Size spectra assess social-ecological impacts on Amazonian fish assemblages

Finding easily accessible indicators to assess the biodiversity conservation is crucial in regions where long-term monitoring data is deficient. This is particularly true for tropical freshwater ecosystems, which house most of the known fish biodiversity and faces multiple natural and anthropogenic threats. Fish sizes serve as a key ecological indicator of ecosystem productivity and responses to environmental changes. Using length measurements of 354,465 fish produced by multiple projects and researchers, we investigated the spatial patterns and main drivers of fish productivity and mortality across 11 widely distributed basins in the Brazilian Amazon. Specifically, we examined if fish mortality and ecosystem productivity vary among environments, water types, Amazon sampling regions, and landscape metrics. Our findings show that fish size structure vary between water types and sampling regions, with Northern regions of the Amazon Basin exhibiting larger fish sizes and lower mortality than Southern region. Human population demography and forest cover also influenced fish productivity and size structure, affecting complexity and diversity of ecosystem trophic chains. We provide evidence that maintaining at least 75% forest cover is critical for supporting fish fauna size structure, highlighting the importance of considering terrestrial-freshwater links in conservation programs aiming to enhance sustainable fishing.