Impact of a hydroelectric power plant on migratory fishes in the Uruguay River

Abstract

Dams alter river functioning and hinders the free movement of migratory fishes. We analysed the effects of a subtropical dam on the reproduction of migratory fishes using data from ichthyoplankton monitoring surveys. These were conducted at six sampling stations upstream, downstream and within the reservoir on a weekly basis during 6 years. The dam had a direct impact on fish movement, leading to the fragmentation of fish populations. Two different and independent reproductive areas became evident: one located in the reservoir headwater area and the other downstream of the dam. The long temporal scale of our study facilitated the detection of the impact of Salto Grande on the species of ichthyoplankton. Freshwater secondary fish species (mainly Lycengraulis grossidens) increased in abundance in the stations located inside the reservoir and closer to the dam, whereas most fishes found in the headwater of the reservoir corresponded to river migratory species (potamodromous fish). These differences are most likely due to changes in the physicochemical characteristics of the water induced by the transformation of a river into a reservoir. During rainy summers, Microcystis spp. densities were lower than 1,000 cells ml−1 and fish densities were higher than 4 ind. m−3, whereas in dry summers, there were important Microcystis spp. blooms (401,573 ± 112,559 cells ml−1) and fish larvae decreased sharply (<1 ind. m−3) in the reservoir headwater area. This could indicate that the discharge of water in the reservoir may also indirectly affect the regulation of fish reproduction. The alternating periods between dry and rainy summers associated with El Niño-Southern Oscillation poses a challenge to ichthyofauna management in the reservoir.