Spatial and temporal patterns of ichthyoplankton assemblages across a large flow-regulated Amazonian river system

Dams may significantly affect fish reproduction and the natural drift of ichthyoplankton. Here, we investigated fish egg and larval dynamics in the area affected by the Belo Monte Dam. Ichthyoplankton was surveyed daily during an entire flood-pulse cycle, across three sectors: upstream of the main reservoir (UP), within the reservoir (MR), and in the reduced-flow sector (RF) downstream of the dam. We analyzed the influence of river level and other covariates, including time of day and environment type. Results revealed strong spatiotemporal variation in ichthyoplankton assemblages. Contrary to expectations, the MR and RF sectors exhibited higher densities and taxonomic richness than the UP sector, indicating that reproductive activity persists near the dam. However, spawning timing differed among sectors: ichthyoplankton in the UP and MR peaked at the onset of flooding, whereas in RF it coincided with high-water conditions. Larval composition also varied, suggesting differential responses to hydrological alteration. The higher abundance near the dam may reflect ichthyoplankton retention in the reservoir due to lower water velocity, as well as the accumulation of mature individuals below the dam. Continued monitoring is essential to better understand how differing hydrological dynamics among sectors influence reproductive asynchrony and to support the optimization of management practices.