Impacts of large and small barriers on fish assemblage composition assessed using environmental DNA metabarcoding

Area of Science:

• Ecology
• Conservation Biology
• Freshwater Science

Background:

• River fragmentation from instream barriers is a major driver of freshwater biodiversity loss, especially impacting migratory fish populations.
• Existing research often prioritizes the effects of large dams, potentially underestimating the cumulative impact of smaller structures on aquatic ecosystems.

Purpose of the Study:

• To assess the combined effects of large and small instream barriers on fish species richness and abundance.
• To investigate the influence of environmental factors, including altitudinal gradients, on fish distribution in a fragmented river system.

Main Methods:

• Utilized environmental DNA (eDNA) metabarcoding to identify fish species (zero-radius operational taxonomic units - zOTUs) along the River Allier, France.
• Analyzed fish species richness and relative abundance in relation to elevation, flow, and barrier characteristics (density and cumulative height).

Main Results:

• Identified 26 fish species across the river's main stem using eDNA metabarcoding.
• Elevation, average flow, and barrier density were key factors influencing fish distribution. The greatest discontinuity in species richness was downstream of the largest dam, not at its location.
• Cumulative effects of numerous small barriers significantly impacted fish species richness and abundance.

Conclusions:

• Both large and small instream barriers cumulatively affect fish biodiversity.
• Managing numerous small barriers could be more beneficial for restoring fish species richness in fragmented rivers than focusing exclusively on large dams.