Pachyurus schomburgkii Günther, 1860 and P. paucirastrus Aguilera, 1983 occur in the Tocantins-Araguaia basin, where the latter is endemic. The species were found in the upper Tocantins, in the region where it was impounded by the Serra da Mesa hydroelectric dam. Fourteen sites were sampled but both species were restricted to two sites (FU and PG) located downstream from the dam. Fishes were bimonthly captured from December 1995 to December 2002 with standardized gillnets. We present data on the distribution, diet, physiological condition and the reproductive status of these species, and focus on the alterations related to the impoundment. Most specimens were adult, and the number of reproductive individuals decreased after damming. Species were classified as benthic invertivores, feeding mainly on immature insects. Niche breadth was significantly higher to P. paucirastrus, and, for both species, during the pre-impoundment phase. Interspecific diet overlap was high; however, resource partitioning was more pronounced in the site immediately downstream from the dam (FU) than partitioning in the more distant site (PG). Values relative to the condition factor indicated that P. schomburgkii must have profited more effectively from the available resources after the environmental changes than P. paucirastrus. Thus, factors related to feeding might have contributed to the disappearance of this species after the beginning of power plant operation. Pachyurus schomburgkii seems to have had a better adjustment to changes, and was more persistent in the area after the damming; however, it remained restricted to the more distant site, where it had been occasionally captured until it was no longer found, approximately four years after dam closure. Considering that many impoundments are planned to the rio Tocantins, we recommend further studies about P. paucirastrus in other stretches of the basin, in order to evaluate the conservation status of this species.
Pachyurus; biology; diet; impoundment; Tocantins River
