Reproductive biology of Oreochromis mossambicus populations of man-made lakes in Sri Lanka: a comparative study

The sex ratio, mean size at maturity (Lm), gonosomatic index (GSI), frequency of distribution of egg diameter of the mature ovaries and fecundity of Oreochromis mossambicus (Peters) populations of 12 man-made lakes were studied using commercial landings from the gill-net fishery. The overall sex ratio was 1·0:0·88 of females to males and that of the individual populations varied from 1·0:0·54 to 1·0:1·34. The Lm ranged from 16·0 to 21·0 cm (TL) and was found to be correlated to the size of the water-body. The GSI of the females in stage V, the stage prior to spawning, did not differ from each other, except in one reservoir. The variations in the mean egg size (μ) of different populations were negatively correlated to the fecundity of a Ash of standard weight (FW200) of the different populations and the relationship was, y =3487·4– 1·14FW200(df = 10; r= -0·64; P < 0·05) The overall fecundity ranged from 318 to 3169 in fish ranging in body length (TL) and body weight from 16·5 to 32·0 cm and 80 to 651 g respectively. Fecundity was linearly related to body weight and curvilinearly to body length. The exponent b of the fecundity (F) length (1) regressions (F = aLb) of the different populations varied between 1·20 and 3·83. The overall relationship of fecundity to body weight (W) and body length (1) for O. mossambicus reservoir populations in Sri Lanka were: F = 3·23W + 357·8 (df = 203; r= 0·73; P < 0·001) F = l·52L2·11(df = 203; r= 0·68; P < 0·001) The variations in the theoretical fecundity calculated for a standard size individual of 200 g in weight (FW200) of the reservoir populations were correlated to the mean fishing pressure (X) exerted on each population according to the equations. (a) FW200= 10–24X+ 896·8 (df 9; r= 0·58; P < 0·05) (b) FW200= 1200 – 422·76e−0·102X (df 9; P < 0·05)