This study investigated the migratory fish spawning within the reservoirs of the Machadinho and Itá dams (upper Uruguay River, Brazil) and its relationship to environmental variables. Sampling was conducted in the lotic region of the river in two sites between the dams' reservoirs: Uruguay (main river) and Ligeiro (tributary). Sampling included nine consecutive reproductive periods (RP) spanning the period from 2001 to 2010 and was conducted at night on the water surface using cylindrical-conical plankton nets (0.5 mm mesh); environmental variables were also recorded. The spawning of the migratory species Salminus brasiliensis, Prochilodus lineatus, and Steindachneridion scriptum was registered: S. brasiliensis and P. lineatus spawned in the tributary river at the end of spring/beginning of summer, during flooding and during periods of high water temperature. Steindachneridion scriptum spawned in the main river at the beginning of spring. The study showed that S. brasiliensis, P. lineatus, and S. scriptum are able to spawn in small lotic river stretches within two reservoirs, but only under very specific and not common environmental conditions.
Ichthyoplankton; Temporal distribution; Water flow; Water temperature
