EFFECTS OF ACUTE AND CHRONIC THERMAL EXPOSURES 721 



DISCUSSION 



The hatching success data from this study have been used to 

 calculate the maximum temperatures to which eggs can be exposed 

 for given lengths of time without sustaining any measurable 

 reductions in hatching success. Such temperatures, termed "safe 

 levels" or "upper thermal protection limits," were determined, 

 following the convention of the U. S. Environmental Protection 

 Agency (1976) and Mihursky (1969), by subtracting 2.2°C from 

 computed TL5 values. The computed safe levels are presented in 

 Table 3. In most cases the 2.2°C safety factor provided estimates of 

 safe levels which are conservative in terms of biological protection. 



The chronic -exposure safe levels for alewife and striped bass eggs 

 coincided at 24°C, despite differences in the acclimation temperature 

 ranges for the two species. This safe level exceeds ambient Hudson 

 River temperature during the alewife and striped bass spring 

 spawning seasons by up to 11 and 10°C, respectively (Table 3). In 

 contrast, the 4.4°C safe level estimated for Atlantic tomcod eggs 

 exceeds ambient during the species' midwinter spawning season by 

 only 2.4 to 4.4°C. 



Acute-exposure safe levels were determined for 5-min (10-min in 

 the case of Atlantic tomcod) and 30-min exposures for each of the 

 three species tested (Table 3). The estimated safe levels increased 

 with advancing developmental stage and decreased with lengthening 

 exposure duration, as noted above for the TL5 data from which the 

 safe levels were derived. The acute safe levels for alewife and striped 

 bass eggs were similar (within 3°C) except in one case, the alewife 

 blastula stage, where the 30-min safe level was 6°C lower than that 

 for striped bass — 23.8 vs. 29.7°C (Table 3). Blastula-stage alewife 

 appeared particularly sensitive to short exposures to elevated 

 temperature. 



The acute safe levels for Atlantic tomcod eggs were only 3.5 to 

 8.7°C less than corresponding safe levels for alewife and striped bass 

 (Table 3). This is in contrast to the chronic case, where the Atlantic 

 tomcod safe level was 20° C less than that estimated for the other 

 two species — 4.4 vs. 24°C. Atlantic tomcod eggs appear quite 

 tolerant of short exposures to elevated temperatures, whereas they 

 appear very intolerant of long exposures. 



Since it was generally observed that safe levels decreased with 

 lengthening exposure duration, the coincidence of the alewife chronic 

 safe level and the alewife blastula-stage 30-min acute safe level, both 

 24°C, was a cause for question. The source of this coincidence 

 appears to have been an artifact of the experimental design; i.e., the 



