Stress Responses in Siamese Fighting Fish (Betta splendens) Subjected to Commercial Confinement Rearing Practices: An Assessment in Welfare

Abstract Siamese fighting fish, which are subject to unique culture practices, are reared individually in confined spaces. This study aims to examine their stress responses, including blood chemistry, stress hormones, and stress-related gene expression under these commercial confinement rearing practices. The fish (mean body weight 0.12 ± 0.04 g; n = 30) were individually housed in 250 mL glass bottles containing 150 mL water for 14 days, with groups designed as control and treatment, the latter receiving 0.5% NaCl. Baseline parameters were measured before confinement, with assessments at 1, 3, 7, and 14 days. Results showed no significant differences in plasma cortisol, glucose, or hematological values between baseline and confinement (controls), except for increased plasma glucose with 0.5% NaCl (treatments) after 7 days. However, significant upregulation of stress-related genes (GR, MR, and HIF-1α) and downregulation of genes HSP70 and HSP90 were observed in confined fish. While certain genes (HSP70, GR, MR, and HIF-1α) showed significant differences after three days, these returned to baseline levels by the end of 14-day period. The addition of 0.5% NaCl significantly reduced plasma cortisol levels and the expression of certain stress-related genes, including HSP70, HSP90, GR, and MR compared to the control groups (p < 0.05). The findings suggest that current confinement methods do not cause significant stress and do not contradict established animal welfare principles. In addition, adding 0.5% NaCl to the water could be a practical and cost-effective method to reduce stress indicators during confinement, offering a viable solution for betta fish husbandry.