The reproductive ecology of Moenkhausia sanctaefilomenae (Steindachner, 1907) was studied at 15 lakes of the Cuiabá river floodplain, Mato Grosso. The sex ratio, the timing of reproduction, spawning type and the length of first gonadal maturation were analyzed. We sampled 212 individuals (108 females and 104 males) from June of 2005 to March of 2006. There was not significant difference in the sex ratio, considering the whole period sampled. The biggest frequency of matured females was found in December in all of the lakes. The mean width of the ovarian follicles varied among the periods in December it was larger than others. What it suggests that the spawning happened, probably, among the end of December or beginning of January. That corresponds to the high water and flood in the area. The frequency distribution of the width of the ovarian follicles showed a polimodal pattern. That suggest parceled out spawning, but restricted to the period of December to March. The mean length of gonadal maturation was estimated in 33.9 mm. There was a positive relationship between length of first maturation and the asymptotic length. In synthesis, the data indicate that the reproduction happens in a synchronous way in all of the lakes sampled.
Gonadal maturation; spawning; sex ratio
