DYNAMICS OF BIOCHEMICAL COMPOSITION IN THE OVARIES OF CAPTIVE-BRED FEMALE RAINBOW TROUT (Oncorhynchus mykiss) DURING GONADAL DEVELOPMENT

The reproductive biology of rainbow trout (Oncorhynchus mykiss) kept in captivity at Laribal hatchery, Dachigam National park, Jammu and Kashmir was studied from February 2018 to March 2019. This study aims to identify and illustrate developmental stages of oocytes and correlate them with seasonal maturity, gonadosomatic and hepatosomatic indices, and with changes in biochemical composition. Ovarian samples were taken and processed for routine histological analysis. Morphological and anatomical changes of the ovaries were recorded. The oocytes at different developmental stages were observed in the ovaries. The entire reproductive phase was divided into four major divisions: - immature phase, developing phase consisting of growing and maturation phase, spawning phase, and post-spawning phase. The six stages of oocyte development identified in this study include chromatin-nucleolus oocytes, peri-nucleolus oocytes, cortical alveolus oocytes, yolk vesicle oocytes, yolk granular oocytes, and mature oocytes. Maximum gonadosomatic index (12.33 ± 1.04%) and minimum hepatosomatic index (0.82 ± 0.08%) were observed in January, indicating that December- February was the peak spawning period for rainbow trout. It was also observed that June-August was the growing/ immature phase, September-November was the developing phase, and March-May was the post-spawning phase. Furthermore, the biochemical compositions of ninety eight captive rainbow trout ovaries were investigated to determine the broodstock requirements for nutrient-rich feed formulations. The highest moisture (75.30 ± 3.54 %) and crude protein contents (16.10 ± 2.66%) were observed in immature ovaries, while the highest lipid (11.0 ± 2.23%) and ash (2.3 ± 0.60 %) contents were found in developing ovaries. This study revealed the inverse relationship between moisture and lipid. The different biochemical composition of the ovaries at different stages of gonadal development is the key to determine the appropriate broodstock feed formulation at the suitable period of gonadal development. Thus, highly nutritious diet rich in protein and lipids having a high level of digestibility are important for female brooders.