Submitted to: Journal of Applied Aquaculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/22/2003
Publication Date: 11/1/2003
Citation: Delaney, M.A., Klesius, P.H., Shelby, R.A. Cortisol responses of Nile tilapia (Oreochromis niloticus (L.)to rapid temperature changes. Journal of Applied Aquaculture. 16: (3/4) 95-104. Interpretive Summary: Fish are stressed both in the wild and in aquaculture operations. Acute stressors typically are caused by activities such as transporting, confinement and handling. Chronic stressors include poor water quality, overcrowding, disease and incorrect diets. Fish respond to stressors by releasing hormones such as catecholamines and cortisol. The majority of the eresearch documenting changes in plasma cortisol in response to stress has been conducted in cold water fish such as salmonids. Very little information is available on the response of warm water fish, especially Nile tilapia to acute thermal stress. Enzyme linked immunosorbent assay (ELISA) was used to determine the levels of the glucocorticoid cortisol circulating in the plasma of juvenile Nile tilapia evaluated during two seperate experiments. In Experiment I, there was no significant difference between cortisol levels in fish when the water temperature was gradually increased from 27deg to 32deg or 40deg C and maintained for 1 hour. In Experiment II, tilapia were subjected to 18deg, 27deg, 30deg, 32deg, 34deg, 36deg, 38deg or 40deg C water for 10 minutes followed by a recovery period of ten minutes at 27deg C. There were no significant differences in cortisol levels in tilapia exposed to 18deg, 27deg, 30deg, 32deg, 34deg, 36 deg or 38deg C; however, a significant difference was observed between the values of these treatments and that of the 40deg C treatment. Based on the cortisol results a gradual temperature increase from 27deg C to either 32 or 40deg or direct exposure to temperatures from 18-38 for ten minutes does not trigger the cortisol mediated stress response, sudden temperature change of 13-40deg C does. Results of this study indicate that juvenile Nile tilapia have transient cortisol responses to acute thermal stress.
Technical Abstract: Enzyme linked immunosorbent assay (ELISA) was used to determine the levels of the glucocorticoid cortisol, circulating in the plasma of juvenile Nile tilapia (Oreochromis niloticus (L.)), during two forms of acute thermal stress. In Experiment I, there was no significant difference (p<0.05) between cortisol levels in tilapia when the water temperature was gradually increased from 27 deg to 32 deg or 40 deg C and maintained for one hour. In Experiment II, tilapia were subjected to 18 deg, 27 deg, 30 deg, 32 deg, 34 deg, 36 deg, 38 deg or 40 deg C water for ten minutes followed by a recovery period of ten minutes at 27 deg C. There was no significant difference in cortisol levels in tilapia (p<0.05) between treatments at 27 deg, 30 deg, 32 deg, 34 deg, 36 deg or 38 deg C; there was a significant difference between these treatments and treatments at 40 deg C. There were significantly higher cortisol levels in tilapia exposed to 27 deg C or 40 deg C for ten minutes, and the 27 deg C treatment with a ten minute recovery period.