The sum of multiple human stressors and weak management as a threat for migratory fish

Habitat fragmentation, species introduction and poor management have disturbed populations of migratory fish worldwide. No study, however, has investigated how the sum of impacts affects Neotropical migratory fish. This study investigated the migratory behavior of an endemic fish (Prochilodus hartii, Jequitinhonha River, Brazil) in a scenario of fragmentation and flow alteration by Irapé Hydropower Dam, hybridization with alien fish and controversial management practices using an integrative framework (radio and acoustic telemetry + genetics). Movements were assessed at four sites: the reservoir (TR1); fish passed manually to the reservoir (TR2), simulating a fish passage system; the tailrace (TR3); and 80 km downstream from the dam (TR4). Overall, fish from TR1 and TR2 moved over shorter distances across the reservoir, and few reached upstream sites. Fish from TR3 remained close to the dam, while fish from TR4 showed a diversity of migratory behaviors; most migrated to a large tributary, and no fish approached the dam. We recorded hybrids in all sites, but the percentage increased (62%) at TR4. Native fish and hybrids showed synchronized migratory behavior. Results depict a difficult conservation problem, considering that the dam isolated populations, the impoundment disturbed migratory dynamics, and fish invasion and hybridization affected the genetic structure of native stocks. In this scenario, the installation of a fish pass would not benefit native populations, with risk of enhancing demographic impacts and species invasions. This study shows how multiple stressors create a complex (but common) scenario where the conservation of migratory fishes is made more difficult. •Habitat fragmentation, species introduction and fish passage interaction•Integrative framework: telemetry (radio and acoustic) and genetics•Native fish and hybrids showed synchronized migratory behavior•Impoundment disturbed migratory dynamics•Fish pass would not benefit native populations