FISHERY BULLETIN: VOL. 74, NO. 2 



maximum temperatures for Saluelinus (Sal- 

 monidae), depending on the time the test was con- 

 ducted. He also noticed that maximum tolerance 

 followed a 24-h cycle and suggested that this was a 

 physiological adaptation to natural habitats with 

 24-h variations in temperature. 



The present experiments on E. mordax were 

 conducted in the fall when anchovy presumably 

 ascend from deep water to warm surface waters in 

 the evening (California Department of Fish and 

 Game 1971). If a circadian cycle of thermal resist- 

 ance existed in anchovy, one might anticipate 

 maximum resistance to high temperatures to 

 occur in the evening. The data in Table 1 suggest 

 that, under laboratory conditions, resistance to 

 high temperature is reduced in the evening. 



The embryonic and larval stages of pelagic 

 fishes are potentially the most vulnerable ones to 

 thermal stresses. While juvenile and adult fishes 

 may detect and avoid unfavorable environmental 

 conditions (Bull 1928; Doudoroff 1938; Alabaster 

 and Robertson 1961; Coutant 1969), the eggs and 

 planktonic larvae of fishes such as E. mordax are 

 at the mercy of currents which might carry them 

 into environments unfavorable for growth or sur- 

 vival. Reviews by de Sylva ( 1969) and Brett ( 1970) 

 have shown that on the average, marine fish lar- 

 vae are one-third to one-half as tolerant to thermal 

 stresses as their conspecific adults. Normal de- 

 velopment of £. mordax is inhibited below 11.5°C 

 and above 27.0°C. Larvae held at temperatures 

 below 11.0°C for short periods become inactive, 

 making little effort to avoid capture by pipette. 



The survival of pelagic larvae is dependent on 

 the early consumption of prey species and the abil- 

 ity to avoid predators (Lasker et al. 1970). The 

 degree to which these two processes can be ac- 

 complished is largely dependent on the optimal 

 development of swimming ability, precise biting 

 reflexes, and visual acuity (Hunter 1972). Since 

 swimming ability is proportional to larval size, the 

 development of maximum growth potential 

 should be of distinct survival value. Maximum 

 growth of larvae in the yolk-sac stage, in turn, is 

 dependent on the efficient utilization of the lim- 

 ited yolk reserve, i.e., its conversion into body 

 tissues. 



Growth of anchovy larvae in the yolk-sac stage 

 is maximal in experimental temperatures be- 

 tween 14° and 20°C. Variation within this range 

 may be highly significant but is difficult to test. 

 Although growiih rates of anchovy larvae in the 

 yolk-sac stage increase with increasing tempera- 



tures, the maximum size attained by the larvae 

 decreased at high temperatures. 



Thermal tolerance limits have been determined 

 for anchovy larvae and juveniles and adults by 

 tests that considered the LD50 as a lethal end 

 point. LD50 temperatures do not represent "safe" 

 levels and have been used merely because of con- 

 vention. Any temperature level that produces a 

 lethal response significantly greater than the 

 maximum response at control temperatures 

 should be considered excessive. This would repre- 

 sent the most realistic end point to insure en- 

 vironmental quality. The thermal death of even a 

 few individuals at any particular temperature 

 level suggests that the survivors are under severe 

 stress, leaving them unable to compete success- 

 fully for limited resources or avoid predation. For 

 acclimation temperatures of 8°, 12°, 16°, 20°, and 

 24°C, a range of temperatures encountered by 

 juveniles and adults in nature, immediate ex- 

 posure to high temperatures less than 23.0°, 24.0°, 

 25.5°, 26.5°, and 27.5°C, respectively, would be 

 tolerated by fish from southern California without 

 significant mortality from the direct effects of 

 temperature alone. Likewise, for the same accli- 

 mation temperatures, juvenile and adult anchovy 

 could tolerate lows of 7.5°, 10.0°, 12.5°, 13.5°, and 

 14.5°C, respectively. Larvae in the yolk-sac stage 

 can tolerate limited exposure (24 h) to any tem- 

 perature <28.0°C and >12.0°C. Regardless of ac- 

 climation temperature, larvae in the yolk-sac 

 stage, juveniles, and adults can endure sudden 

 temperature increases and decreases between the 

 limits of 14.5° and 23.0°C wdthout significant le- 

 thality from direct temperature effects alone. 



Although the gross effects of high and low tem- 

 perature extremes have been quantified, the 

 physiological and biochemical factors that are 

 responsible for thermal death and temperature 

 acclimation are poorly understood. Various 

 mechanisms to account for these phenomena have 

 been discussed by Hochachka and Somero (1971) 

 and Hazel and Prosser (1974). Evidence suggests 

 that qualitatively different enzymes (isoenzymes) 

 may be synthesized during thermal acclimation, 

 and "warm" and "cold" enzyme variants have been 

 described (Hochachka 1967; Hochachka and 

 Somero 1968; Hebb et al. 1969). Enzyme inactiva- 

 tion has been suggested as a cause of thermal 

 death, but it is ". . . undoubtedly more subtle than 

 gross protein denaturation" (Hochachka and 

 Somero 1971:139). The reaction velocities (Kr,,) of 

 enzymes may drop below certain critical levels at 



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