The critical importance of an undammed river segment to the reproductive cycle of a migratory Neotropical fish

Biotelemetry, ichthyoplankton and genetic data can provide detailed information about the migratory dynamics and reproductive cycle of freshwater fishes. However, few studies have combined these techniques in Neotropical systems. The objective of this study was to examine the migratory and reproductive dynamics of Prochilodus costatus in the Sao Francisco River watershed, south-east Brazil, by comparing the ecological importance of two rivers to the species, an undammed segment of the Sao Francisco River and a dammed segment of one of its main tributaries, the Para River. In total, 215 fish were radio-tagged over three years (2014-16). Eggs and larvae were sampled at seven locations and analysed by PCR to identify Prochilodus spp. ichthyoplankton. Most radio-tagged individuals (97%) used the undammed segment of the Sao Francisco River as spawning migration route, even those captured and released in the Para River. Fish migrated to spawn from late September to late November with the arrival of the rains and returned to feeding sites from December to May after spawning. The highest densities of fish eggs and larvae were recorded in the upper reaches of the Sao Francisco River during months of peak river discharge. Returning fish showed high fidelity to sites occupied before spawning migration. Fish spent roughly 71% of the year at feeding sites, 25% at spawning sites and 4% moving between them. This study provides novel information about the migratory dynamics of Neotropical fishes and underscores the key role of undammed river segments for the conservation of Neotropical migratory fish species.