CONICET | Buscador de Institutos y Recursos Humanos

Dams alter river functioning and hinders the free movement of migratory fishes. Weanalysed the effects of a subtropical dam on the reproduction of migratory fishes usingdata from ichthyoplankton monitoring surveys. These were conducted at six samplingstations upstream, downstream and within the reservoir on a weekly basis during6 years. The dam had a direct impact on fish movement, leading to the fragmentationof fish populations. Two different and independent reproductive areas became evident:one located in the reservoir headwater area and the other downstream of thedam. The long temporal scale of our study facilitated the detection of the impact ofSalto Grande on the species of ichthyoplankton. Freshwater secondary fish species(mainly Lycengraulis grossidens) increased in abundance in the stations located insidethe reservoir and closer to the dam, whereas most fishes found in the headwater ofthe reservoir corresponded to river migratory species (potamodromous fish). These differencesare most likely due to changes in the physicochemical characteristics of thewater 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 werehigher 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 (