Size structure, sex ratio and onset of sexual maturity of two species of Hypostomus

Length-frequency analyses showed that male Hypostomus affinis are bigger than females, but H. luetkeni showed no such differences. The sex ratio was estimated at 1:1 for both species. The temporal variation of sex ratio indicated a decrease in the male population when GSI was at its maximum value, suggesting that males are less prone to capture during the spawning season. According to the decrease of males during the reproductive period, paternal behaviour is proposed for both species. H. affinis reached sexual maturity at a smaller size than H. luetkeni. Considering the length weight relationship and the mortality rate of the reproductive females in both species, the physiological consequences of delay or precocity of the first maturation are discussed.