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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.

The understanding of the environmental preferences and life history of a species in its native range provides insights for assessing its potential success in a novel area. Hemiodus orthonops is a migratory fish from the Paraguay and Middle Parana ŕivers that invaded the Upper Parana ´River through a fish pass, constructed in 2002. The invasion of this fish was analyzed based on habitat attributes and its life history in the native range and in the novel habitats. The native and novel habitats presented similar features. The population showed exponential growth in the years following the invasion, with relevant alterations in somatic growth, proportions of items in the diet, and in reproductive investment. The successful invasion appears to be related to similarities in water chemistry and habitat types between the native and novel range and the species' high dispersion ability. The species also showed earlier maturation and use of poor quality, but highly available feeding resources. Comparing life histories there was indication of a trade-off between investment in reproduction and somatic growth, with an increase in the latter in the novel habitats. In addition, the reproductive investment was more associated with offspring survival (oocyte size) than fecundity.

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Fish reproductive guilds were used to evaluate the responses of species with different reproductive strategies during two different periods of post-dam construction. The data used for the comparisons were collected in the upper Paraná River floodplain (Brazil), downstream of the Porto Primavera dam, 2 and 10 years after impoundment. The abundance (catch per unit effort, CPUE), species richness, evenness and structure of communities, all within reproductive guilds, were used to test the hypothesis that these metrics vary spatially and temporally. The influence of damming on species structure and the diversity of fish reproductive guilds varied spatiotemporally, and species with opportunistic reproductive strategies tended to be less affected. Conversely, long-distance migratory species responded more markedly to spatiotemporal variations, indicating that the ecosystem dynamics exert greater effects on populations of these species. Thus, the effects of a dam, even if attenuated, may extend over several years, especially downstream. This finding emphasizes the importance of maintaining large undammed tributaries downstream of reservoirs.