Barrier features, fish traits, and river flows drive fragmentation of freshwater fish

Abstract

Instream structures such as dams and weirs create artificial barriers to the passage of riverine fish, fragmenting their communities and contributing to global declines in freshwater fish biodiversity. Preventing further declines requires the remediation of barriers to restore fish passage, but analysis of fragmented fish communities is necessary to prioritize locations and fish taxa for remediation. Additionally, the potential for high flow events to facilitate barrier drown-out and reduce fragmentation remains unresolved. We used a meta-regression analysis to investigate the severity of fish fragmentation in relation to barrier features, fish traits, and river flows, quantifying fragmentation with a novel log response ratio metric reflecting the asymmetry of fish populations around barriers. We discovered that high barriers, barriers which separate different sized habitats, and clusters of sequential barriers cause more severe fragmentation and should be prioritized for remediation. Currently, barrier remediation is focused on improving passage for mobile fishes, but taxa which migrate short distances and have poor swimming performance were most fragmented, suggesting efforts are warranted to improve passage for less vagile fishes. We found evidence that fragmentation was reduced by large river flows which spill onto the floodplain and provide additional connectivity around barriers, particularly in highly regulated sections of stream with many sequential barriers. The findings of this study can be applied to improve the management of fish passage in rivers, an area of increasing relevance with the worsening discontinuity of rivers due to climate change and the continued construction of barriers.