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

## TL;DR

Barriers in rivers can reduce population synchrony in some fish species, which helps stabilize their populations, suggesting that conservation should focus on maintaining complex river structures.

## Contribution

The study empirically shows species-specific effects of barriers on population synchrony and metapopulation stability in riverine fish.

## Key findings

- Barriers decrease synchrony in brown trout and Eurasian minnow but not in northern pike.
- Asynchrony stabilizes metapopulation dynamics in brown trout through a portfolio effect.
- Higher synchrony reduces stability in brown trout and Eurasian minnow populations.

## Abstract

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 (Salmo trutta) and Eurasian minnow (Phoxinus phoxinus), but not in northern pike (Esox lucius), 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 S. trutta. Higher fragment synchrony also made S. trutta and P. phoxinus 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.