The effect of levothyroxine sodium on hatch and survival rate of rainbow trout (Oncorhynchus mykiss) at early developmental stage

BACKGROUND: Thyroxine hormone appears to play some role at early stages of development and survival of fish larvae. Objectives: The purpose of the present study was to investigate the effect of levothyroxine sodium on hatching and survival rate at early developmental stage of rainbow trout (Oncorhynchus mykiss) larvae. Methods: This study was carried out in Dalkhan propagation and cultivation farm, Sepidan, Shiraz. Fertilized eggs were bathed in levothyroxine sodium with concentrations of 0, 0.1, 0.5 and 1.0 mg/L (each one, triplicates) for six hours. Then they were transferred into fiberglass trough trays. After hatching, all larvae were placed in the mentioned levothyroxine sodium concentration bath for another six hours and then transferred into troughs (without tray of running water). Larvae stayed in troughs until the yolk- sac was absorbed. Finally, live rainbow trout larvae were counted in separated plates. Results: The results indicated that the highest ratio of hatched eggs (81.42%± 1.04) and survival larvae after the yolk - sac was absorbed (72.90%± 0.75) were observed in 0.5 mg/L of levothyroxine sodium concentration (p<0.05). However, hatching and survival rate at early growth stage, in 0 and 1 mg/L of levothyroxine sodium concentration, didn't indicate significant difference (p>0.05). Conclusions: It can be concluded that 0.5 mg/L of levothyroxine sodium concentration has positive effect on hatching and survival rate of rainbow trout larvae.