A synthesis of the diversity of freshwater fish migrations in the Amazon basin

In the Amazon, the world's largest river basin, migrations within freshwater habitats are one of the predominant life history strategies for fishes. The flood pulse and the extensive river network provide aquatic organisms with temporal and spatial accessibility to a mosaic of freshwater habitats. Although migratory fish species are central to freshwater ecosystems and fisheries, the knowledge of species and migratory patterns has traditionally relied on anecdotal and scattered information, lacking a unifying methodological and conceptual framework. We quantitatively synthesize the evidence about this biological phenomenom in the Amazon basin through a systematic literature review. We constructed a reference database of migratory events in the Amazon basin, including species, life stages, purposes, direction, habitats and subbasins. We found that 223 species were documented in 90 references as performing migrations distributed across eight orders and 31 families. Migration is a conserved trait in the evolutionary history of Amazonian fish fauna, suggesting that similar to 41% of migratory species are likely unreported. We noted a geographical bias in the report of migratory events towards 13 of the 27 major subbasins of the Amazon. We found a significant association between the hydrological timing at the beginning and end of migrations across species, including reproduction as the most commonly reported purpose. However, most species lack the application of robust methods (e.g. telemetry, otolith microchemistry) to classify them as migratory, relying upon secondary sources of information (i.e. reviews or species checklists). Further, we discuss future opportunities and challenges to continue the study of fish migrations in the Amazon basin.