Brown trout prove that beaver dams don't always limit fish movement

Brown trout have long been considered vulnerable to beaver dams, which have traditionally been seen as obstacles to fish migration.

A new study refutes this idea, revealing that these resourceful fish can actually make their way through such obstacles in certain conditions.

Dams often trap fish in low water levels, preventing or delaying access to key spawning grounds. Researchers warn that global warming, with its extended periods of dryness, could intensify these issues in the future.

Tracking brown trout movement

In the current study, researchers studied trout movement in two Scottish streams – one modified by four beaver dams and the other unaltered.

Using telemetry technology, they tracked how the fish responded to these natural barriers during the critical spawning season from October to December.

The method involved tagging trout with microchips, which were then detected by antennae placed across the dam structures.

Fish overcoming beaver dams

The results showed that high river flows, often triggered by rainfall, significantly increased the likelihood of successful upstream passage. Larger fish also had better chances of overcoming the dams.

On the other hand, low water flow created more substantial obstacles, sometimes preventing migration altogether.

Dr. Robert Needham is the restoration manager at Beaver Trust and former researcher at the University of Southampton.

“Our findings highlight how adaptable brown trout are under favorable conditions, regularly passing beaver dams and with certain individuals making multiple repeat passes,” said Dr. Needham.

“However, as climate change continues to bring warmer and drier weather, the risk of migratory barriers may become a concern on certain rivers.”

The return of beavers to Britain

Beavers were once widespread across Great Britain, but were hunted to extinction by the 16th century. Their recent reintroduction has been hailed as a conservation success, with around 1.2 million beavers now found across Europe.

In Great Britain, populations remain much smaller, with the largest groups in Scotland and some isolated populations in southern England.

Recently, DEFRA authorized the licensed release of beavers into the wild in England.

Concerns about beaver activity

Beaver activity creates biodiversity benefits, but concerns remain about its impact on fisheries.

The construction of dams can potentially block rivers and contribute to localized flooding. This research highlights the need for proactive management strategies to balance conservation efforts with the needs of fish populations.

“Our findings indicate how fish response to river modification through the construction of dams by beaver can be nuanced,” said Professor Paul Kemp, the project lead at the University of Southampton.

“In general, and if rivers are allowed to respond naturally, the benefits of beaver activity can be substantial from an ecological perspective. However, under some circumstances, beaver dams can pose barriers to fish movement, particularly under low flows.”

“More research is now needed to understand how beaver dams might impede fish movements in more modified lowland rivers, such as in the south of England.”

A changing landscape

This study was conducted in collaboration with the Game and Wildlife Conservation Trust (GWCT), Trout and Salmon Association, and NatureScot.

Understanding how beaver reintroduction affects migratory fish like trout and salmon is crucial for conservation planning.

“Beavers are now back in England and Scotland, but the landscape is very different to when they were last here 500 years ago,” said Dylan Roberts, head of GWCT Fisheries.

“With populations of many migratory fish, which need free passage up and downstream to access spawning areas and the sea, now at crisis point and classified as endangered. Such studies are crucial to better understand the interactions between beavers and migratory fish like brown trout and salmon.”

Debate over beaver dams and fish

Dr. Martin Gaywood, Species Projects Manager at NatureScot, noted that the interactions between beavers and fish have been the subject of great debate over many years, especially within Scotland.

“We were pleased to support this work, which makes another important contribution to our understanding of these complex relationships,” said Dr. Gaywood.

“This kind of evidence is vital in helping us and others to plan the continued restoration of beavers to Scotland, including how we use appropriate management and mitigation when necessary.”

Conservation and fisheries management

This research highlights the importance of balancing conservation with practical fisheries management.

As beavers continue to reshape aquatic ecosystems, understanding their impact on fish migration will be crucial.

Scientists emphasize the need for further studies, particularly in lowland rivers where environmental conditions differ from the study areas in Scotland.

With climate change affecting water levels, future conservation efforts must consider how both beavers and fish can thrive in shared ecosystems.

The full study was published in the journal PLOS One.

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