THERMOREGULATORY BEHAVIOR AND 



DIEL ACTIVITY PATTERNS OF BLUEGILL, 



LEPOMIS MACROCHIRUS, FOLLOWING THERMAL SHOCK 



Thomas L. Beitinger^ 



ABSTRACT 



Individual bluegill were allowed to thermoregulate for 3 days in a temperature-preference apparatus 

 and then were exposed for 30 min to one of three temperature treatments: 21.0°, 31.0°, or 36.1°C. Fish 

 exposed to 31°C served as controls for handling procedures. Thermoregulatory performance of surviv- 

 ing fish was monitored for an additional 3 days. Pretreatment results indicated mean lower and upper 

 avoidance temperatures of 29.3° and 33. 1°, and 3 1 .2°C as the midpoint of the preferred range. All 20 fish 

 exposed to 21° and 31°C survived treatment and demonstrated no significant differences between 

 pretreatment and posttreatment thermoregulatory performance. Thirty-five percent offish (7 of 20) 

 exposed to 36.1°C died during treatment. Fish surviving the 36.1°C treatment retained the ability to 

 thermoregulate; however, their mean lower and upper avoidance temperatures increased 0.6° and 

 0.7°C, respectively. Activity patterns were typically diurnal, but variable, in all three treatment 

 groups. Immediately after treatment, the activity offish exposed to 21° and 36.1°C was markedly 

 decreased. Thereafter, activity tended to be higher in the 21°C group and lower in the 36.1°C group 

 than during the pretreatment p)eriod. 



Opportunities for temperature shock occur 

 wherever sharp temperature gradients are pres- 

 ent. Fish may be exposed to a sudden temperature 

 change when penetrating the thermocline; in 

 areas containing springs, upwellings or natural 

 allochthonous inputs of water; in sharp horizontal 

 temperature gradients characteristic of shallow 

 waters; and during the passage of weatherfronts. 

 Also, fish species migrating through or residing 

 within waters under the influence of electric 

 generating companies may be subjected to sudden 

 temperature changes. 



Investigations concerning thermal shock of 

 fishes have mainly been limited to descriptions of 

 morbidity stages and determinations of species' 

 lethal temperatures. Noteworthy exceptions are 

 studies by Sylvester ( 1972) and Coutant ( 1972a, b, 

 1973) that demonstrate enhanced vulnerability of 

 thermally stressed fishes to predation. Fish mor- 

 talities owing to natural and artificially induced 

 temperature shock have been reported in marine, 

 estuarine, and freshwater environments 

 (Gunther, 1936; Gunter, 1941; Huntsman, 1942; 

 Gunter and Hildebrand, 1951; Bailey, 1955; 

 Threinen, 1958; Colton, 1959; Alabaster, 1963; 

 Clark, 1969), but the majority of work has in- 



'Laboratory of Limnology, Department of Zoology, Univer- 

 sity of Wisconsin, Madison, WI 53706. 



Manuscript accepted December 1973. 

 FISHERY BULLETIN: VOL. 72, NO. 3, 1974. 



volved laboratory determinations of thermal re- 

 sistance. 



An important question is: does a sublethal 

 temperature shock disrupt subsequent ther- 

 moregulatory behavior of a fish? The objective of 

 this study was to assess effects of temperature 

 shock upon the thermoregulatory ability, selected 

 temperatures, and locomotor activity patterns of 

 individual hluegiW, Lepomis macrochirus. 



MATERIALS AND METHODS 

 Specimen 



Juvenile bluegill (mean length 86.3 mm, range 

 72-105 mm) were captured during summer with 

 electroshocking gear from Lake Wingra (Dane 

 County, Wis.) and maintained in the laboratory at 

 25°C under constant photoperiod (LD 14:10 with 

 0.5 h dawn and dusk intervals) for at least 2 wk 

 prior to experimentation. Throughout the preex- 

 perimental and experimental periods fish were fed 

 pelleted food daily at 1630 h ± 15 min. 



Apparatus 



The thermoregulatory apparatus was derived 

 from that of Neill and Magnuson (in press) with 

 temperature control and rate-change modi- 



1087 



