Stress Response in Mozambique tilapia (Oreochromis mossambicus): Temporal and Inverse Interaction of Cortisol and Thyroid Hormone when Confined to Net

Thyroid hormones and cortisol are vital for the regulation of metabolic and hydromineral homeostasis in fish. The levels of triiodothyronine (T₃), thyroxine (T₄) and cortisol in the plasma and the indices of metabolic and hydromineral regulations were quantified in fresh water tilapia after confining them to net for varied time intervals (2, 6, 12, 24 h) to examine whether thyroid and interrenal interact during net-confinement. A time-dependent increase (P<0.001) in plasma cortisol occurred after net-confinement with a maximum increase at 12 h, indicating an induction of stress response in this fish. Confinement of tilapia to net for 6 and 12 h did not alter plasma T₃ but significantly decreased (P<0.05) its level at 24 h. Plasma T₄ remained unaffected at all intervals tested. Net-confinement produced a substantial increase in the plasma glucose (P<0.01) at all intervals tested and a maximum rise was found at 6 h. Branchial Na⁺, K⁺-ATPase activity increased (P<0.01) and renal Na⁺, K⁺-ATPase activity decreased (P<0.01) after 12 and 24 h net-confinement, with the maximum rise at 12 h. Plasma Na⁺ and plasma osmolality declined significantly (P<0.05) at 24 h net-confinement. Overall, the results indicate that net-confinement evokes stress response in tilapia, which includes a temporal and inverse interaction between T₃ and cortisol. The data thus support the hypothesis of a lead role of cortisol in stress response of tilapia.