724 KELLOGG, SALERNO, AND LATIMER 



cooling waters at the point of intake or discharge. Since the effects 

 of the acute exposure on fish eggs depend on both the exposure 

 duration and temperature, as noted by this study and by Lauer et al. 

 (1974) and Frank (1974), both these factors must be considered in 

 the acute impact analysis. Once the time— temperature characteristics 

 of a given entrainment exposure are specified, the appropriate acute 

 safe level can be used to determine whether or not a reduction in 

 hatching success would be expected as a result of the thermal 

 exposure. 



This approach has been used in the analysis of the thermal 

 impact of Hudson River power plants. The 30-min safe level is 

 applicable to plants with surface outfalls, where the relatively slow 

 rate of temperature decay in the thermal plume may prolong the 

 period of exposure to elevated temperatures. On the other hand, 

 plants with high-velocity diffuser discharges have comparatively short 

 exposure periods, and consequently the 5-min safe level is often 

 applicable. For example, in the former case, blastula-stage alewife 

 eggs, with a 30-min safe level of 23.8°C (Table 3), would be capable 

 of tolerating maximum entrainment temperature elevations of 8 to 

 11°C, based on ambient temperatures during the spawning season 

 (13 to 16°C). In the latter case, with a 5-min safe level of 28.1°C, the 

 maximum tolerable AT values would range from 12 to 15°C. At 

 temperature elevations above these levels, reductions in hatching 

 success would be expected. 



The assessment of acute thermal impact requires calculation of 

 the total losses of eggs during a spawning season due to entrainment. 

 Total losses can be determined after computing (1) the total time 

 during the spawning season when discharge temperatures occur in 

 excess of the appropriate safe levels and (2) the average density of 

 eggs passing through the cooling system during those times of 

 excessive temperatures. The biological significance of total losses of 

 eggs must then be determined, one possible approach being the 

 conversion of egg losses to equivalent fish losses. 



In conclusion, hatching-success data obtained from acute and 

 chronic thermal-exposure experiments can be used in the assessment 

 of the impact of once-through cooling on fish eggs. When combined 

 with thermal-effects data for other life stages, the impact of a power 

 plant on a given species can be assessed. 



ACKNOWLEDGMENTS 



Funding for these studies was made available by Central Hudson 

 Gas & Electric Corporation, Consolidated Edison Company of New 



