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Background. Usage of biosciences in increasing fish production needs to have a proper understanding and knowledge of endocrine physiology of fish reproduction. The most suitable method of determining the reproductive cycle in fishes is to observe seasonal development changes in the gonads. The present paper describes morphological changes in gonads of Mystus montanus. Materials and Methods. The individuals of M. montanus were sampled monthly in captive condition throughout one year to determine the changes occurring in gonadal histology and reproductive status. The stages of gonadal maturation and the seasonal changes in the proportion of oocyte development within the ovaries were noticed and maturity stages were assessed by microscopic and also macroscopic observations. Results. The peak spawning period of Mystus montanus was noticed during November-January in male and October-December in female. The size at first maturity was 10-11 cm (8-12 g) in male and 13-14 cm (14-16 g) in female, respectively. M. montanus spawned only once in a year with the onset of northeast monsoon. The gonadosomatic index (GSI) ranged from 2.8 to 8.5 in males and from 4 to 16 in females and their condition factor (CF) was 4-6 and 6-9, respectively. Conclusion. The sequence of gonadal maturation in M. montanus is morphologically and histologically divided into five stages viz. immature, maturing, mature, matured, and spent or rest.

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