A groundbreaking study, led by the University of Southampton (UK), has revealed that brown trout can successfully navigate beaver dams — obstacles which were previously thought to pose a significant barrier to their movement.
However, researchers say the situation can be complex, with low water flows in some rivers holding fish below the dams, delaying them from accessing spawning areas. They suggest prolonged dry spells, due to climate change, may increase this problem in certain areas of Great Britain in the future.
Findings are published in the journal PLOS One.
The research monitored trout movements in two streams in Scotland — one modified by a series of four beaver dams and the other unaltered.
The scientists observed the trout as they navigated the barriers during critical spawning periods (October to December). The fish were tracked using telemetry technology, where trout are tagged with microchips that are read by antennae spanning the dam structures.
The researchers found that high river flows, triggered by rainfall, significantly increased the likelihood of successful upstream passage. Additionally, larger fish had greater success at navigating the dams. Conversely, during low flow periods, beaver dams posed a more significant obstacle, delaying or sometimes preventing trout movement upstream.
Dr Robert Needham, Restoration Manager at Beaver Trust and former University of Southampton researcher, said: “Our findings highlight how adaptable brown trout are under favourable conditions, regularly passing beaver dams and with certain individuals making multiple repeat passes. However, as climate change continues to bring warmer and drier weather, the risk of migratory barriers may become a concern on certain rivers.”
Beavers were once common throughout Great Britain until they were hunted to extinction around the 16th Century. Their reintroduction in recent years has been heralded as a conservation success story with some 1.2 million of the animals now living wild across Europe. Populations in Great Britain are much smaller, the largest being in Scotland, with isolated populations in the south of England — although DEFRA has just authorised the licenced release of beavers into the wild in England.
The natural engineering of beaver habitats creates biodiversity benefits but has also sparked concerns about potential impacts on local fisheries, such as blocking of rivers and localised flooding. This new research underscores the importance of proactive management strategies to mitigate potential future challenges for fish migration, particularly in a shifting climate.
“Our findings indicate how fish response to river modification through the construction of dams by beaver can be nuanced,” says 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.”
The project was conducted in collaboration with the Game and Wildlife Conservation Trust (GWCT), Trout and Salmon Association, and NatureScot.
Dylan Roberts, Head of GWCT Fisheries, comments: “Beavers are now back in England and Scotland, but the landscape is very different to when they were last here 500 years ago. 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.”
Dr Martin Gaywood, Species Projects Manager, NatureScot adds: “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. 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.”
The research was funded by the collaborating organisations, Natural Environmental Research Council (NERC) and the University of Southampton’ s doctoral research programme SPITFIRE.
