THERMAL TOLERANCE AND RESISTANCE OF THE NORTHERN ANCHOVY, ENGRAULIS MORDAX

Abstract

An experimental, flow-through seawater system, constructed to maintainjuvenile 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 ofacclimation, and some effects oftemperature on the development and growth ofthe 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 wasnearly 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 ofcontrols (incubated at 16°C) only between temperatures of 27.0° and U.5°C. Embryos in the blastodisc stage proved most sensitive toacute temperature increases when comparedto 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 oftemperature 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; Tarzwell1972). 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