BREWER: THERMAL TOLERANCE AND RESISTANCE 



Embryos and Larvae 



Twenty-four Hour Tolerance 



Over 600 larvae were tested in the 24-h toler- 

 ance experiments. Generally, 10 or more larvae 

 were tested at each temperature. The percentage 

 (normal) survival for controls ranged from 72.7 at 

 the 12°C acclimation level to 86.7 at 16° and 20°C 

 acclimation temperatures. 



Apparently the physiological mechanisms for 

 thermal acclimation are little developed in E. 

 mordax larvae in the yolk-sac stage. Figure 7 

 shows the 24-h lethal temperature relations with 

 percent adjusted mortality plotted against test 

 temperatures for acclimation temperatures of 12° 

 and 24°C. Rearing the yolk-sac larvae in warm 

 and cold water does little to increase or decrease 

 their upper or lower lethal temperatures, respec- 

 tively. Potential effects of parental acclimation 

 temperatures (Hubbs and Bryan 1974) or the ex- 

 posure of eggs to acclimation temperatures at the 

 time of fertilization require investigation. 



10 12 14 16 18 20 22 24 26 28 30 32 

 ACCLIMATION TEMPERATURE t 



Figure 8. — Thermal tolerance polygon for Engraulis mordax 

 larvae in the yolk-sac stage. Those points where the extrapolated 

 LDso levels intersect the diagonal represent the extreme toler- 

 ance limits and correspond to 7.0° and 30.2°C. 



IfMP(R*IUHf "^ 



FIGURE 7.— Effects of acclimation temperatures of 12° and 24°C 

 on the upper and lower lethal temperatures of Engraulis mordax 

 larvae in the yolk-sac stage (original data in Brewer 1975b). 



In Figure 8, adjusted upper and lower LDso 

 temperatures are plotted against respective ac- 

 climation temperatures in the construction of a 

 thermal tolerance polygon. Ultimate upper and 

 lower lethal temperatures are estimated to be 

 30.2° and 7.0°C, respectively. 



Hatching and Developmental 

 Temperature Limits 



Results of this experiment are given in Table 3. 

 Although hatching was observed at temperatures 



Table 3. — Effects of temperature on hatching and development 

 of Engraulis mordax through yolk-sac absorption and eye pig- 

 mentation of larvae. Temperatures were maintained within 

 ±0.5°C of those shown below. 



Rearing 



With normal development Adjusted 



'Control. 



as high as 29.5°C and as low as 8.5°C, 50% (ad- 

 justed) normal development occurred between 

 11.0° and 28.5°C. Only below 27.0°C and above 

 11.5°C did the percentages of hatching and normal 

 development approach those for the controls. 



439 



