Beyond Biodiversity Hotspots: Free-Flowing Tributaries as Ecological Refugia and Benchmarks for River Conservation and Sustainable Basin Governance

Empirical evidence integrating multiple biological levels on freshwater biodiversity persistence in regulated river basins remains scarce for Neotropical systems. We evaluated fish assemblage structure, reproductive strategies, and life-cycle support for migratory species across lotic and lentic habitats in three tributaries of the Porto Primavera Reservoir, Upper Paraná basin, Brazil, over a 12-year monitoring period (2007–2018), encompassing 104 species and a Life-cycle Support Index (LSI) based on occurrence of eggs, larvae, juveniles, and adults. Marginal lagoons functioned as biodiversity hotspots, with higher richness and diversity than river channels; reproductive guilds were habitat-partitioned, with opportunistic and parental-care strategists in lentic environments and periodic/migratory strategists in lotic habitats. Tributary identity was the strongest predictor of assemblage composition: the Aguapeí River, partially protected, showed the highest LSI values, whereas the Verde River provided the strongest evidence of reproduction for conservation-priority taxa, including the critically endangered Brycon orbignyanus, whose adults occurred exclusively there, and the vulnerable Pseudoplatystoma corruscans, for which adults, eggs, and larvae co-occurred. Three proposed hydropower developments on the Verde River would fragment a 200-km free-flowing stretch, threatening these conditions. Undammed tributaries are irreplaceable for freshwater biodiversity and should be recognized as conservation priorities in hydropower licensing and basin governance.