Defining an ideal temperature range for the northern subpopulation of Pacific sardine, Sardinops sagax caeruleus

The aim of this study was to determine a physiologically “ideal” temperature range of Pacific sardine ( Sardinops sagax caeruleus ) from the northern stock by assessing the effects of cumulatively increasing or decreasing temperature on their stress physiology. Sardines collected off the coast of Southern California during late spring and late fall were exposed to cumulatively increasing or decreasing temperatures that reached +/− 8 °C from their acclimation temperature over a period of approximately one month. Blood plasma and tissue samples were collected at every 2 °C temperature change. Measurements included plasma cortisol, expression of heat shock proteins (Hsp70, Hsp90, HOP) in liver tissue, expression of an immune gene (IgM) in liver tissue, and Michaelis-Menton (K m ) determinations of lactate dehydrogenase (LDH) and citrate synthase (CS) from white and red muscle tissue, respectively. Critical thermal minimum and maximum temperatures were also measured to identify the outer edges of their thermal tolerance. Feeding behavior ceased and plasma cortisol was elevated at the lowest temperature, while Hsp90 and K m pyr (LDH) values were elevated at the highest temperatures to which sardines were exposed. An additional finding was that V max of LDH increased with increasing condition factor, indicating the occurrence of metabolic scaling in Pacific sardine. Critical thermal minimums and maximums for sardines acclimated to 15 °C and 17 °C were 3.4 °C – 29.1 °C and 4.8 °C – 29.9 °C, respectively. This study indicates Pacific sardine from the northern stock have a physiologic ideal temperature range of 9 °C – 19 °C for 15 °C acclimated sardines and 11 °C – 21 °C for 17 °C acclimated sardines.