El Niño Effects and Biological Parameter Comparisons of an Estuarine Resident Fish Occurring in the Sea

Abiotic factors and hydrological disturbances associated with climatic phenomena are challenges to be overcome by many estuarine resident fish. This study investigates whether periods of intense rainfall and continental discharge associated with El Niño events increased the occurrence of estuarine resident fish at adjacent marine sites and possible changes in their body shape and size when caught in the marine area. We used a long-term time series (1996–2019) of environmental factors and abundance of an estuarine resident fish ( Atherinella brasiliensis ) and comparisons of their body measurements (morphometry) in a subtropical estuary and its adjacent marine zone. Generalized linear models (GLM) showed that A. brasiliensis abundance increased in the marine zone during periods of higher rainfall associated with moderate and very strong El Niño events in relation to neutral and weak events. The species showed differences in body size between estuarine and marine regions, but no substantial changes related to body shape. El Niño events with higher intensity seem to be influencing the displacement of the larger specimens to the marine surf zone over time. More specifically, hydrological disturbance (intense rainfall and river flow) triggered by El Niño of greater intensity might explain the abundance of A. brasiliensis in the marine area and its high tolerance for salinity changes may explain the maintenance outside its estuarine habitat. These findings may help to understand the mechanisms used by estuarine fish to tolerate environmental stress and changes in their habitats, especially in estuaries subject to climatic disturbances caused by El Niño events that are predicted to become more frequent and intense with global warming. Results suggests that long-term changes in flow regimes trigged by strong climatic phenomena like El Niño might alter estuarine specimens’ distribution in the Patos Lagoon Estuary.