EFFECTS OF ACUTE AND CHRONIC THERMAL EXPOSURES 723 



8- to 12-hr delay following fertilization in the start of the alewife 

 chronic test incubations may have prevented the most thermally 

 sensitive developmental stages from being exposed to elevated 

 temperature. Test incubations should have been initiated as soon 

 after fertilization as possible to keep this condition from occurring. 

 The chronic safe level for alewife eggs reported in Table 3 therefore 

 probably overestimates a safe level appropriate for the entire 

 embryonic period. The same may be true for striped bass eggs, 

 although the delay was only 8 hr in this case. Considering the length 

 of the embryonic period of Atlantic tomcod, up to 39 days at 

 ambient temperatures, the 2-hr delay in the start of the test 

 incubations for this species most likely did not bias the estimated 

 chronic safe level. 



Applications 



Acute- and chronic-exposure hatching-success data can be used to 

 assess the thermal impact of power plants on fish eggs. 



Chronic-exposure safe levels can be used to identify maximum 

 thermal-plume temperatures compatible with successful embryonic 

 development. With knowledge of the spatial characteristics of a 

 thermal plume, such as the surface area enclosed by each plume 

 isotherm, the area of potential habitat in which temperatures exceed 

 the safe level can be determined. Within that area, successful 

 embryonic development is excluded. The assessment of the impact of 

 that exclusion area is completed by considering its biological 

 significance. This may be done by examining a species' spawning 

 habits, spawning locations, egg characteristics, and distribution of 

 eggs in the water body, to determine the relative importance of the 

 excluded area to the reproductive success of the species in the water 

 body. 



The chronic-exposure safe levels computed in this study have 

 been used to evaluate the potential impact of the Hudson River 

 thermal discharges. For example, the 4.4°C safe level estimated for 

 Atlantic tomcod (Table 3) indicates that plume temperatures only a 

 few degrees above ambient (0 to 2°C) during the spawning season 

 would exclude successful embryonic development. As a result, the 

 potential for impact within a thermal plume is relatively high. In 

 contrast, the computed safe levels for alewife and striped bass, both 

 about 24°C, considerably exceed river ambient temperatures during 

 the two species' respective spring spawning seasons. The potential for 

 impact is very low in both cases. 



Acute safe levels can be used to assess the impact of thermal 

 exposures experienced by fish eggs entrained into power-plant 



