EFFECTS OF ACUTE AND CHRONIC THERMAL EXPOSURES 717 



each test and was maintained in an ambient-temperature test bath 

 during the exposure period. Following exposure, all samples were 

 returned to ambient temperature and observed through hatch. The 

 observed percentage of normal hatch for each sample was then 

 computed and corrected for control mortality by the following 

 equation: 



/-I ^ J u 4- u o7 observed hatch, % ^ „ „^ 



Corrected hatch, % = 7 — , , ^ , ' x 100% 



control hatch, % 



The corrected hatch percentages were then plotted vs. test tempera- 

 ture, and the TL5 was determined by linear regression as the 

 temperature corresponding to 50% corrected hatch. 



RESULTS 



Chronic Effects 



The results of the long-term hatching-success experiments are 

 shown in Table 1. The percentages of total and normal hatch 

 corresponding to each test temperature are shown for each of the 

 three anadromous species examined. 



For alewife acclimated to 13 to 14°C, results obtained with eggs 

 from four replicate females were combined. Differences in hatching 

 success between replicate female alewife occurred, but the relative 

 response to temperature between females was similar. The highest 

 percentage of total and normal hatch occurred at 20,8°C (Table 1). 

 Test temperatures from' 12.7°C, the lowest test temperature, to 

 23.9°C were within the "optimum hatching range." The TL5 for 

 normal hatch was computed as 26.5°C. The difference between the 

 percentage of total and normal hatch increased at the higher test 

 temperatures, with the greatest deformed hatch for alewife eggs 

 occurring at 26.7°C. 



For striped bass acclimated to 18°C, results for eggs from a single 

 female indicated that the highest percentage of total and normal 

 hatch occurred at 22.2° C (Table 1). At temperatures from 18.8 to 

 25.8°C, normal hatching success remained at levels at least 70% of 

 the highest value recorded. In this case, however, no optimum range 

 was defined since at test temperatures above and below 22.2°C 

 normal hatching success was significantly less than the highest value 

 recorded (P= 0.05). The TLg for normal hatch was 26.3°C. At the 

 two highest test temperatures, 27.2 and 29.7°C, the normal hatch 

 percentages were both zero. 



