The present study was conducted on gonad histology and length at first maturity of the rainbow trout Oncorhynchus mykiss Walbaum 1792. The length at first maturity recorded was at 28.7cm. Histological snaps of immature phase revealed the ovigerous lamellae containing oogonial nests. Early maturing phase showed vascularisation and initiation of yolk formation with the ovaries start occupying the space within abdominal cavity. The oocytes show increase in yolk deposition in the form of globules and appearance of zona pellucida membrane with ooplasmic content was the highlight of the advance maturing phase. Considerable percentage of vittellogenic oocytes were observed within the advance maturing phase. The spawning phase revealed developed ovaries that reflected maturation of oocytes justified by increase in their size, volume, weight and entire abdominal cavity was filled with mature ovaries surrounded by thinnest tunica albuginea and zona pellucida. The presence of the late maturing oocytes and post ovulatory follicles was seen in late maturing phase with conspicuous blood vessels. The histology of spent phase revealed ruptured post-ovulatory follicles and showed irregular folds of zona pellucida and post-ovulatory follicles. During this phase atretic oocytes were also observed that is highlighting the atresia incase of trout fishes.
Histology, O. mykiss, Trout farms, Kashmir, Coldwater
