Free‐flowing tributaries are important for the maintenance of fish diversity in dammed systems. In relation to the reproduction of fish species, the role of a free‐flowing tributary was evaluated and compared with that of a dammed tributary downstream of large dams in the Madeira River, one of the main tributaries of the Amazon River. Two hypotheses were tested: (i) the densities of the ichthyoplankton differ between the dammed and the free‐flowing tributaries; and (ii) the free‐flowing tributary contributes a greater diversity of fish larvae species to the assemblage structure of the main stem than the dammed tributary. Fish eggs and larvae were sampled at five sites: one in each tributary (Jamari – dammed – and Machado – free‐flowing) and three in the main stem (Madeira River), upstream and downstream from the mouth of each tributary. Fitted Bayesian models showed that the densities both of eggs and larvae were more than twice as high in the free‐flowing tributary than in the dammed tributary. The results also indicated that the free‐flowing tributary makes a major contribution to the fish assemblage structure of the Madeira River, whereas the dammed tributary does not have a significant influence on the main stem assemblage structure. The results demonstrate the importance of conservation and maintenance not only of the Machado River but also of other free‐flowing tributaries in dammed basins, especially those basins with high biodiversity, such as the Amazon. Moreover, this study helps inform the decision‐making process regarding the location of new dams, which is critical in fish diversity conservation.

Little is known about the seasonal movements of fish that inhabit large rivers in South America, which makes it difficult to identify potential threats to fish populations associated with the proliferation of hydropower developments. Dorados Salminus brasiliensis (Characiformes) are large riverine piscivores that are targeted by recreational and commercial fishers and are considered regionally 'vulnerable' in Brazil due to overfishing, pollution, and habitat fragmentation. Here, we used radio telemetry to study the seasonal movements of dorados in the upper Uruguay River, Brazil, to provide the first information on large-scale migratory biology and to inform management and conservation actions. From November 2001 to July 2003, 73 dorados were radio-tracked using aerial surveys and 7 fixed radio telemetry stations installed in a section of the upper Uruguay River covering ~400 km. Despite use of an extensive radio telemetry array and aerial tracking, nearly 40% of fish tagged at the downstream site were never detected, suggesting unreported harvest, post-release mortality, or migration to tributaries or downstream reaches that extended beyond the tracking area, emphasizing the challenges of working in such a large study system in jurisdictions where research capacity and funding are limited. Nonetheless, this study yielded the first data on the migratory biology of dorados and revealed that a segment of the population is quite mobile and thus could be negatively impacted by river fragmentation, suggesting the need for management strategies that maintain connectivity (e.g. fish passage facilities).

We used a combination of radio and acoustic telemetry to assess the movements of large catfish (Pimelodidae) in the Xingu River, a clearwater tributary of the Amazon River in Brazil. A total of 121 Phractocephalus hemioliopterus and 61 Pseudoplatystoma punctifer were tagged for monitoring within a 685 km segment, including the Belo Monte Hydroelectric Complex (BMHC), between February 2013 and July 2015. Long distance upstream movements were detected for P . hemioliopterus (up to 347 km) and for P . punctifer (up to 164 km) mainly during the transition between dry season and the rising water period. Both species moved through a long segment of rapids previously thought to function as barriers to migration. Several individuals exhibited long-distance bidirectional movements. Some tagged fish never left the release zone, indicating mortality, tag loss or resident individuals, which would characterize partial migration. The findings show evidence of migratory behaviour for large catfish within the Xingu River, emphasizing the influence of the hydrologic cycle on their movements. As part of the study area has become partially dewatered due to the BMHC, findings support the need of adequate management strategies to allow the movements of large catfish between spawning and feeding sites in the Xingu River.

No abstract

No abstract