Abstract
Migratory fish species are generally the most important species for the fishing industry as well as for artesanal fishing. In the lower Tocantins, two species of migratory fish dominate the captures: the mapará (Hypophthalmus marginatus) and the curimatã (Prochilodus nigricans). These two species were studied before the daming of this river by the Tucuri hydroelectric dam as part of a larger study of commercial fisheries encompassing the Tocantins basin from the mouth to Marabá (Pará). Based principally on landing records, this study describes the migration cycles of the two species. A classical and simple pattern of migration can be recognized: an upstream movement of immature and adults in the upper part of the distribution area, and then, the reverse or downstream movement of larvae and adults in a dispersion phase. The main difference between these two species lies in the size of distribution area. The mapará is limited to part of the river downstream from the hydroelectric dam of Tucurui, and so, would not be affected by its presence. The curimatã has a much larger geographic range in the Tocantins basin. The part of population living downsteam will probably be profundly pertubed by the dam due to the blockage of its migratory route.
