Coastal freshwater stream fish fauna from a threatened estuarine lagoon complex in northeastern Brazil

Freshwater ecosystems play a vital role in sustaining human populations; however, these environments are increasingly subject to human interference, driven by land use modifications, species introductions, pollution, and habitat loss. We sampled the ichthyofauna and collected environmental variables from 24 coastal streams in northeastern Brazil. Fish composition, abundance, and biomass served as the response variables, while physicochemical data, hydrological attributes, riparian characteristics, and substrate composition were considered as predictor variables. Our main objective was to evaluate the impact of a riparian land use gradient, ranging from conserved to degraded scenarios (i.e., forested, grassy, and urban streams), on the structure of fish assemblages. To achieve this, the graphical relationship of ABC plots and their W values between the three sets of streams was evaluated, and a Hellinger transformation-based Redundancy Analysis (tb-RDA) was conducted and we compared fish composition among the stream categories using a PERMANOVA test. We identified that the W values of forested and urban streams were significantly different, indicating a shift from k to r-strategists; the tb-RDA revealed three stream categories: (i) forested streams and their association with Anablepsoides bahianus and Parotocinclus cf. jumbo, (ii) grassy streams and their association with Poecilia vivipara and Astyanax cf. bimaculatus, and (iii) urban streams and their relationship with Poecilia reticulata and Oreochromis niloticus. We also found important differences between fish composition along the riparian land use gradient, with urban streams showing a significant divergence from grassy and forested streams. We observed a loss of native species and an introduction of exotic species in the evaluated gradient, consistent with the premises of niche theory.