Cyclical Changes In The Histology Of The Gonads And The Biology Of Clarias Mossambicus (peters) In The Nyanza Gulf Of Lake Victoria.
Owiti, Dickson Otieno
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Histological and seasonal changes in the gonadal development and the biology of the cat-fish Clarias mossambicus (Peters), in the Nyanza Gulf of Lake Victoria were studied during a period of twelve months. In the females, germ cells proliferatte through mitotic divisions and become transformed into oocytes. Growth of oocytes as a result of synthesis of protoplasmic, and then trophic substances, eventually result in maturation. In the males,maturation is brought about as a result of a series of divisions of the germ cells giving rise, through intermediate stages, to ripe spermatozoa. Anaysis of the seasonal trends in the gonadosomatic index (GSI), changes in histological conditions and variations in the percentage occurrence of different maturity stages of the gonads reveal that the peak breeding ativity of the fish is confined to be time of maximum,rainfall in the months of March, April and May, and extend up to June The flood-plains, swamps and temporary streams and rivers draining into the gulf were identified as the spawning grounds of the species, and, during the breeding time, the species undertook spawning migration to these areas. Six maturity stages were identified in the females and five in the males. The minimum size at sexual maturity was in the size range of 40-45 cm in both the males and females. The females dominated over the males in the population just before the breeding season (in February) and immediately after the peak breeding time (in May). During the peak breeding period, however, the sex ratio was 1:1. The males then dominated over the females during the rest of the period Fecundity estimates for 27 females ranged from 7,966 eggs to 229,648 eggs in fishes measuring 46.0 cm and 87.0 cm respectively, with a mean of 78,152 eggs. There was a positive linear relationship between fecundity and (i) total length (r,~ 0.92; P < 0.001) (ii) body weight (r =,0~~4;'P < 0.001) and (iii) ovary weight (r = 0.~9·; p < 0.001) .. The regression equations for these relationships are described. From logarithmic equations describing the length-weight relationships, it seems that growth in the species is isometric. The relative condition was generally high and both males and females reached maximum condition just Prior to spawning.