RESPONSE OF MOSQUITOFISH TO THERMAL EFFLUENT 509 



that environmental temperature cannot account directly for differ- 

 ences in body composition. 



Mosquitofish in Pond C may be living near their maximum 

 thermal tolerance (Falke and Smith, 1974). On the basis of our 

 observations, mosquitofish at PCI and PC2 were inshore inhabitants 

 when the reactor was operating. Fish had to be forced to swim to the 

 deeper, warmer waters (45°C), and this resulted in nearly instanta- 

 neous loss of equilibrium and mortality. Shoreline dispersal was 

 prevented by extreme temperatures. When temperatures decreased, 

 however, bass and sunfish preyed on the mosquitofish. 



Observations on temperature selection indicated that mosquito- 

 fish selected 39.5°C water when cooler waters were available for 

 habitation. This temperature was 2°C higher than the reported upper 

 tolerance limit for Gamhusia acclimated to 20 and 35°C (Hart, 1952) 

 and 12°C higher than temperatures selected by mosquitofish in a 

 thermal gradient tank (Bacon, Neill, and Kilambi, 1967). 



Our results show that G. af finis is a eury thermal species that can 

 adapt to conditions of severe thermal stress. Mosquitofish from 

 thermally affected areas were found to have similar population size 

 structure, percentage of body fat, proportion of females reproducing, 

 size at sexual maturity, sex ratios, and numbers of eyed embryos per 

 unit length of female. Reproductive activity was increased in the 

 thermally affected areas, however. Because of the eurythermal nature 

 of Gambusia, management agencies should consider introducing the 

 mosquitofish into reactor cooling ponds because of its economic 

 importance as a predator on mosquito larvae and its biological value 

 as a forage species. 



ACKIMOWLEDGMENTS 



The research reported here was supported by contract EY-76C- 

 09-0819 between the U.S. Department of Energy and the University 

 of Georgia. Manuscript preparation was aided by the College of 

 Forestry, Wildlife and Range Sciences, University of Idaho. 



REFERENCES 



Bacon, E. J., Jr., W. H. Neill, Jr., and R. V. Kilambi, 1967, Temperature 

 Selection and Heat Resistance of the Mosquitofish, Gambusia affinis, Proc, 

 Anna. Conf. Southeast. Assoc. Game Fish Comm., 21 : 411-415. 



Blair, W. F., A. P. Blair, P. Brodkorb, F. R. Cagle, and G. A. Moore, 1968, 

 Vertebrates of the United States, 2nd ed., pp. 117-118, McGraw-Hill Book 

 Company, New York. 



