Barriers decouple population dynamics of riverine fish, and asynchrony of subpopulations promotes stability within fragments.

The spatial synchrony framework suggests that asynchrony among subpopulations in different branches of a river network should stabilize the metapopulation. However, how barriers affect this framework remains poorly understood. This is a significant knowledge gap given that population synchrony arises from dispersal and environmental similarity, both of which are influenced by barriers. We empirically evaluated how barriers impact fish population synchrony and, subsequently, the associations between synchrony and metapopulation persistence, productivity, stability and trajectory within fragments. We found that barriers demographically decouple populations by decreasing synchrony in brown trout () and Eurasian minnow (), but not in northern pike (), suggesting species-specific responses to fragmentation. Additionally, asynchrony had a stabilizing portfolio effect on metapopulation stability at the fragment level that was statistically significant for . Higher fragment synchrony also made and populations less stable. The impact of barriers on riverine fish population synchrony emphasizes the need to include barriers in future studies on the causes and consequences of synchrony in rivers. That asynchrony stabilizes populations in some riverine fishes suggests that conservation priorities should lie in restoring or retaining larger fragment sizes and higher branching complexity with intact connectivity.