Regulation of rivers by dams transforms previously lotic reaches above the dam into lentic ones and limits or prevents longitudinal connectivity, which impairs access to suitable habitats for the reproduction of many migratory fish species. Frequently, unregulated tributaries can provide important habitat heterogeneity to a regulated river and may mitigate the influence of impoundments on the mainstem river. We evaluated the importance of tributaries to spawning of migratory fish species over three spawning seasons, by comparing several abiotic conditions and larval fish distributions in four rivers that are tributaries to an impounded reach of the Upper Paraná River, Brazil. Our study confirmed reproduction of at least 8 long‐distance migrators, likely nine, out of a total of 19 occurring in the Upper Paraná River. Total larval densities and percentage species composition differed among tributaries, but the differences were not consistent among spawning seasons and unexpectedly were not strongly related to annual differences in temperature and hydrology. We hypothesize that under present conditions, densities of larvae of migratory species may be better related to efficiency of fish passage facilities than to temperature and hydrology. Our study indicates that adult fish are finding suitable habitat for spawning in tributaries, fish eggs are developing into larvae, and larvae are finding suitable rearing space in lagoons adjacent to the tributaries. Our findings also suggest the need for establishment of protected areas in unregulated and lightly regulated tributaries to preserve essential spawning and nursery habitats. Copyright © 2014 John Wiley & Sons, Ltd.

1. Conservation of fish assemblages in severely impounded rivers has often focused on providing longitudinal and lateral connectivity along the main stem. Less attention has been given to tributaries, some of which remain unimpounded.2. This study shows that the biodiversity of ichthyoplankton in tributaries to the largest reservoir in the Paraná River, Brazil, is similar to that of tributaries of the few remaining unimpounded stretches of the Paraná River.3. Annual and seasonal variability in discharge within and among tributaries attracts and supports a diverse mix of species in each spawning season.4. Tributaries enrich the fish biodiversity of the entire system by providing a variety of environmental conditions, access to spawning habitat, refugia for early life stages, and by promoting ecological heterogeneity. Thus, tributaries can mitigate the adverse influence of an impounded main stem. 5. Tributaries also support populations of endangered and threatened species. A focus on tributaries offers conservation alternatives that are more feasible given the scale and large number of impoundments constructed on the main stem.6. Conservation in the main stem may focus on maintaining passage through dams and access to large floodplains. Conversely, in tributaries conservation may focus on managing the quality of inflows from catchments, preserving suitable instream habitats and flows, and preserving access to smaller but more numerous floodplains.

Gymnogeophagus setequedas is a rare and rheophilic species of tribe Geophagini, considered endangered in Brazilian red lists. Its previously known geographical distribution range was the Paraná River basin, in Paraguay, and a tributary of the Itaipu Reservoir in Brazil. Since its description no specimens have been collected in the original known distribution area. However, recent records of G. setequedas in the lower Iguaçu River, in a region considered highly endemic for the ichthyofauna, extended the known geographical distribution and may represent one of the last remnants of the species. The aim of this study was to estimate the genetic diversity and population structure of G. setequedas, using microsatellite markers and mitochondrial haplotypes, in order to test the hypothesis of low genetic diversity in this restricted population. Muscular tissue samples of 86 specimens were obtained from nine locations in the Lower Iguaçu River basin, between upstream of the Iguaçu Falls and downstream of the Salto Caxias Reservoir. Seven microsatellites loci were examined and a total of 120 different alleles were obtained. The number of alleles per locus (NA) was 17.429, effective alleles (NE) 6.644, expected heterozygosity (HE) 0.675, observed (HO) heterozygosity 0.592, and inbreeding coefficient (FIS) 0.128. Twelve haplotypes in the D-Loop region were revealed, with values of h (0.7642) and π (0.00729), suggesting a large and stable population with a long evolutionary history. Thus, both molecular markers revealed high levels of genetic diversity and indicated the occurrence of a single G. setequedas population distributed along a stretch of approximately 200 km. The pattern of mismatch distribution was multimodal, which is usually ascribed to populations in demographic equilibrium. Nevertheless, the construction of a new hydroelectric power plant, already underway between the Salto Caxias Reservoir and Iguaçu Falls, could fragment this population, causing loss of genetic diversity and population decline, and for this reason it is necessary to maintain the Iguaçu River tributaries and downstream area from the Lower Iguaçu Reservoir free of additional dams, to guarantee the survival of this species.

This study investigated the external body morphology (based on morphometric data) and the swimming efficiency in fish passages (based on the ascent time in a fish ladder) of two Neotropical long-distance migratory species. The fish were collected in the fish ladder of the Porto Primavera Hydroelectric Power Plant (known as Engenheiro Sergio Motta), upper Paraná River, Brazil. The species differ in several morphometric ratios related to swimming performance. Prochilodus lineatus exhibited a higher muscle ratio and caudal fin length ratio. However, Leporinus elongatus showed higher values for the fineness ratio, a higher length from snout to pectoral fin base ratio, and a higher caudal peduncle depth factor. Although both species show good swimming performance, the characteristics listed above may enable L. elongatus to apply greater power and propulsion, allowing it to reach a high swimming speed and to exhibit greater efficiency than P. lineatus. Leporinus elongatus moves faster than P. lineatus and has a lower mean ascent time (1.39 hours) than P. lineatus (3.76 hours). Prochilodus lineatus would require a somewhat longer time to pass through the fish ladder.Este estudo investigou a relação entre a morfologia externa do corpo (baseado em dados morfométricos) e eficiência de natação em passagem para peixes (baseado no tempo de ascensão em uma escada para peixes) de duas espécies neotropicais migradoras de longas distâncias. Os peixes foram coletados na escada para peixes da Usina Hidrelétrica de Porto Primavera (conhecida como Engenheiro Sérgio Motta), alto Rio Paraná, Brasil. As espécies diferem em algumas razões morfométricas referentes ao desempenho de natação: Prochilodus lineatus apresentou valores mais elevados para razão muscular e razão do comprimento da nadadeira caudal. No entanto, Leporinus elongatus apresentou valores mais elevados para razão fineza, para razão do comprimento do focinho à base da nadadeira peitoral e para fator de largura do pedúnculo caudal. Embora ambas as espécies tenham um bom desempenho natatório, essas características podem permitir a L. elongatus aplicar uma maior potência e propulsão, permitindo atingir velocidades de natação elevadas e apresentar uma melhor eficiência em relação a P. Lineatus. Leporinus elongatus movimenta-se mais rápido, com um tempo médio de ascensão (1,39 horas) inferior ao de P. lineatus (3,76 horas), que requer um tempo um pouco maior para passar através da escada para peixes.

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