Larval Paralichthys dentatus of only one size class (4 mm TL) were tested. 

 The results stand in contrast to the findings with older Menidia larvae. A 

 significant increase in escape ability (P < .01) occurred in P. dentatus larvae 

 following thermal shock (Table 19-1). 



DISCUSSION 



Relationship of Results to Upper Thermal Limits 



Results of tests with four and six week old M. menidia indicate a possible 

 adverse effect following an acute thermal increase to 30.0°C from an 

 acclimation temperature of 20.5^C. The magnitude of this thermal elevation is 

 close to the one hour TLM value of 31.4°C given by Hoff and Westman (14) 

 for juvenile M. menidia acclimated to 20°C. The present study points to the 

 increased sensitivity of behavioral stress indices to monitor effects of 

 short-term or low level pollutant stress. Indeed, these findings strongly contrast 

 the view of Austin et al{\). Based on mortaHty studies of a 13 minute shock of 

 14°C above a 20°C acclimation temperature, he concluded that this treatment 

 would not have any important effects on survival of larvae of this species. 



The absence of significant differences in escapes in newly hatched and two 

 week old stressed M. menidia vs. the controls may be real, or could be due to 

 the low number of tests run and the high variability observed within the shock 

 groups. More data are necessary before final conclusions can be made on the 

 sensitivity of these younger larvae to thermal shock. 



The increase in escape probability following thermal shock with larvae of P. 

 dentatus may be due to an increase in altertness or in frequency of locomotory 

 movements. Because the ambient water temperature was lower in tests with P. 

 dentatus (IS.l'^C), the thermal shock did not approach reported lethal levels 

 (32.0°C CTM at 15°C acclimation, Hoss et al (25)). Increased escape ability 

 has been reported by Coutant (8) in juvenile Salmo gairdneri when thermal 

 shock temperatures are well below lethal levels. 



Potential Mechanisms of Thermal Shock Effects on Predator 

 Avoidance 



Although the phenomenon of differential predation in thermally shocked 

 fish is now well documented (8, 25, 27), causal mechanisms for changes in 

 vulnerability following thermal shock are not known. It has been demonstrated 

 that the central nervous system is highly sensitive to temperature fluctuations 

 (4, 23). The thermal sensory receptors are believed to consist of cutaneous free 

 nerve endings (3), yet behavioral response to thermal shock is not necessarily 

 limited to free nerve endings. Blood chemistry, membrane permeability, and 



294 



