Deciphering the complex life cycle and partial migration of an ecological engineer and critical Neotropical fishery species, <scp> <i>Prochilodus costatus</i> </scp>

Understanding complex migration patterns, including drivers of partial migration and habitat use, is challenging but essential for conservation, as it determines a species' adaptative capacity in the face of environmental change and anthropogenic threats. Therefore, we investigated the traits of partial migration and spawning use in the long-distance migratory and ecosystem engineering fish Prochilodus costatus (pioa). Data from 200 adult individuals tagged with radio transmitters at their feeding and spawning sites, combined with egg and larvae sampling identified through metabarcoding, revealed strong fidelity to both feeding and spawning sites. Providing a comprehensive overview of the pioa life cycle, this study shows that only fish present at specific upstream sites reproduce, indicating that populations isolated downstream, or whose migrations are blocked for any reason, fail to reproduce. Adults may also remain resident at spawning grounds, but these residents are smaller and have lower body condition, suggesting suboptimal feeding conditions at these sites. Use of spawning grounds was not evenly distributed: approximately half of the tagged fish performed annual upstream reproductive migrations, with 58.6% migrating to Bambuí River and 41.49% to the upper Sao Francisco River basin spawning sites, displaying homing behaviour. Hormone levels, body weight, and condition factor were not related to migration decisions, suggesting that the drivers of partial migration remain unclear. Our findings reveal putative population structuring, site fidelity, habitat use, and partial migratory behaviours shared with other phylogenetically distant fish groups, indicating that these results may be generalizable to other Neotropical migratory species.