Physiological Stress Responses of Starry Flounder (Platichthys stellatus) to Stocking Density under High-Temperature Conditions

This study investigated the effects of stocking density on the survival, growth, and physiological stress responses of starry flounder (Platichthys stellatus) under high-temperature conditions. Fish were assigned to four stocking-density treatments: a control group maintained at the recommended stocking density, two high-density groups stocked at 30% and 60% above the control density, and a low-density group stocked at 30% below the control density. Following acclimation at 22°C, the water temperature was increased by 1°C per day until it reached 28°C and was then maintained at 28°C for 28 days. Feeding and starvation conditions were evaluated separately. Survival, condition factor, weight loss, plasma cortisol, glucose, aspartate aminotransferase (AST), alanine aminotransferase (ALT), total protein (TP), total cholesterol (TCHO), and triglyceride (TG) levels were assessed, along with dissolved nutrient concentrations in the rearing water. Under feeding conditions, survival at the end of the experiment was markedly lower in the 60% and 30% high-density groups (35.3% and 48.6%, respectively) than in the control (92.9%) and low-density (96.0%) groups. The 60% high-density group also exhibited a lower condition factor and greater weight loss than the control and low-density groups. Plasma cortisol levels generally peaked on day 7 after exposure to 28°C, whereas AST levels increased with prolonged high-temperature exposure and reached their highest level on day 28. TP, TCHO, and TG levels generally decreased during prolonged exposure, suggesting progressive changes in nutritional and energy status under thermal stress. Dissolved nutrient concentrations, particularly NH4, NO3, and dissolved inorganic nitrogen, tended to increase with stocking density and were generally higher at 22°C than at 28°C. Overall, high stocking density aggravated the adverse effects of prolonged high-temperature exposure, resulting in reduced survival, poorer body condition, greater weight loss, and altered physiological stress responses in starry flounder. These findings indicate that maintaining an appropriate stocking density is important for reducing physiological stress and mortality of P. stellatus during periods of elevated water temperature.