THERMAL TOLERANCE OF MOSQUITOFISH 787 



temperature to follow environmental test temperature without a 

 significant time lag. In our study, however, exposure to high 

 temperature continued until opercular beating ceased. In the second 

 experiment irradiated Gambusia were exposed to a lethal tempera- 

 ture bath, and their mean survival time was determined. 



Fish used in the CTM type experiment were from a Gambusia 

 population that had been maintained at 25 ± 1°C under laboratory 

 conditions for over 2 years. The Gambusia for the survival-time 

 experiment were collected from a local population and acclimated to 

 laboratory conditions before irradiation. Both experiments made use 

 of female Gambusia not obviously laden with young and ranging in 

 length from 2.7 to 3.5 cm. 



The fish were irradiated in a Gammacell 200 (Atomic Energy of 

 Canada Limited) containing ^°Co at a dose rate of 2.5 rads/sec. 

 Doses of either 500, 1500, 3000, or 5000 rads were given. 



After irradiation the fish were maintained at 25 + 1°C and fed 

 freeze-dried brine shrimp until tested. Gambusia for the lethal- 

 temperature-bath experiment were maintained in the same manner, 

 except that after 8 days the acclimation temperature was inadver- 

 tently changed to 27 ± 1°C and remained at that temperature until 

 after day 15, when it was returned to 25°C. 



Gambusia that had received a dose of either 0, 500, or 1500 rads 

 were subjected to a CTM type test at 5, 15, and 30 days after 

 irradiation. Gambusia that had received doses of either 3000 or 5000 

 rads were tested 3 days after irradiation. Critical thermal maximum 

 tests were conducted on 5 to 10 Gambusia at each dose by placing an 

 individual fish in a 1000-ml flask containing 800 ml of water from 

 the tank in which the fish had been maintained after irradiation. The 

 flask was heated by a hemispherical heater controlled by a variable 

 voltage transformer. The rate of temperature elevation was 

 1.0°C/min. The temperature at which Gambusia lost their ability to 

 escape (CTM) and the temperature at which opercular beating ceased 

 were recorded. Cessation of opercular beating was chosen as the 

 biological end point for comparative purposes because less variability 

 was observed in the results. 



In the second experiment, Gambusia that had received doses of 

 either 0, 1500, 3000, or 5000 rads were exposed to a lethal 

 temperature bath 5, 15, and 30 days after irradiation. During the test 

 the fish were kept in small plastic containers (18 cm in diameter by 

 14 cm high), which were submerged in 5.5-gal aquariums main- 

 tained at 39.5°C. Each container was divided into two compartments 

 by a stainless-steel screen, and groups of either three or four 

 Gambusia, the maximum number that could be accurately observed, 



