RESPONSE OF MOSQUITOFISH TO THERMAL EFFLUENT 499 



tures on aquatic life. Early thermal studies concentrated on 

 establishing upper tolerance limits for various species, and more 

 recently ecological impacts of thermal effluents have been evaluated 

 on-site. As more information on the effects of thermal effluents 

 becomes available, it is apparent that some fishes can adapt to 

 relatively high temperature conditions better than others. 



Mosquitofish, Gambusia affinis, is one species that has been 

 reported to have a relatively high thermal tolerance (Hart, 1952). 

 Since Gambusia sp. are indigenous to areas that have warm climates 

 (Blair et al., 1968) aiid generally inhabit shallow, nonmoving waters 

 throughout their life cycle, they are naturally exposed to warmer 

 waters than are deepwater forms of aquatic life (Falke and Smith, 

 1974). Data on the life history of G. affinis from areas unaffected by 

 thermal discharge are available (Krumholz, 1948; Rosen and Bailey, 

 1963), but little is known about mosquitofish in thermally affected 

 waters. 



The U. S. Department of Energy's Savannah River Plant (SRP) is 

 an area of diverse thermal research. Specifically, the Par Pond system 

 has been the site of extensive thermal ecological evaluations on 

 turtles, largemouth bass, alligators, and aquatic plants (Gibbons 

 et al., 1974). The Par Pond system was constructed in the late 1950s 

 to provide cooling water for nuclear production reactors. Water is 

 pumped to the reactor, heated, and discharged into a canal system 

 where it gravitates through several cooling ponds before entering a 

 cove in Par Pond about 5 km from the pumping site. One cooling 

 pond in the system is Pond C, where water temperatures commonly 

 exceed 40° C (Ferens and Murphy, 1974; Clugston, 1973). This 

 provides an ideal site for field studies to evaluate the effects of 

 thermal discharges on biota in receiving waters. 



The presence of mosquitofish inhabiting waters in Pond C at 

 temperatures near their reported upper tolerance limit prompted 

 comparative field and laboratory studies to determine their mecha- 

 nisms for survival in an abnormally warm and widely fluctuating 

 thermal environment. The objectives of this study were to compare 

 size structure, percentage of fat, reproductive activity, sex ratios, and 

 size at sexual maturity of mosquitofish populations inhabiting 

 thermally stressed areas with those from unaffected waters. 



STUDY AREAS 



Mosquitofish were sampled from five locations at the Savannah 

 River Plant. Three populations (at stations PCI, PC2, and PC3) were 

 sampled in Pond C, a primary coohng pond in the Par Pond system. 



