effect of fasting and re-feeding on thyroidal hormone concentrations and growth performance of rainbow trout (oncorhynchus mykiss)

the aim of this study was to show the effect of four feeding regimes on thyroid hormones concentrations, growth performance and food coefficient in rainbow trout applied through 4 treatments and 3 replications in winter 2006. in this research, samples of rainbow trout with an initial mean weight (so) 47.19±0.42 used in each rearing unite. fish were fed twice a day ad libitum as follows: treatment a (ta): which was the control treatment; continues feeding, treatment b (tb): 4 weeks of starvation and 5 weeks of re-feeding, treatment c (tc): 3 weeks of starvation and 5 weeks of re-feeding, and treatment 0 (to): 2 weeks of starvation and 5 weeks of re-feeding. indexes like food coefficient ratio (fcr), specific growth rate (sgr), daily food intake and thyroid hormones (t3, t4) were also examined during the experiment. blood samples were taken for thyroid hormones concentration at the beginning of the experiment and at the end of the starvation and every 12 days in re-feeding periods. tb and tc had significant difference (p0.05). there were no significant differences between the treatments in fcr (p>0.05). t3 concentration came down in comparison with the control treatment at the end of the starvation (day 29) (p in there-feeding periods. t4 concentrations of fasting groups were lower than the control group at the end of fasting periods (p<0.05), but t4 trend in re-feeding periods increased. according to the results, different treatments of feeding regimes could affect the thyroid hormones concentrations. fasting periods  reduced  and  re-feeding  periods  increased  thyroid  hormones  concentrations.  tb  and  tc showed more indexes of cg in comparison with ta and to. we concluded that thyroid hormones alone are enough to assess cg and we suggest to use other growth relating physiological elements in different feeding diets and regimes in future studies to complete the evaluation.