Evaluation of the physiological status of the Pacific sardine, <Emphasis Type="Italic">Sardinops sagax caeruleus</Emphasis>, acclimated to different thermal regimes based on selected blood parameters

Despite the widespread recognition of the importance of temperature in the population dynamics and distribution of Pacific sardine (Sardinops sagax caeruleus), few studies have examined the species’ physiological response to temperature under controlled conditions. The effect of fluctuating thermal regimes on blood plasma cortisol, glucose, aspartate aminotransferase (ALT), alanine aminotransferase (AST) and red blood cell (RBC) counts of Pacific sardine was evaluated to monitor physiological response. Sardines from the southern subpopulation that inhabits waters off Baja California, Mexico, were collected during fall 2008. Acclimation to four fluctuating temperature regimes (two symmetric and two asymmetric) and a constant, optimal temperature occurred over 20 d. Symmetric and asymmetric regimes were designed to simulate regional summer (18–23°C) and winter (13–18°C) temperature ranges within 24 h. In the optimal regime (OR), sardines were acclimated to 18°C. In symmetric regimes, sardines were exposed for 7 h to the high and low temperatures, while in asymmetric regimes exposure to the high temperature was shorter (4 vs.10 h). Blood sampling took place during the low and high temperature exposure, and we tested for differences in blood parameters compared to OR. Plasma cortisol and glucose concentration of sardines from summer regimes were only significantly higher at 23°C. AST activities were significantly higher than OR during the high and low temperatures of both summer treatments. No differences were found in individuals exposed to the winter regimes. ALT activities were significantly higher than OR only during the high and low temperature of the symmetric summer regime. RBC were significantly higher than OR for both summer regimes. Sardines from southern subpopulation exhibit a negative physiological response to high temperatures, yet they can acclimate to the lower (≤18°C) temperatures typical of the California Current System. Temperatures ≥23°C may be considered a detrimental sublethal temperature, particularly for long exposure periods.