The invasion of an alien characiform fish and the decline of a native congener in a Neotropical river-floodplain system.
Using a long time series, we analyzed trends in abundance of two characiform fishes in the upper Paraná River and its floodplain: the native Schizodon altoparanae and its alien congener Schizodon borellii, which invaded the floodplain in the last decades. We aimed to contribute to the understanding of the invasive species success and its relationship with its native congener. We compared feeding-related morphology, reproductive investment, and spatial and temporal variations in abundance, searching for possible negative interactions between them. S. altoparanae presented a marked abundance decline between 1986 and 2018, varying with river systems and habitats, but also depending significantly on the abundance of the alien. The native was more abundant in rivers and channels, whereas S. borellii thrived in all habitats, but particularly in channels. Moreover, the alien species, when compared to S. altoparanae, presented relatively larger head depths and mouth gape, and higher values of relative gonadal weight for smaller females. Overall, our results suggest a more generalist habitat use, greater investment in reproduction at small sizes, and larger mouth gape for the invader, which might provide competitive advantages. Although multiple disturbances impact the floodplain, our results suggest that competitive interactions may have contributed to the decline of the native S. altoparanae.