EFFECTS OF ACUTE AND CHRONIC THERMAL EXPOSURES 719 



For Atlantic tomcod embryos acclimated to 2°C, the optimum 

 hatching range extended from 2.0 to 4.9°C (Table 1). The TL5 o for 

 normal hatch was 6.6° C. As with alewife, differences in percent 

 hatch between eggs of the three replicate females occurred, but the 

 relative response to temperature between females was similar. 



Acute Effects 



The results of the short-exposure hatching-success experiments 

 are shown in Table 2. The TL^ q values for normal hatch at one or 

 more exposure durations are shown for several developmental stages 

 of each of the three anadromous species studied. 



For alewife embryos acclimated to 12 to 13°C, short-exposure 

 TL5 values for normal hatch ranged from 26.0 to 37.0°C, 

 depending on developmental stage and exposure duration (Table 2). 

 The TL50 values for the blastula stage were the lowest, and, as 

 developmental stage advanced, the corresponding TL5 values in- 

 creased. The TL5 values for tail-bud and tail-free embryos exceeded 

 those for blastula by 6.0 to 7.9°C. In all cases, TL5 values for 

 30-min exposures were about 3 to 4°C less than those for 5-min 

 exposures. The TL5 q values varied up to 2.4°C among samples from 

 replicate females; however, no consistent trends were visible among 

 replicates (i.e., there were no consistent differences in thermal 

 tolerance due to female stock). 



For striped bass embryos acclimated to 16 to 19°C, short 

 exposure TL5 values for normal hatch generally increased with 

 advaiicing developmental stage and decreased with lengthening 

 exposure duration, as was the case with alewife. Contrary to the 

 general trend, however, striped bass embryos in the late embryo stage 

 were slightly less tolerant of short-term temperature elevations than 

 were embryos in the next younger stage — tail-free embryo. Differ- 

 ences in TL5 values as great as 4.5°C with developmental stage were 

 observed. The TL5 values for 30-min exposures were up to 5.5°C 

 less than corresponding TL5 values for 5-min exposures. The overall 

 range of TL5 values observed was 31.5 to 38.5°C. 



Short-exposure TL5 values for normal hatch for Atlantic 

 tomcod embryos acclimated to 2 to 3°C varied from 19.8 to 31.2°C, 

 depending again on developmental stage and exposure duration. The 

 trends in TL5 in relation to these two variables were similar to 

 those for alewife and striped bass. In contrast to the other two 

 species tested, TL5 values were available for only relatively 

 advanced embryonic stages of Atlantic tomcod. 



