The Critical Thermal Maximum of Juvenile Red Drum Reared for Out‐of‐Season Stocking in Texas

Abstract

AbstractHatchery‐propagated red drum Sciaenops ocellatus are released by the Texas Parks and Wildlife Department (TPWD) during the spring, summer, and fall to supplement natural stocks. Reported success of red drum stocked out of season in summer has been low or nonexistent; several potential causes, including mortality due to high temperatures, have been suggested. The present study was devised to determine the critical thermal maximum (CTM) of pond‐raised juvenile red drum that had previously been exposed to diurnal temperatures of the summer season. Laboratory studies designed to mimic the rate of temperature increase (0.25°C per hour) typical of TPWD rearing ponds were used to examine rates of mortality in juvenile red drum that were collected from three separate ponds exposed to the ambient fluctuating temperatures of the summer season. The CTM (temperature that was lethal to 50% of the test fish [LT50]) was significantly lower (P < 0.0001) for two of the fish trials (LT50 = 37.1°C and 37.0°C) than for a third trial (LT50 = 38.7°C). This difference correlated with the size of fish in each trial; the two trials with smaller‐sized fish (range = 18–32 mm total length) had the lower CTMs, whereas the trial with larger‐sized fish (range = 31–50 mm total length) had the higher CTM, suggesting positive size dependence in the CTM. The CTMs obtained here are higher than the maximum temperatures encountered in grow‐out ponds during summer, indicating that high temperature exposure may not be the sole cause for the low success of red drum stocked out of season.