The present study explored the interaction between the upriver migration of fish and the blockage of their migration routes by dam construction. Specifically, we studied (i) the capacity of migratory fish to locate alternative routes in the presence of an obstacle, and (ii) the behavior of the fish after they were artificially transferred to the reservoir. With the use of the markrecapture technique (tagging), the study was carried out near Porto Primavera Dam (UHE Engenheiro Sérgio Motta) between 1994 and 1999, a period prior to the closure of the floodgates and the installation and operation of the fish pass facilities. The fish were caught in the dam forebay downstream, marked with LEA type tags, and released upstream (5113 individuals; 14 species) and downstream (1491; 12) from the dam. The recaptures were carried out by local professional and amateur fishermen. A total of 188 individuals (2.8%) were recaptured, mostly the curimba Prochilodus lineatus. Nearly half of the recaptures downstream occurred in tributaries, especially in the Paranapanema River, indicating that in the presence of an obstacle the fish are able to locate alternative migration routes. The remainder stayed in the main channel of the Paraná River, at a mean distance of less than 50 km from the release point. Of the fish released upriver from the dam, approximately half were recaptured downriver. Although the river was only partly dammed, the movement of the fish downriver suggests that they became disoriented after being transferred. Those that remained upriver avoided the reservoir and moved, rather rapidly, toward the lotic stretches farther upstream. From these results it is clear that, in the course of the decision process in installing fish passes, it is necessary to take into account the existence of spawning and nursery areas downriver and upriver from the reservoir.O presente estudo visa explorar a interação entre a migração ascendente de peixes e a obstrução de vias migratórias, pela construção de barragens. Especificamente, foi estudada (i) a capacidade de peixes migradores em localizar rotas alternativas na presença de um obstáculo, e (ii) o comportamento dos peixes após serem transpostos artificialmente para o reservatório. Com o uso da técnica de marcação e recaptura, o estudo foi desenvolvido nas imediações da barragem da hidrelétrica de Porto Primavera, entre 1994 e 1999, período anterior à instalação e operação de mecanismos de transposição. Os peixes foram capturados a jusante, nas imediações da barragem, receberam marcas do tipo "LEA" e foram liberados a montante (5113 indivíduos; 14 espécies) e a jusante (1491; 12) da barragem, sendo as recapturas realizadas por pescadores profissionais e amadores da região. No total, foram recapturados 188 indivíduos (2,8%), a maioria de curimba Prochilodus lineatus. Praticamente metade das recapturas de jusante ocorreram em tributários, especialmente no rio Paranapanema, indicando que na presença de um obstáculo os peixes são capazes de localizar rotas alternativas de migração. O res...

The importance of subaquatic structures in determining fish assemblages is well documented in the literature, but few studies have examined the influence of submerged vegetation resulting from river impoundments in which arboreal vegetation was not removed. The purpose of this study was to evaluate the role of submerged trunks determining abundance and composition of fish assemblages by comparing habitats with and without them in two reservoirs of different ages. The results of this study demonstrated the importance of structured areas (with submerged vegetation) to the abundance of fish in both reservoirs. However, the role of these structures in maintaining species richness and diversity was significant only during the initial years after the impoundment. The occupation of these habitats by small invertivorous fish indicates that the nonremoval of arboreal vegetation in these reservoirs contributes to the increase of their biogenic capacity. In addition, vegetation provides shelter against predation. Considering the conservation of fishery resources, we conclude that the pre-removal of arboreal vegetation in new reservoirs should be allowed only to the extent necessary to maintain acceptable water quality, even considering transitory and localized hypoxic processes.

This work aimed to determine which population features were important to the colonization process of