Efeitos do El Niño Southern Oscillation (ENSO) nas assembleias de peixes de uma planície de inundação Neotropical.

Abstract

We evaluated the effects of El Niño Southern Oscillation in the fish assemblages of the upper Paraná River floodplain. Specifically, we tested if: (i) reservoirs regulate ENSO effects on the hydrometric level in the rivers that compose the plain (Rio Paraná – intensely dammed and Rio Ivinhema – not dammed); (ii) ENSO events influence the structure of the fish assemblages of the study rivers; and that (iii) the hydrometric level and the ENSO events will act synergistically on the structure of the fish assemblage of the Rio Paraná and Rio Ivinhema. ENSO data was gathered at the National Oceanic and Atmospheric Administration (NOAA), using as proxy the "Oceanic Niño Index" (ONI). Water level data were gathered from the hydrological stations located in the sampled area. Fish samplings were carried out quarterly between the years 2000 and 2018. In general, ENSO events were correlated with the hydrometric level of the Rio Paraná and Rio Ivinhema, but with lower correlation for the Rio Paraná, probably due to the effects of dams upstream the plain. The fish assemblages of the Rio Paraná and Rio Ivinhema presented distinct structures, and the ENSO events appeared to influence this differentiation. Migratory species were negatively correlated with ONI, but the greatest abundances were found one year after the events of El Niño ("Lag" of one year). Higher values of the attributes of the fish assemblages were recorded for the Rio Ivinhema, in all events of ENSO (La Niña, Neutral and El Niño), probably because it presents more pristine conditions. The hydrometric level and the ENSO events act synergistically, but distinctively in the study rivers, which should be related to their degree of impacts. Thus, the influence of ENSO in the studied area is apparently minimized, especially in the Rio Paraná, due to the flow control imposed by dams located upstream of the area.