THE NEOTROPICAL MIGRATORY TELEOST PROCHILODUS ARGENTEUS IN THE SÃO FRANCISCO RIVER, DOWNSTREAM FROM THE TRÊS MARIAS

The curimatã-pacu Prochilodus argenteus is an important characiform from the São Francisco River basin that performs long-distance migrations for spawning upstream during the rainy season, when the temperature and photoperiod are elevated. Despite the interruption of the migratory routes by the Três Marias Dam and accentuated decline in fishing, the curimatã-pacu still sustains the fisheries at the Três Marias region in recent decades. The objective of this study was to evaluate the reproductive activity of P. argenteus in two sections of the São Francisco River, downstream from the Três Marias Dam, during the rainy season. In the first 34 km of the river, immediately below the dam, most of the females were in gonadal resting. At 34–54 km downstream from the dam, following the confluence with a medium-sized tributary, the Abaete ́ River, there was a high fre-quency of males and females in reproductive activity. Follicular atresia was more frequent in the upper section of the river while postovulatory follicles occurred predominantly in the lower section. Fulton’s condition factor and gonadosomatic index indi-cated that the females were in a better physiological and reproductive condition below the confluence with the Abaete ́ River. In contrast to the females, the males were less affected by damming, and testicular maturation was largely achieved in two river sections. Thus, although the section of the São Francisco River immediately below the Três Marias Dam was found to be unfa