Fish assemblages in Neotropical reservoirs: Colonization patterns, impacts and management

Brazil has more than 700 large reservoirs distributed in all of the major river basins of South America. Most dams were constructed to produce electricity. Although these reservoirs favor the development of local and regional economies, they seriously impact the aquatic biota. An unavoidable consequence is the change in the composition and abundance of species, with the proliferation of some and reduction or even local extinction of others. The intensity and nature of these changes are related to peculiarities of the local biota and the location, morphometric and hydrological characteristics of the reservoir, dam operation and interactions with other uses of the basin, including other reservoirs. These impacts exhibit substantial spatiotemporal variations. The filling phase is marked by abrupt and intense changes in the key attributes of aquatic habitats, followed by predominantly heterotrophic processes, with possible thermal stratification and anoxic conditions. Fish richness increases soon after filling and decreases in subsequent years. Trophic depletion is expected, and diversity gradients are intensified toward more lentic stretches, the average length of fish decreases, and the fish fauna becomes dominated by species with sedentary strategies and/or parental care. The virtual absence of species with pre-adaptations to inhabit lentic areas of large reservoirs leads to a concentration of biomass in shallow littoral areas. Long-distance migratory species are the most affected, which include larger fish with high market value. Migratory species require different biotopes to fulfill their life cycles and strongly depend on the seasonal flood regime, which is altered due to dam operation. In this study, we discuss the details of these trends as well as the mitigation measures and management actions that are practiced in Brazil. We conclude that these actions have not promoted the conservation of fish; on the contrary, some of them have generated additional impacts. As a consequence, the conservation of Neotropical fish and aquatic resources is severely threatened.