Abstract

Beaver reintroductions and beaver dam structures are an increasingly utilized ecological tool for rehabilitating degraded streams, yet beaver dams can potentially impact upstream fish migrations. We collected two years of data on Arctic grayling movement through a series of beaver dams in a low gradient mountain stream, utilizing radio‐telemetry techniques, to determine how hydrology, dam characteristics, and fish attributes impeded passage and movement rates of spawning grayling. We compared fish movement between a “normal” flow year and a “low” flow year, determined grayling passage probabilities over dams in relation to a suite of factors, and predicted daily movement rates in relation to the number of dams each fish passed and distance between dams during upstream migration to spawning areas. We found that the average passage probability over unbreached beaver dams was 88%, though we found that it fell below 50% at specific dams. Upstream passage of grayling was affected by three main characteristics: (a) temperature, (b) breach status, and (c) hydrologic linkages that connect sections of stream above and below the dam. Other variables influence passage, but to a lesser degree. Cumulative passage varied with distance upstream and total number of dams passed in low versus normal flow years, while movement rates upstream slowed as fish swam closer to dams. Our findings demonstrate that upstream passage of fish over beaver dams is strongly correlated with hydrologic conditions with moderate controls by dam‐ and fish‐level characteristics. Our results provide a framework that can be applied to reduce barrier effects when and where beaver dams pose a significant threat to the upstream migration of fish populations while maintaining the diverse ecological benefits of beaver activity when dams are not a threat to fish passage.

Highlights

  • Natural resource managers often note the difficulty of conserving multiple species and ecological processes in ecosystems, especially when species are perceived to have competing habitat requirements

  • Our study examined the effect of beaver dams on grayling movement and was designed to determine relationships for hydrology, dam features, and fish attributes that could impede fish passage and affect movement rates of spawning grayling

  • It is important to note the average passage probability over unbreached dams, after controlling for other factors affecting passage at the dam level, was 88% (Figure 4), showing that Arctic grayling are clearly capable of navigating across beaver dams

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Summary

| INTRODUCTION

Natural resource managers often note the difficulty of conserving multiple species and ecological processes in ecosystems, especially when species are perceived to have competing habitat requirements One example of this phenomenon occurs in managing for migratory fish in landscapes extensively influenced by beavers. We predicted that barrier events during snowmelt runoff are greater at lower flows relative to higher flows, both within and among years, which may reduce the ability of grayling to pass many dams and to reach upper basin spawning areas. In order to address these hypotheses and predictions, we conducted a 2‐year study of Arctic grayling spawning passage and movement utilizing radio‐telemetry techniques in a low gradient mountain stream with abundant beaver dams

| MATERIALS AND METHODS
Findings
| DISCUSSION

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