Serum cortisol concentrations change in tiger grouper, Epinephelus fuscoguttatus in response to water temperature and salinity stress

The present exposition was designed to evaluate blood serum changes in tiger grouper, Epinephelus fuscoguttatus in response to changes in water temperature and salinity. Uniformly sized fingerlings were randomly distributed into different groups at fifteen fish per tank, in two replicates. Each group represented experimental fishes subjected to water temperatures of 18, 24, 28 and 30°C (control), and salinity of 10, 20 and 30 (control) ppt respectively. Replicate groups of fishes in each tank were exposed to these physiological stressors for 4 and 36 hours. At the end of experiment, blood samples were collected via caudal vein. The collected blood was centrifuged to obtain serum, andanalyzed for cortisol and glucose concentrations, using ELISA method. Results showed that decrease (to 28, 24 and 18°C) in water temperature from the control (30°C) and salinity from 30 ppt to 20 and 10 ppt for 4 and 36 hours influenced changes in the physical appearances (skin coloration) and behaviors (swimming vigor, opercula movement and schooling) of fish, to suggest responses to stress. Further, analyses of serum cortisol revealed consistently higher concentrations at the lower temperatures tested (to 28, 24 and 18°C), than the optimum tolerated by the species. Interestingly, fish maintained at 28°C for 36 hours contained lower serum cortisol concentrations compared to control groups. Meanwhile, the increment of the serum cortisol concentration occurs at decreased water salinity from 30 ppt to 20 and 10 ppt for 4 and 36 hours. It is concluded that: 1) water temperature and salinity are important physiological stressors; 2) sudden alteration of these factors leads to stress, and should therefore be avoided or minimized; 3) results of the present trial suggest that the blood serum factors are reliable parameters for evaluating the level of stress in fish.