POSSIBLE ADAPTIVE SIGNIFICANCE OF SEX RATIO AND SEXUAL DIFFERENCES IN SIZE AT MATURITY IN THE ORANGE RIVER LABEO, LABEO CAPENSIS

Abstract

Labeo capensis spawns on inundated vegetation during flood periods. Suitable conditions for spawning are of short duration and do not allow time for extended breeding behaviour. The success of fertilisation is therefore probably independent of the size of males. This species grows rapidly in length during its first two years of life. At a length of 20 – 30 cm it becomes relatively immune to predation, its gain in mass per unit length then increases proportionately in preparation for the reproductive stage of its life cycle. Males and females reach sexual maturity at lengths of 28 and 36 cm or four and six years of age respectively. Females grow significantly faster than males from their fourth year of life onwards. Fecundity increases with increase in size. The sex ratio changes rapidly in favour of females at the length at which females reach sexual maturity.