| Summary: | Tambaqui Colossoma macropomum (CUVIER 1818) is a large Amazonian characid highly appreciated for its meat and therefore over explored by the fishing activity. For the last decades, tambaqui has been the first native fish produced by the Brazilian aquaculture industry. The Amazon Basin is characterized by the presence of acidic and basic waters with pH ranging from 4 to 7.2. In the wild, tambaqui migrates from alkaline white waters where it spawns, to regions of black and acidic water during the rainy season. During this migration, tambaqui swim through waters with different hydrogen potentials that could easily have some effect on its sexual determination process, which has not yet been identified. Hence, the present study aimed at unveiling the possible effect of a peculiar Amazonian environmental feature, pH, on the sex determination of tambaqui. For this, tambaqui larvae at 12 days (post hatch) were kept at pH 6.7, 7.5 (control) and 8.2 for 45 days (until they reached 4 cm of standard length), and then transferred to ponds to grow for sex identification by histology. Although the group treated with more acidic water showed a slight bias towards males, there was no significant difference in both treatments compared to the control group. Therefore, we concluded that the water pH does not influence the sex determination mechanism on C. macropomum.
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