Low-Temperature Tolerance of Juvenile Spotted Seatrout, Cynoscion Nebulosus, in South Carolina

Mortality due to low temperatures impacts population dynamics by reducing total stock size, influencing recruitment, and differentially weakening annual cohort strength.  In South Carolina, spotted seatrout are periodically affected by rapid changes in temperature during the winter.  Spotted seatrout are vulnerable to winter kills when air temperature drops, causing the shallow water to chill rapidly.  In these conditions, temperature changes are so sudden that fish may be immobilized and killed due to osmotic imbalance before they are able to escape the cold.  Significant population declines have been observed in South Carolina following extreme low temperatures in the winter.  In this study, I used the chronic lethal method (CLM) to determine the temperatures that are lethal to spotted seatrout when exposed to a -1 C/day change in temperature.  Spotted seatrout were found to experience loss of equilibrium at a mean (±SD) temperature of 3.57 ± 0.24 C, and mortality at a mean temperature of 3.08 ± 0.31 C.  In the next phase of my study I will evaluate how spotted seatrout are affected by sub-lethal temperatures when exposed over a prolonged period of time, and how seatrout mortality is affected by temporary cooling and warming periods.  This experiment will use a modified acclimated chronic exposure (ACE) method, which allows acclimation to changing temperatures (-1 C/day) until a pre-determined base temperature is reached.  The ACE method will be performed with both static and fluctuating temperatures, at two base temperatures.  These experimental temperature regimes mimic the natural variability observed in the environment.  Under current management in South Carolina, recommendations concerning the harvesting of spotted seatrout are not made until stocks are estimated by surveying in the spring.   If a local population is significantly reduced due to winter mortality and an emergency closure of the recreational fishery is needed, management decisions may be made too late to be fully effective if they are delayed by lack of data.  The results of this study will provide data that can be used to develop predictive management tools, and will be shared with fishery managers and recreational fishermen.