Brown trout can successfully navigate beaver dams in defiance of many people’s expectations, according to a study.
The research monitored trout movements in two streams in northern Scotland, Allt Coire an t-Seilich and Allt a’Choilich – with the former modified by a series of four beaver dams and the latter unaltered.
Scientists at the University of Southampton observed the trout as they navigated barriers during critical spawning periods from 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 research found high river flows, triggered by rainfall, significantly increased the likelihood of successful upstream passage, and that larger fish had greater success navigating the dams.
During low flow periods, however, beaver dams posed a more significant obstacle to trout, delaying or sometimes preventing their movement upstream.
A beaver dam on the Allt Coire an t-Seilich, one of the research sites (Beaver Trust/PA)
The findings have been published in the journal PLOS One.
Dr Robert Needham, restoration manager at the Beaver Trust and a 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.”
Professor Paul Kemp, the project lead at the university, said: “Our findings indicate how fish response to river modification through the construction of dams by beaver can be nuanced.
“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.”
Beavers are back in the wild in Scotland and England (Andrew Matthews/PA)
The project was conducted in collaboration with the Game and Wildlife Conservation Trust (GWCT), the Trout and Salmon Association and NatureScot.
Dylan Roberts, head of fisheries at GWCT, said: “Beavers are now back in England and Scotland, but the landscape is very different to when they were last here 500 years ago.
“Populations of many migratory fish, which need free passage up and downstream to access spawning areas and the sea, are 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 at NatureScot, said: “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, the Natural Environmental Research Council and the University of Southampton’s doctoral research programme Spitfire.