Mortality, Cortisol, Glucose and Plasma Ion Responses of <i>Oreochromis variabilis</i> to Transport Stress Under Low Oxygen and High Loading Density

Abstract

Live fish transportation is an essential component of aquaculture supply chains, yet it exposes fish to acute stressors including confinement, crowding, mechanical disturbance, and progressive deterioration of water quality typically associated with sealed transport systems. These stressors can impair welfare, disrupt physiological homeostasis, and increase mortality. This study quantified transport stress responses in <i>Oreochromis variabilis</i> by evaluating mortality and changes in endocrine, metabolic, and osmoregulatory indicators under low oxygen and high loading density conditions during road transport in sealed polythene bags. Fish were packaged at five loading weights (1, 3, 5, 7, and 9 kg) under varying oxygen supply levels. Blood sampling was conducted at the start and end of transportation to determine serum cortisol and glucose concentrations and plasma sodium (Na⁺) and chloride (Cl⁻) ions. Mortalities remained low and did not differ significantly among treatments at 1–5 kg loading weights (p > 0.05). However, mortality increased significantly at 7 and 9 kg loading weights (p < 0.05), indicating that excessive loading density was the dominant risk factor for survival regardless of oxygen level. Cortisol and glucose concentrations increased markedly after transport across all treatments compared with baseline (p < 0.01), confirming activation of the hypothalamic–pituitary–inter-renal axis and stress-related metabolic mobilisation. Plasma Na⁺ significantly declined post-transport (p < 0.05), while Cl⁻ concentrations differed significantly between baseline and post-transport and varied with oxygen supply (p < 0.05), suggesting impaired ion regulation in response to transport-induced hypoxia and crowding. Overall, the findings demonstrate that high loading density intensifies transport stress in <i>O. variabilis</i> and compromises survival and physiological stability. Optimisation of loading density and oxygen management is therefore recommended to improve welfare and reduce transport losses in culture systems.