Effects of Long Light Photoperiods on the Delay of Gonadal Development in Female Rainbow Trout (Oncorhynchus mykiss): 1. Structural Changes

Effects of two long-light photoperiod regimes on oocyte development and structural changes in female rainbow trout ovary were investigated. Two-years old females with a mean initial weight of 279.94 ± 2.25 g were subjected to 24L:0D and 18L:6D as long light photoperiods and also natural photoperiod (as control) for 5 months. Fish were fed formulated pellet food from a local factory in 2% BW daily. The oocytes of control group were significantly larger than those of the other groups (P< 0.05) with a mean diameter of 2.34 ± 0.07 mm (mean ± SE). Application of 24L:0D and 18L:6D caused the lowest development in follicular layer compared to the control group. The number of granulose cells were 349 ± 21 and 406 ± 23 in 24L: 0D and 18L:6D, respectively, which were significantly lower (P<0.05) than the control group (773 ± 20). Similarly, granulose cell area of both artificial regimes were significantly lower (P<0.05) than the control. The mean granulose cell area were recorded 16.58 ± 0.60 and 16.73 ± 0.33 µm2 in 24L:0D and 18L:6D, respectively and 19.28 ± 0.42 µm2 in control. In conclusion, in the period of gonadal growth and development, artificial long-light photoperiods can be used as an effective managerial tool to delay the reproduction of female rainbow trout.