Influence of a continuous light period on the first sexual maturation of the rainbow trout (Oncorhynchus mykiss).

Sexual maturation rate at the age of 2 years was significantly lower in a g roup of rainbow trout (Oncorhynchus mykiss) under continuous light from the end of the vesicle resorption stage than in the control. This inhibition w as almost total in females, while sexual maturation rate was reduced by hal f in males. The same phenomenon has been observed in a group of females mai ntained under constant illumination from the first day of 0+ July to the fi rst day of 1+ July. The analysis of the physiological parameters has shown that the exogenous vitellogenesis did not start during the 1+ year. Moreove r, it was confirmed that the continuous light exposure has a beneficial eff ect on the growth rate of the treated fish in comparison with control.