The study was undertaken to investigate the effects of rainy and dry periods on the gross and histological features of ovaries of Oreochromis niloticus in Zaria, Kaduna State, Nigeria. Twenty-four (24) adult wild tilapia fish with average weight of 300.17 ± 25.71 gm and 229.94 ± 23.68 gm was used during rainy and dry periods, respectively. They measured a standard body length of 24.00 ± 0.76 cm and 23.83±0.60 cm during rainy and dry periods, respectively. The fish were sourced from Zaria dam and transported live in open plastic troughs containing clean water to the Gross Anatomy Laboratory, Department of Veterinary Anatomy, Ahmadu Bello University Zaria. Each fish was euthanized using tricaine anaesthetic at 8 drops/litre of water. The ovaries were extracted, weighed and fixed in Bouins fluid for 24 hours and processed histologically. Grossly, the ovaries of O. niloticus were highly vascularized and well developed during both rainy and dry periods. The average weight of both ovaries showed insignificant variation across the periods (0.125). The Gonado-somatic index value was insignificantly higher during rainy period (0.115) compared to dry period. Histologically, the ovaries were predominated by vitellogenic and post-vitellogenic follicles across both periods of the year. The tunica albuginea and interstitial connective tissues were thin during both periods. In conclusion, the results in this study revealed that adult wild tilapia fish had developed ovaries with predominant mature follicles during both rainy and dry periods suggesting that adult wild tilapia fish spawns during both rainy and dry periods in Zaria, Nigeria
Key words: Follicles, Gonado-Somatic Index (GSI), Hematoxylin & Eosin, ovaries, Wild Tilapia, Zaria dam
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