BEITINGER: THERMOREGULATORY BEHAVIOR OF BLUEGILL 



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2rC EXPOSURE 



31°C EXPOSURE 

 iCONTROLSi 



36rC EXPOSURE 



Figure 3. — Medians (circles) and 95% confidence limits for pre- 

 treatment and posttreatment bluegill hourly activities for the 

 three treatment groups. Day and night activities are presented 

 separately. 



calculated midpoint of the preferred range, 

 31.2°C, is similar to that of Neill and Magnuson 

 (in press), 30.4° and 0.8°C below the final tem- 

 perature preferendum for bluegill reported by 

 Fry and Pearson (1952).^ The effect of sudden 

 temperature stress on thermoregulatory behavior 

 has not been previously examined. However, 

 several other external factors have been reported 

 to influence temperature preferenda, including 

 season (Sullivan and Fisher, 1953; Zahn, 1963; 

 Barans and Tubb, 1973), light intensity (Sullivan 

 and Fisher, 1954), starvation (Javaid and 

 Anderson, 1967) and exposure to chlorinated 

 hydrocarbons (Ogilvie and Anderson, 1965; 

 Peterson, 1973). 



Pretreatment and posttreatment comparisons 

 in the grouped control data clearly indicate no 

 change in thermoregulatory performance. The 



^Fry, F. E. J., and B. Pearson. 1952. Some temperature 

 relations of the pumpkinseed and bluegill sunfish. Unpubl. 

 manuscr., 10 p. Ont. Fish. Res. Lab. R.R. 2, Maple, Ont., Can. 



A t experienced by control fish was minimal and 

 changes, if any, could be ascribed to handling or 

 time-dependent variations in temperature 

 preference. 



Cold- and heat-treated fish actually experienced 

 two temperature shocks. One occurred when the 

 fish were introduced into the treatment chambers 

 and the second when they were returned to their 

 experimental tanks. Both cold and heat treat- 

 ments were conducted at temperatures actively 

 avoided by fish while in their thermoregulatory 

 tanks. 



Cold exposure did not significantly change any 

 of the four measured parameters of thermo- 

 regulatory behavior. These fish experienced a 

 At of approximately 10°C, but the exposure 

 temperature, 21°C, is near the middle of the toler- 

 ance zone for bluegill. That these fish did not select 

 lower temperatures was expected, owing to the 

 slow rate of downward temperature acclimation 

 characteristic of fishes (Brett, 1944, 1946). The 

 lethal rates of temperature increase are at least 

 20 times the corresponding lethal rates of tem- 

 perature decrease for bluegill (Speakman and 

 Krenkel, 1971). Apparently, the 30-min exposure 

 was not sufficient to change the acclimation state 

 and, hence, the preferred temperature range of 

 these fish. 



Fish exposed to 36. 1°C experienced a smaller A t 

 (about 5''C) during treatment, but this exposure 

 was to within approximately 0.5°C of the bluegill's 

 incipient upper lethal temperature (Hart, 1952; 

 Cairns, 1956). The ability of surviving bluegills to 

 thermoregulate was not deleteriously affected by 

 the 30-min exposure to 36.1°C, however, statisti- 

 cally significant changes in avoidance and mid- 

 point temperatures did occur. Thus, the ther- 

 moregulatory performance of bluegill was 

 influenced more by exposure either to 1) tem- 

 peratures closer to lethal limits than exposure to 

 large Afs per se or to 2) temperatures above 

 rather than those below, the acclimation state of 

 the fish. 



Of the three exposure temperatures, only 

 36.1°C resulted in fish mortality. The pretreat- 

 ment midpoint temperatures of these fish were 

 significantly lower than those of fish surviving 

 heat treatment, indicating a relationship be- 

 tween preferred and upper lethal temperatures. 

 That a considerable proportion, 35^^ , of fishes 

 exposed to 36.1°C died, is more important to the 

 population than the observation that surviving 



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