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Treatment of one-day old yolksac larvae of tilapia (Oreochromis mossambicus) by immersion in 0.05 ppm T4 or 0.01 ppm T3 significantly accelerated the differentiation and growth of all the fins, particularly pectoral and tail fins. Both the treatments also significantly accelerated yolk absorption and transition to free-swimming activity in the larvae. The treatments also significantly accelerated the growth of the larvae, with T3 at 0.01 ppm having a greater effect than T4 at 0.05 ppm. The yolk conversion efficiency was found not to be significantly affected by the hormone treatments but the treated larvae exhibited an increased heart beat, suggesting metabolic stimulation by the hormones.On the other hand, yolk absorption and free-swimming activity were significantly delayed in tilapia larvae immersed in 25 ppm solution of an antithyroid drug, phenylthiocarbamide (PTC). PTC also retarded the growth of the larvae. T4 (0.05 and 0.10 ppm) or T3 (0.01 and 0.02 ppm) therapy removed the PTC-inhibition,albeit not completely, suggesting that thyroid hormones are involved in the larval growth and development of tilapia.

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This study examines the effect of the steroid analogue, RU486, on the physiological responses of fed and chronically fasted rainbow trout to an acute handling stressor. This potent ligand of the glucocorticoid receptor was administered as a slow-release implant either alone, or in combination with cortisol. There were temporal changes in plasma cortisol concentrations following administration of cortisol implants in both fed and fasted trout. By day 14, plasma cortisol levels in fed fish were similar in all treatment groups, but in fasted fish, the effect of cortisol administration on plasma cortisol concentrations was still evident; RU486 administered with cortisol, did not affect this response. Cortisol administration also elicited a small, but significant increase in plasma GH concentrations in fed rainbow trout and in plasma glucose concentrations in Fasted animals. RU486-treatment prevented these responses. Conversely, whereas RU486 alone had no effect on hepatic 5'-monodeiodinase activity, when administered with cortisol it enhanced the marked suppressive effect of cortisol evident in both fed and fasted groups, suggesting that it may exert an interactive effect with cortisol on this process. Stressor-related changes in plasma cortisol, glucose, GH and thyroid hormone concentrations were evident in both fed and fasted groups; however, there was no evidence of a suppressive effect of RU486 treatment on any of the measured plasma parameters. Although RU486 did not prevent the stressor-related changes, the post-stressor cortisol profiles in RU-treated trout were extremely erratic compared with the oil-treated controls. This implies a disturbance of the normal interactions of the components of the hypothalamus-pituitary-interrenal tissue axis.