Low-Temperature Tolerance of Juvenile Spotted Seatrout in South Carolina

Spotted Seatrout Cynoscion nebulosus are vulnerable to mortality in winter when water temperature decreases rapidly. Temporary fishery closures in response to population declines should be based on a thorough understanding of the magnitude of winter mortality. We used three methods to estimate the low-temperature tolerance of Spotted Seatrout: the chronic lethal method (CLM) and the acclimated chronic exposure (ACE) method using both static and fluctuating temperatures. All three methods use an environmentally realistic temperature change of 1 degrees C/d. Fish acclimated to 14 degrees C (n = 24) for 10 d and subsequently exposed to constantly declining temperatures using the CLM experienced loss of equilibrium at a temperature of 3.57 +/- 0.24 degrees C (mean +/- SD) and mortality at a temperature of 3.08 +/- 0.31 degrees C. The ACE method, which uses changing temperatures until a base temperature is reached, was used to evaluate how Spotted Seatrout are affected by prolonged, sublethal temperatures. The ACE method was performed with static and fluctuating temperatures at two base temperatures (5.25 degrees C and 4.25 degrees C). Fish exposed to static temperatures of 5.25 degrees C (n = 46) and 4.25 degrees C (n = 46) had significantly different survival rates (P < 0.001) and 4% and 91% mortality, respectively. Temperature fluctuations around a 5.25 degrees C base temperature did not significantly affect loss of equilibrium or mortality compared with the static treatment at 5.25 degrees C. Temperature fluctuations at the 4.25 degrees C base temperature did not cause greater mortality, but did cause fish to lose equilibrium significantly faster than in the static treatment (P < 0.001). Results of this study suggest that temperatures below 5 degrees C should be cause for concern for this species, though the duration of cold temperatures and the spatial distribution of temperature conditions will determine the severity of a winter mortality event. These data can be used to develop predictive mortality estimates for juvenile Spotted Seatrout (175-220 mm).