Influence of the Abaete River on the reproductive success of the neotropical migratory teleost Prochilodus argenteus in the Sao Francisco River, downstream from the Tres Marias Dam, southeastern Brazil

The curimata-pacu Prochilodus argenteus is an important characiform from the Sao 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 Tres Marias Dam and accentuated decline in fishing, the curimata-pacu still sustains the fisheries at the Tres Marias region in recent decades. The objective of this study was to evaluate the reproductive activity of P argenteus in two sections of the Sao Francisco River, downstream from the Tres 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 frequency 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 indicated 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 Sao Francisco River immediately below the Tres Marias Dam was found to be unfavourable for the reproduction of the migratory fishes due principally to the hypolimnetic water from the reservoir, reproductive success of P. argenteus was achieved below the Abaete River. In this section, the species encountered appropriate conditions for maturation and spawning, i.e. warm temperatures above 24 degrees C, high water flow and dissolved oxygen, and low water transparency. These results indicate the importance of a non-regulated tributary to minimize the ecological impact of a dam on the downstream native fish communities. Copyright (c) 2005 John Wiley & Sons, Ltd.