Dietary γ-Aminobutyric Acid (GABA) Improves Growth Performance and Increases Plasma Cortisol During Recovery from Short-Term Acute Thermal Exposure in Juvenile Rainbow Trout (Oncorhynchus mykiss)

γ-aminobutyric acid (GABA) is a functional feed additive that may improve growth and stress resilience. The study evaluated the effects of dietary GABA on growth performance and recovery-phase physiological responses after short-term acute thermal exposure in juvenile rainbow trout. Two hundred twenty-five fish (18.9 ± 0.1 g) were fed five diets for 8 weeks: a control diet containing 68 mg/kg (GABA68) and diets with 108 (GABA108), 149 (GABA149), 183 (GABA183), and 218 (GABA218) mg/kg GABA. Fish were then subjected to short-term acute thermal exposure (20 °C for 2 h) followed by 2 h recovery at 15 °C. Dietary GABA significantly improved weight gain and specific growth rate, with the provisional model-based dietary GABA level for weight gain estimated to be approximately 198–210 mg/kg diet based on broken-line regression models, and increased feed and protein efficiency (p < 0.05). Fish fed the GABA183 and GABA218 diets had significantly higher plasma cortisol concentrations than those fed GABA68 and GABA108 when averaged across temperature treatments, with a significant temperature × GABA interaction (p < 0.05). During recovery, hepatic hsp70α and hsp90βα expression was significantly lower following 20 °C exposure than in fish maintained at 15 °C, whereas dietary GABA and its interaction with temperature had no significant effects on the stress-related genes analyzed. Dietary GABA improved growth performance and feed utilization; however, cortisol measured at a single recovery time point cannot distinguish a stronger stress response, delayed recovery, or altered cortisol clearance and does not demonstrate improved thermal tolerance or a faster recovery rate.