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A few fishes breed only once in their long life spawn eg. Pacific salmon . The fresh water eel breeds only once in 10-14 years . Most of the fishes breed in one season every year, they are called seasonal breeders, but some breed in all seasons, through out the year, and they are called year round spawners . - Seasonal breeders exhibit rhythmic changes in the structure and physiology of ovary and testes in different seasons. These changes are demarcated into five phases - 1. Resting phase (December - January) 2. Preparatory phase(February - April) 3. Maturing phase (may - June) 4. Spawning phase (July - September) 5. Post spawning phase(October - November) GONADOSOMATIC INDEX (GSI) - It is a ratio between body weight and the weight of gonad showing the status of ovary and testis in terms of maturity and denotes the phase of the reproductive cycle- GSI = Weight of the gonad × 100 Weight of fish-weight of the gonad - GSI of fish increases with the maturation of the fish i.e. lowest during post spawning phase and highest at the peak of maturity i.e. spawning phase.FEMALE REPRODUCTIVE ORGAN - Unlike mammals, the female reproductive system of teleosts.

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