RESPONSE OF MOSQUITOFISH TO THERMAL EFFLUENT 501 



TABLE 1 



WATER TEMPERATURES (°C) AT THE SITES OF 

 COLLECTION OF SAMPLE Gamhusia af finis* 



*Stations PCI, PC2, and PC3 are in areas affected by thermal 

 effluent; RP and PP are at temperatures natural for the area. 



Temperatures and observations of behavior were recorded at the 

 sampling stations when samples were collected and at other times 

 throughout the year. 



RESULTS 



Water-temperature regimes at the five sampling stations differed 

 among and within stations during the study period (Table 1). Water 

 temperatures at PCI and PC2 were significantly higher than those at 

 the unaffected stations (PP and RP); temperatures at PCS were 

 intermediate. Water temperatures at PCI, PC2, and PCS often 

 changed 20°C during a 24-hr period with changes in reactor power 

 levels. Nearshore areas inhabited by mosquitofish were generally 5 to 

 10°C lower than waters 1 to 2 m from shore. 



At stations PCI and PC2 mosquitofish survived by inhabiting the 

 shallow, cooler waters near shore which were slightly sheltered from 

 the high temperatures. Fish at these stations usually inhabited the 

 upper few centimeters of water immediately adjacent to shore, being 

 restricted to these areas by high temperatures that precluded lateral, 

 shoreline dispersal. When lethal temperatures prevailed at PCI and 

 PC2, individuals who were physically forced to swim from the shore 

 to deeper waters immediately lost equilibrium and died within a few 

 seconds. Shoreline observations made at PC2 revealed that mosquito- 

 fish seemed to "select" temperatures at S9.5°C when higher and 

 lower temperature waters were available. 



