FISHERY BULLETIN: VOL. 74, NO. 2 



Rates of Thermal Acclimation 



Results suggest that acclimation from 12° to 

 20°C nears completion within 2 days of exposure to 

 the higher temperature. Mean resistance times for 

 day 2 and day 5 samples exceeded the mean resist- 

 ance time for the control sample. However, 

 analysis of variance shows that the variation be- 

 tween day 2, day 5, and control samples is not 

 significant (P>0.05). The relatively high resist- 

 ance of some fish in the day 2 and day 5 samples, 

 which exceeded the resistance of control fish, may 

 be due to slight temperature variations (± 0.2°C) in 

 the test aquaria, or possibly to "physiological 

 overshoots" to the acclimation process (Prosser 

 1973). Figure 5 shows the progress toward accli- 

 mation with contined exposure to the higher 

 temperature. Most noticeable is the change in 

 shape of the resistance curves with acclimation. 

 Nonacclimated fish succumb to the lethal 30°C 

 temperature quickly, probably as a result of 

 "shock effects" (Scott 1964; Tyler 1966; Allen and 

 Strawn 1971). Acclimation to the higher tempera- 

 ture diminishes the shock effects. Apparently, ac- 

 climated fish die from secondary causes termed 

 "direct effects" by Fry (1971). The physiological 

 basis of the shock and direct effects is not clear. 

 Acclimation from warm to cool water (20° to 12°C) 

 appears to be nearly complete by day 5 (Figure 6). 

 As acclimation progresses and resistance to low 

 temperatures is increased, death rate becomes in- 



MINUTCS EXPOSURE 



Figure 6. — Cumulative percent mortality ofEngraulis mordax 

 juveniles and adults as a function of exposure to 6°C. The re- 

 sponse of a 12°C acclimated control group (C) is compared with 

 that of a 20°C acclimated group after 0-, 2-, 5-, and 9-day expo- 

 sures to 12°C (original date in Brewer 1975b). 



creasingly regular; the graphs approach a straight 

 line and the effects of the initial cold shock are 

 largely eliminated. Because these tests used small 

 sample sizes, statistical differences cannot be 

 demonstrated. 



Effects of Cycled Temperatures on 

 Resistance 



MINUTES EXPOSURE 



Figure 5. — Cumulative percent mortality oiEngraulis mordax 

 juveniles and adults as a function of exposure to 30°C. The 

 response of a 20°C acclimated control group (C) is compared with 

 that of a 12°C acclimated group after 0-, 1-, 2-, and 4-day expo- 

 sures to 20°C (original data in Brewer 1975b). 



Results are summarized in Table 2. Mean re- 

 sistance times to 6° and 30°C for fish subjected to 



Table 2. — Resistance times (minutes until death) of juvenile 

 and adult Engraulis mordax to 6° and 30°C after being subjected 

 to temperature fluctuations between 12° and 20°C on a 48-h cycle 

 for a period of 25 days. 



periodic changes in temperature between 12° and 

 20°C were well below the mean resistance times of 

 fish acclimated to a constant 12°C and constant 

 20°C, respectively. However, the fish have greater 

 high temperature resistance than those accli- 

 mated to 12° and greater low temperature resist- 

 ance than those acclimated to 20°C. 



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