MAY: EFFECTS OF ACCLIMATION 



36 



o ^ 



32 



LiJ 



a. 



30 



26 



• 15 %o 



o 25%, 



A 36 %o 



i 45 %o 



-~--i- 



-4— 



A 



?"- 



I 



3 6 12 24 



DURATION OF EXPOSURE (hours) 



48 72 



Figure l.-Heat tolerance of larval bairdiella acclimated to 21°C 

 in various salinities. The upper median tolerance limits (TLm) 

 are plotted for various durations of exposure. The time scale is 

 logarithmic, and lines were fitted by eye. 



(Brett 1956), but in several curves there is a 

 suggestion of an early plateau during the first few 

 hours of exposure to the test conditions (Figures 1, 

 3, 4). At any given time the TLm was usually 

 higher in the lower salinities. Since the highest 

 test temperature used in the experiments was only 

 36°C, at acclimation temperatures above 21 °C the 

 50% mortality point was usually not reached until 3 

 or more hours after exposure to the test condi- 

 tions. Survival was very poor among larvae from 

 eggs maintained at 30°C (a temperature highly 

 stressful to eggs-May 1975) in 30°/oo, and at 24 h, 

 survival in this group was below 50% at all test 

 temperatures. For purposes of comparison, the 



o 



o 



E 

 _i 



H 



(E 

 LiJ 



a. 



Q- 



3 



3 6 12 24 



DURATION OF EXPOSURE (hours) 



48 72 



Figure 3.-Heat tolerance of larval bairdiella acclimated to 27°C 

 in various salinties. 



24-h upper TLm has been plotted against salinity 

 for various acclimation temperatures (Figure 5); 

 in this graph the increase in TLm at lower salini- 

 ties is clear, as is the general increase in TLm 

 effected by higher acclimation temperatures. In a 

 salinity of 15^/00, all larvae alive at 12 and 24 h in 

 the 21°C acclimation group were moribund in test 

 temperatures of 30°C and higher, i.e., they were 

 contorted and totally immobile and unresponsive 

 to touch, although their hearts were beating and 

 they were not opaque. These larvae have been 

 considered dead for the purpose of data presen- 

 tation; if considered alive, they would raise the 

 calculated 12-h upper TLm from 29.5° to 32.1°C 

 (Figures 1, 5). At the salinity of normal seawater, 

 the 24-h upper TLm of larval bairdiella lies 



36 



334 



E 



LU 



Q. 



13 



32 



30 - 



28 



I 3 6 12 24 48 72 



DURATION OF EXPOSURE (hours) 



Figure 2.-Heat tolerance of larval bairdiella acclimated to 24°C 

 in various salinities. 



I 3 6 12 24 48 72 



DURATION OF EXPOSURE (hours) 



Figure 4.— Heat tolerance of larval bairdiella acclimated to 30°C 

 in two salinities. 



251 



