorded on an event recorder, were utilized as a 

 measure of fish activity. 



PROCEDURE 



One fish was introduced per tank and allowed to 

 experience the static system for 2.5 days with the 

 tank halves set at 24° and 26°C. The test period 



then began and tank temperature control was re- 

 linquished to each fish. Thermoregulatory per- 

 formance during the second, third, and fourth 

 days constituted the pretreatment data. Then fish 

 were removed and individually subjected to a sud- 

 den temperature change in 3.5-liter cylindrical 

 chambers. The water in each chamber was well 

 aerated and "conditioned" with 150 ml of that 

 fish's thermoregulatory tank water. High temper- 

 ature treatment was 36.1 ± 0.1°C and low temper- 

 ature treatment was 21.0 ± 0.1°C. For control pur- 

 poses, a third group offish was treated at 31. 0± 

 O.lC, a temperature approximating the preferred 

 range midpoint for bluegill. A series of cursory 

 experiments indicated that fish body tempera- 

 tures equilibrated to the treatment temperature 

 during exposure. Fish were randomly allocated to 

 the three treatment temperatures. Following a 

 30-min exposure, each surviving fish was re- 

 turned to its respective thermoregulatory tank for 

 an additional 3-day posttreatment period. Ther- 

 moregulatory tank temperatures at fish reentry 

 were the same as those at fish removal. Finally, 

 fish were isolated and observed for 1 wk for possi- 

 ble latent effects. 



RESULTS 



Prior to treatment, there were no statistically 

 significant differences in thermoregulatory per- 

 formance among the three groups (Kruskal- 

 Wallis one way analysis of variance; Siegel, 1956; 

 lower and upper avoidance temperatures, mid- 

 point temperature, and width of preferred range, 

 all F>0.20). Fish had mean lower and upper 

 avoidance temperatures of 29.3° and 33.1°C 

 and mean preferred range width of 3.8°C. The 



Table 1. — Lower and upper avoidance temperatures, preferred range midpoint and width, pretreat- 

 ment and posttreatment, for each of the three groups. Means ± standard deviations are given. 



'One fish survived treatment but died durmg the first posttreatment night, owing to electronic failure. 



1088 



