BREWER: THERMAL TOLERANCE AND RESISTANCE 



physiological compensation begin to break down 

 at temperatures above 25°C. 



Reid's (1967) observation that E. mordax may 

 overwinter at temperatures of 7° or 8°C off British 

 Columbia is of special interest. These fish may be 

 within less than 1°C of their lower lethal tempera- 

 ture. Juvenile and adult anchovy acclimated to 

 8°C in the laboratory and transferred to 7°C made 

 no effort to consume food offered to them after 5 

 days at the lower temperature. I have not 

 confirmed this by stomach examination, but feed- 

 ing, if it takes place at all, is minimal at this low 

 temperature. 



It is important to consider the possibility that 

 the thermal tolerance and resistance of £■. mordax 

 may be different for northern, central, and south- 

 ern populations. Apparently genetically dis- 

 tinct, these populations were first identified on the 

 basis of meristic characters by McHugh ( 195 1 ) and 

 later on by serum transferrin analysis conducted 

 by Vrooman and Smith (1971). If the thermal re- 

 quirements of these populations were distinct, I 

 would anticipate their reproductive temperature 

 ranges to vary accordingly. Richardson's (1973) 

 data on anchovy spawning off Oregon discount 

 this. In any case, thermal resistance experiments 



on samples from each population would be of 

 interest. 



Experiments on the resistance of juvenile and 

 adult anchovy to a high lethal temperature 

 showed no significant difference in the mean re- 

 sistance times for fish of different sizes or for fish 

 maintained under different photoperiods. How- 

 ever, females were more resistant than males, and 

 animals tested in the morning showed greater re- 

 sistance than those tested in the evening. Inves- 

 tigators have variously shown significant differ- 

 ences in one or more of the factors tested here, 

 depending on the species. Thermal resistance has 

 been found to vary according to size, with large 

 Oncorhynchus (Salmonidae) and Carassius (Cyp- 

 rinidae) more resistant to cold (Brett 1952; Hoar 

 1955, respectively) and large Clupea (Clupeidae) 

 less resistant to heat (Brawn 1960). Carassius 

 maintained under long photoperiods were more 

 resistant to high temperatures than fish main- 

 tained under short photoperiods, while resistance 

 to cold temperatures was greater for the short 

 photoperiod fish (Hoar 1956). Hoar discovered that 

 male Carassius were more resistant to low tem- 

 perature extremes than females. Heath ( 1963) ob- 

 served slight differences in critical thermal 



°c 6 



8 



10 12 14 16 18 20 22 24 26 28 30 



-Spawning Limits - 



Range of Larvae - 



-Range of Adults- 



FlGURE 10.— Field and laboratory deduced thermal limits for the distribution and survival ofEngraulis mordax. 



441 



