When a freshwater invader meets the estuary: the peacock bass and fish assemblages in the São João river, Brazil.
Peacock basses (Cichla spp.) are native to the Amazon basin but introduced to different parts of the world. Almost thirty years ago, Cichla kelberi was introduced in an impoundment of the São João River, a coastal system in southeastern Brazil. Recently, this cichlid invaded the estuarine section of the basin. This study aims to analyze spatial and temporal variations in catch of C. kelberi and fish assemblage structure along the estuarine stretch of this river and how abiotic variables affect their distribution. Sampling was performed in four segments downstream of the dam. Principal component analysis revealed that abiotic variables displayed temporal and spatial variation, in part due to the salinity gradient, that were more pronounced in the dry season. Cichla kelberi occurred in all segments, but mainly in shallow and vegetated habitats of the middle course and barely in the most downstream. Eighty-one fish species were recorded, nine of which were non-native, representing 33.4% of total catch. A redundancy analysis indicated that the fish assemblages showed marked spatial variation, mostly related to the salinity gradient. The lowermost segment of the river was dominated by marine species, the only locality where non-native species summed less than 40% of the catches. In the upstream segments, higher oxygen levels and lower temperatures influenced the occurrence of most species. Higher salinity of the estuary seems to limit the spread of C. kelberi, but the invader may reach adjacent basins in years of exceptional floods. The eurihalinity and piscivory of C. kelberi partly explain its invasive success.