Environmental predictors of fish assemblages in insular streams of the eastern Amazon

Abstract Understanding fish species distributions remains a key challenge in ecology, particularly in complex environments such as small Amazonian streams. These data are essential for developing and supporting effective management and conservation strategies, especially in highly diverse regions like the Amazon Basin, where substantial gaps in biodiversity knowledge persist. We assessed the structure and composition of fish assemblages in 22 intra-insular streams in the Marajó Archipelago, eastern Brazilian Amazon, testing their responses to local environmental conditions (i.e., environmental variables), land use and land cover, and fluvial distances between streams. Sampling occurred during the dry season, in October 2022 and June 2023. Relationships between assemblage structure and predictors were analyzed using Linear Models and Redundancy Analysis (RDA). We collected 7813 individuals representing 54 species, 26 families, and nine orders. The linear model explained 79% of the variation in species richness, with canopy cover, temperature, pH, and forest cover as key predictors. RDA indicated local environmental conditions as the main drivers of species composition. These findings highlight the need to account for multiple environmental gradients in Amazonian fish conservation.