786 BLAYLOCK AND FRANK 



reducing postirradiation temperatures prolonged the survival of fish, 

 but other investigations (Etoh and Egami, 1967; Aoki, 1964; Aoki, 

 Egami, and Arai, 1966; Egami, 1969) revealed that radiation lesions 

 continued to develop at cold temperatures and were expressed when 

 the fish were returned to a warmer environment. A small change 

 (5°C) in the postirradiation temperature can significantly influence 

 the expression of radiation effects in fish (Blaylock and Mitchell, 

 1969). 



Although all these studies addressed the effects of radiation and 

 temperature on aquatic organisms, only an experiment by Ophel and 

 Judd (1966) investigated the converse; i.e., the effects of radiation 

 on the thermal tolerance of fish. In their study goldfish exposed to 

 9 gj.^ I 3 1 J Qj. 9 gj. ^ 1 3 1 J yjQYQ subjected to a lethal temperature. In 

 15 of 17 tests, goldfish that had accumulated a maximum estimated 

 dose from internal emitters of 10'* rads to the bone and tissue and 

 10^ rads to the thyroid survived longer than did control fish. In this 

 case the radionuclide burden did not impair the ability of the fish to 

 w'thstand lethal temperatures. In fact, under the experimental 

 conditions the survival time was increased. 



The purpose of our study v/as to determine the effects of acute 

 external exposures to ionizing radiation on the thermal tolerance of 

 the mosquitofish, Gamhusia af finis. Our results should help to 

 answer questions arising at public hearings on nuclear power reactors 

 concerning the effects on aquatic biota of the interaction of 

 temperature and radiation. Although the estimated radiation dose 

 accumulated by aquatic biota from routine release of radionuclides 

 at operating nuclear power plants would be insufficient to produce 

 detectable radiation effects (Blaylock and Witherspoon, 1976), the 

 effects of radiation on the thermal tolerance of aquatic organisms 

 should be documented to establish the synergistic effects of radiation 

 and temperature at high and at low temperatures. 



MATERIALS AND METHODS 



Two experiments were conducted to determine the effect of 

 ionizing radiation on the ability of Gamhusia to tolerate high 

 temperatures. In the first experiment Gamhusia were irradiated and 

 subjected to a critical thermal maximum type test (CTM) at intervals 

 up to 30 days postirradiation. Hutchison (1961) described the CTM 

 as the arithmetic means of the collective thermal points at which an 

 animal loses its ability to escape from conditions that will promptly 

 lead to its death when it is heated from a previous acclimation 

 temperature at a constant rate just fast enough to allow body 



