RESPONSE OF MOSQUITOFISH 

 TO THERMAL EFFLUENT 



DAVID H. BENNETT* and C. PHILLIP GOODYEARf 

 Savannah River Ecology Laboratory, Aiken, South Carohna 



ABSTRACT 



Reproductive activity, sex ratios, size at sexual maturity, percentage of fat, and 

 size structure were examined in five populations of mosquitofish {Gambusia 

 affjnis) from lentic environments experiencing different thermal conditions at the 

 Savannah River Plant near Aiken, S. C. Populations were sampled during 

 February, June, October, and December 1970. Mosquitofish from thi-ee 

 thermally affected stations inhabited waters from 28 to 40 C, whereas 

 mosquitofish at the unaffected stations were found in waters from 12 to 29 C. 

 Periods of reproductive activity were different for thermally affected and 

 unaffected populations. Females with eyed embryos were found in mosquitofish 

 populations from unaffected waters in June; eyed embryos were found in the 

 affected populations throughout the study period. Mean brood size of 

 populations from thermally affected areas generally varied inversely with water 

 temperature. Size at sexual maturity and sex ratios were generally similar in the 

 five populations. Body fat content varied seasonally for each population. 

 Percentages of fat were significantly higher in male and juvenile fish from 

 thermally affected waters than in those from unaffected waters, whereas fat 

 content was similar in females. Mean size of females from unaffected waters was 

 directly related to water temperature, whereas the size structure of mosquitofish 

 populations from affected waters varied inversely with water temperature. This 

 study indicates that G. affinis is a eurythermal species that can adapt to 

 conditions of severe thermal stress. 



Recent increases in electrical power demands and concomitant 

 increases in release of waste heat into natural waters have stimulated 

 an intensive evaluation of the effects of abnormally high tempera- 



*Present address: College of Forestry, Wildlife and Range Sciences, 

 University of Idaho, Moscow, Idaho. 



tPresent address: U. S. Fish and Wildlife Service, Ann Arbor, Michigan. 



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