Effects of El Niño-Southern Oscillation (ENSO) on the reproduction of migratory fishes in a large South American reservoir

The ENSO (El Niño-Southern Oscillation) phenomenon affects, through climate teleconnections, hydrometric variables of water bodies worldwide, influencing biota communities. The aim of this work is to analyze ENSO effects on water discharge, water level, cyanobacterial blooms and, consequently, on the reproduction of migratory fishes with high-frequency monitoring of ichthyoplankton data from Salto Grande, a large South American reservoir. Two highest peaks of ichthyoplankton density were observed; the first one (2015–2016) occurred during a very strong El Niño event, while the second one (2019–2020) was preceded by weak El Niño events that lasted 18 months. These results suggest that ichthyoplankton abundance may be affected by the intensity and temporal extent of the episodes. In contrast, mean fish larval and egg densities were reduced by 65% and 85%, respectively during La Niña periods. Cross-correlation function (CCF) analysis indicates that water discharge, water level, and ichthyoplankton abundance were positively related to the Oceanic Niño Index, while cyanobacteria showed an inverse relationship. Cyanobacterial density increased by 574% during La Niña compared with El Niño events. Based on an uninterrupted monitoring of ichthyoplankton over 11 years, it seems reasonable to consider ENSO as a predictive tool for the intensity of reproductive activity of migratory fishes.