BREWER: THERMAL TOLERANCE AND RESISTANCE 



intersect a diagonal at the upper and lower ex- 

 tremes, which represents those points where 

 the lethal temperatures equal the acclimation 

 temperatures. Fish cannot be acclimated to tem- 

 peratures above or below these experimentally 

 determined ultimate upper and lower lethal 

 temperatures, respectively. 



Juvenile and adult E. mordax between 45 and 

 139 mm SL (standard length) were held for a 

 minimum of 3 wk at constant temperatures of 8°, 

 12°, 16°, 20°, 24°, and 28°C and under a light cycle 

 of 12 h light and 12 h dark. Unless otherwise 

 noted, the term "acclimated fish" designates E. 

 mordax held under such conditions. "Juvenile" re- 

 fers to metamorphosed fish less than 100 mm SL, 

 while "adult" refers to fish over 100 mm. 



The fish's susceptability to mechanical damage 

 increased at high and low acclimation tempera- 

 tures. Therefore, each acclimation temperature- 

 test series was accompanied by a series of strict 

 control transfers, and the observed mortalities for 

 each series were adjusted separately, based on the 

 respective control mortalities. Fish were consid- 

 ered dead and were removed when all swimming 

 movements ceased. Ninety-six hour LDso (mean 

 lethal dose) temperatures (i.e., incipient lethal 

 levels) were estimated from regression lines plot- 

 ted on probit paper (Sokal and Rohlf 1969). Exper- 

 iments were conducted between February 1973 

 and November 1974 and included all seasons. 

 About 20 fish were used for each test. 



Resistance as a Function of Size, 

 Sex, Time, and Photoperiod 



To test the potential influence of photoperiod, 

 sex, size, and diel effects, anchovies were accli- 

 mated to 20°C, tested by direct transfer to 30°C, 

 and the time to death (resistance time) deter- 

 mined. As the fish died, they were removed from 

 the test aquaria, measured, and adult fish were 

 sexed. Identical tests were conducted in the morn- 

 ing (0900 h) and in the evening (2100 h). Tests 

 were also conducted after fish had been held under 

 a short-day photoperiod (8 h light) and a long-day 

 photoperiod ( 16 h light) for periods of 3 wk each. 

 All thermal resistance tests were run during the 

 summer and fall. 



Rates of Thermal Acclimation 



Juvenile and adult anchovy acclimated to 12°C 

 were subjected to an 8°C temperature change over 



a 24-h period to 20°C, and then tested for resist- 

 ance to 30°C on the same day and after 1-, 2-, and 

 4-day exposure to the 20°C temperature. 

 Moreover, fish acclimated to 20°C were subjected 

 to a temperature decrease to 12°C over a 24-h 

 period, and then tested for resistance to 6°C on 

 the same day and after 2-, 5-, and 9-day exposure 

 to the 12°C temperature. As the fish become accli- 

 mated to the new higher or lower temperature, one 

 would expect the mean resistance times for these 

 fish to approach and eventually equal the mean 

 resistance times (e.g., reach a steady-state) offish 

 acclimated to 20° and 12°C and tested at 30° and 

 6°C, respectively (controls). 



Effects of Cycled Temperatures 

 on Resistance 



In view of the observations by Mais (1974) that 

 E. mordax may undergo diel vertical migrations 

 and consequently experience fluctuating tempera- 

 tures, I examined the relative thermal resistance 

 of anchovies subjected to regular changes in tem- 

 perature from 12° to 20°C over 48-h intervals. Fish 

 acclimated to 20°C were gradually subjected to 

 decreasing temperatures to 12°C over 24 h and 

 then back to 20°C over the next 24 h. The cycle was 

 repeated for 25 days, at which time a sample offish 

 which had just reached 20°C was tested for resist- 

 ance to 30°C. The following morning, as the re- 

 maining fish reached 12°C, a sample was tested for 

 resistance to 6°C. 



Embryos and Larvae 



Engraulis mordax eggs, caught in plankton 

 tows in or near the Los Angeles-Long Beach Har- 

 bor throughout the year, were utilized for experi- 

 ments on embryos and larvae. Water tempera- 

 tures, at time of capture, varied between 13° and 

 18°C. In the laboratory, eggs in the blastodisc 

 stage were placed into 2-liter glass jars and main- 

 tained at 12°, 16°, 20°, or 24 °C until transferred to 

 incubation or test vessels which consisted of 

 250-ml jars containing 60 ml of seawater. Not 

 more than five eggs or larvae were tested per jar. 



Twenty-four Hour Tolerance 



Larvae in the yolk-sac stage were tested within 

 1 day after hatching at each acclimation tempera- 

 ture. Larvae were pipetted from each acclimation 

 temperature directly into test vessels ranging 



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