Effects of the El Niño Southern Oscillation (ENSO) on fish assemblages in a Neotropical floodplain

Abstract

We evaluated the association between the El Nino Southern Oscillation (ENSO), an important climatic phenomenon on Earth, with water level variation and fish assemblage attributes (i.e., abundance, species richness and structure of the assemblage) of the Parana (dammed) and Ivinhema (undammed) rivers, located in the upper Parana River floodplain, Brazil. Cross-Correlation Function analyses were performed using the Oceanic Nino Index (ONI; which indexed ENSO events) as the influential time series on water level and fish assemblage attributes time series. ONI were positively associated with water level and with most of the fish assemblage attributes tested, but these patterns differed between the two rivers. This indicates that El Nino events affect the structure of the fish assemblages, mainly in the Ivinhema River, which presented more assemblage attributes associated with ONI. In addition, dams may play a regulatory effect over the water level of the Parana River, which probably explains the different patterns of assemblage attributes in both rivers. Thus, ENSO events are associated with the composition and structure of the fish assemblages in the studied area, and their effects are apparently minimized in the Parana River due to the presence of upstream dams.