THERMAL TOLERANCE AND RESISTANCE OF 

 THE NORTHERN ANCHOVY, ENGRAULIS MORDAX 



Gary D. Brewer^ 



ABSTRACT 



An experimental, flow-through seawater system, constructed to maintain juvenile and adult northern 

 anchovy, Engraulis mordax, and rear embryos and larvae through yolk-sac absorption under con- 

 trolled temperature and photoperiod regimes, was used to determine aspects of thermal tolerance, 

 resistance, rates of acclimation, and some effects of temperature on the development and growth of the 

 anchovy. 



Thermal tolerance was determined for juvenile and adult fish, acclimated to six constant tempera- 

 tures between 8° and 28°C. Thermal resistance (minutes until death for fish exposed to a lethal 

 temperature) was independent of photoperiod and fish size; however, females proved more resistant 

 than males, and resistance decreased at night. Acclimation (as measured by resistance) from 12° to 

 20°C was nearly complete after 2-day exposure to the higher temperature; acclimation from 20° to 12°C 

 was nearly complete after 5-day exposure to the lower temperature. Fish subjected to fluctuating water 

 temperatures between 12° and 20°C proved less resistant to cold than a 12°C (constant) acclimated 

 group and less resistant to heat than a 20°C (constant) acclimated group. 



Thermal tolerance was determined for larvae in the yolk-sac stage, acclimated to four constant 

 temperatures between 12° and 24°C. Although hatching occurred at temperatures as high as 29.5°C 

 and as low as 8.5°C, the percentage of normally developed larvae equaled that of controls (incubated at 

 16°C) only between temperatures of 27.0° and 11.5°C. Embryos in the blastodisc stage proved most 

 sensitive to acute temperature increases when compared to embryos in the blastopore closure stage and 

 larvae in the yolk-sac stage. These same three stages proved insensitive to acute temperature de- 

 creases to 0.5°C for 60-min exposure periods. 



Temperature is discussed in relation to anchovy distribution and survival under natural and 

 artificially created thermal conditions. 



Research on the effects of temperature on aquatic 

 organisms has been given impetus in recent years 

 as numerous lakes and streams are considered 

 potential heat reservoirs by electric power 

 generating plants and other industrial concerns. 

 As the demands for water as a heat transfer 

 medium continue to increase dramatically, more 

 attention will be turned to the marine environ- 

 ment for large volumes of water and surface areas 

 necessary for the dissipation of excess heat 

 (Naylor 1965; de Sylva 1969; Tarzwell 1972). Un- 

 checked thermal loading of freshwater and near- 

 shore marine ecosystems will inevitably pose a 

 serious threat to the homeostasis and well-being of 

 aquatic communities unless realistic guidelines 

 are established and enforced. Such guidelines 

 must be based on knowledge of how aquatic or- 

 ganisms respond to both acute and chronic tem- 

 perature changes. 



This study details aspects of thermal tolerance 

 and resistance (as defined by Fry 1971) on the 



'Allan Hancock Foundation, University of Southern Cali- 

 fornia, University Peirk, Los Angeles, CA 90007. 



Manuscripted accepted October 1975. 

 FISHERY BULLETIN: VOL. 74, NO. 2, 1976. 



embryo, larval, juvenile, and adult stages of the 

 northern anchovy, Engraulis mordax Girard. The 

 study was prompted by the proposed discharge of 

 thermal effluent into the Los Angeles-Long Beach 

 Harbor. The biology and fishery of the northern 

 anchovy in the Los Angeles-Long Beach Harbor 

 were described by Brewer (1975a). 



The general biology of the northern anchovy has 

 been summarized by Baxter (1967), Messersmith 

 et al. (1969), the California Department of Fish 

 and Game (1971), and Brewer (1975a). A dramatic 

 increase in abundance of E. mordax during the 

 past 20 yr (Ahlstrom 1967; Smith 1972) has 

 prompted an intense interest in the biology and 

 fishery potential of this clupeoid. The California 

 Department of Fish and Game (1971:48) consid- 

 ered the anchovy ". . . the most abundant species 

 with immediate harvest potential in the Califor- 

 nia Current system." 



MATERIALS AND METHODS 



Experiments were conducted in a small, tem- 



433 



