Chronic Thermal Effects on Growth, Osmoregulation, and Stress Physiology in Chum Salmon (Oncorhynchus keta) Smolt

Chum salmon (Oncorhynchus keta) is undergoing aquaculture development in the Republic of Korea (ROK). Understanding its thermal biology, including the identification of optimal and suboptimal temperature ranges, is essential for sustainable aquaculture, particularly in a warming marine environment. In this study, we aimed to assess the optimal temperature range and high-temperature effects in chum salmon smolt reared at 10, 14, 18, and 22 °C for 6 weeks. Specifically, we evaluated growth performance, osmoregulatory capacity, health indicators, and endocrine and cellular stress responses after 3 and 6 weeks of exposure. After 6 weeks, the growth performance peaked at 18 °C, whereas both growth and body lipid reserve significantly declined at 22 °C despite sustained appetite. Growth was also significantly lower at 10 °C. Plasma osmolality and ion concentration did not change with increasing temperature. While hematocrit (Hct) and red blood cell count (RBCC) significantly decreased at 18 °C and 22 °C, hemoglobin concentration did not change significantly. The typical endocrine stress response was not observed; rather, cortisol levels decreased at 22 °C, whereas hepatic Heat Shock Protein (HSP)70 and HSP90 mRNA expressions were significantly upregulated at both 18 °C and 22 °C, with the markedly higher induction at 22 °C. These findings collectively indicate the onset of cellular stress at temperatures of 18 °C or higher. Despite the peak growth performance and competent osmoregulation performance at 18 °C, the concurrent induction of heat shock responses and decreases in Hct and RBCC suggest that the physiological optimum lies below 18 °C. Taken together, these findings suggest that maintaining rearing temperatures above 10 °C and below 18 °C is advisable to promote growth while minimizing cellular stress in aquaculture settings.