Abstract: Longitudinal and lateral connectivity of rivers is necessary for fish to successfully fulfill their life cycle. Three hydraulic constructions prevent fish migrating from the sea to the upstream part of the river Demer. In support of the plan to enhance fish migration along the third barrier, upstream migration of chub and catfish was studied during the spring and summer of 2014 using acoustic telemetry (Vemco technology: V13 transmitters, VR2W data loggers). Beside the barrier, the potential of a small tributary as a natural bypass was studied. Only large chub and catfish succeeded in migrating upstream along the barrier after many trials, indicating a very small chance for fish to pass this barrier. Although the attractiveness of the tributary was smaller, it was more efficient than the barrier, suggesting its potential as a natural bypass channel to upstream spawning habitat in the river Demer. Research on the relation with discharge might further support the local water managers in enhancing the attractiveness of the tributary and its potential as a natural bypass for upstream migration.