Effects of Confinement Rearing and Sodium Chloride Treatment on Stress Hormones and Gene Expression in Siamese Fighting Fish ( <i>Betta splendens</i> )

Siamese fighting fish are typically reared individually in confined spaces under commercial practices. This study examined stress responses-plasma cortisol, blood chemistry, and stress-related gene expression-during confinement and evaluated sodium chloride (NaCl) as a cost-effective mitigation strategy. Fish (0.12 ± 0.04 g) were individually housed in 250 mL bottles with 150 mL water for 14 days. Control fish were kept in freshwater, while treated fish received 0.5% NaCl. Baseline parameters were collected before confinement, with further assessments at days 1, 3, 7, and 14. No significant differences in plasma cortisol, glucose, or hematological values were found between baseline and confined controls. However, confined fish showed upregulation of <i>GR</i>, <i>MR</i>, and <i>HIF-1α</i> and downregulation of <i>HSP70</i> and <i>HSP90</i>, particularly at day 3, with levels returning to baseline by day 14. The NaCl treatment significantly reduced cortisol levels and the expression of HSP70, HSP90, GR, and MR (<i>p</i> < 0.05). These findings suggest that while confinement induces transient molecular stress responses, it does not significantly affect physiological stress markers, and 0.5% NaCl can help alleviate stress during confinement.