Effects of 1997–1998 El Niño on the dynamics of the shallow-water fish assemblage of the Patos Lagoon Estuary (Brazil)

High rainfall associated with El Nin˜ o events significantly increases runoff and stream discharge in southern Brazil. High freshwater discharge changes salinity, temperature, and water circulation patterns that can affect the fish estuarine assemblage. Using long-term data obtained from standardized surveys, we analyzed fish assemblage structure and dynamics in shallow waters of the Patos Lagoon estuary in southern Brazil before, during, and after the 1997–1998 El Nin˜ o event. Overall, the relative abundance of all the fish groups in the estuary was about five times lower during the El Nin˜ o than before and after. Freshwater vagrants were the only group with greater abundance during El Nin˜ o. Fish species richness was higher in the estuary during the El Nin˜ o event, when many freshwater species expanded their ranges into the Patos Lagoon estuary, than before or after the El Nin˜ o. El Nin˜ oinduced assemblage changes were not highly persistent, and the estuarine fish assemblage returned to its pre-El Nin˜ o state within 18 months after the El Nin˜ o period. Densities of many marine and estuarine fishes increased to pre-El Nin˜ o levels within 3–6 months of the end of the El Nin˜ o period. We suggested that the rapid recovery of fish estuarine populations after the 1997–1998 El Nin˜ o may have been caused by one or some combination of: (a) enhanced productivity stimulated by nutrients contained in newly deposited alluvial sediments, and (b) enhanced larvae transport in the large saltwater intrusion that followed the El Nin˜ o event. Clearly, fish population dynamics and assemblage structure of the Patos Lagoon estuary can neither be interpreted nor predicted on a long-term basis without explicit consideration of El Nin˜ o Southern Oscillation patterns