Importance of tributaries for fish reproduction and conservation in a highly anthropized river basin

This study evaluated the role of tributaries as spawning grounds for fish in the Velhas River basin, a highly impacted tributary of the São Francisco River. Ichthyoplankton was sampled every three days between November 2022 and February 2023, in six tributaries with varying levels of environmental preservation. Molecular techniques (DNA barcoding and metabarcoding) identified 36 taxa, including all known migratory species of the Velhas River basin. Eggs and larvae were found in all tributaries, with a predominance of early developmental stages. The best-preserved tributaries with higher flow rates, particularly the Paraúna/Cipó and Curimataí rivers, were identified as key spawning sites, supporting higher densities of ichthyoplankton and hosting exclusive reproductive events of some long-distance migratory species. Spawning peaks were associated with flood pulses in the Velhas River, but synchrony among tributaries was rare. The results highlight the ecological importance of preserved tributaries for fish reproduction and the necessity of maintaining hydrological connectivity. Particularly, the conservation of the Paraúna/Cipó River is crucial for sustaining the fish diversity and recruitment processes in the Velhas River basin, especially given ongoing threats such as pollution and potential river fragmentation.

Keywords:
Ichthyoplankton; Migratory species; Spawning sites; Velhas River