When trout and salmon come up against an obstacle built by a beaver, they have a spectacular solution.
When beavers were re-introduced to the UK after being nearly wiped out, some worried that their dams would cause problems for fish swimming through the rivers. But footage has shown just how unfazed fish, such as trout, are when faced with a dam in their path.
“The Eurasian beaver was once widespread across Scotland, but by the 16th century it had been driven to extinction,” says Elliot McCandless, communications manager at Beaver Trust.
“Beavers were too valuable for their own good,” he adds. People hunted them for their waterproof pelts, meat, and an oil called castoreum, which was used in perfumes, flavourings and even medicine. “As hunting pressure increased to meet this demand, beaver populations declined steadily until they disappeared entirely from Scotland,” says McCandless.
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In recent years, there have been efforts to reintroduce beavers to UK habitats but some people worried that this could have a negative impact. “Fishery stakeholders, particularly those focused on trout and salmon, worried that beaver dams could obstruct the movement of migratory fish to and from their spawning grounds at critical times of year,” he says.
“These concerns are especially acute given that wild Atlantic salmon have declined by around 70% in the last 25 years and are now classified as endangered in Great Britain.”
Beaver Trust also cares about the health of wild fish populations, says McCandless, and has been studying the impact of beavers on these other species. “It all started with our now Restoration Manager, Dr Rob Needham,” he says.
Needham worked in restoration as well as studying the interaction between beavers and salmonids. His research found a positive relationship between beaver reintroduction and brown trout and that the fish are "adept at passing beaver dams".
This is perfectly illustrated in a video that shows a trout leaping over a beaver dam to continue on its way. Its secret? Its strong swimming skills.
“They don't use any special tricks,” says McCandless. “Atlantic salmon and trout are physiologically adapted to overcome significant physical obstacles through powerful swimming and, when necessary, leaping.”
Some blockages might be harder to overcome than others, depending on specific conditions such as height, depth and the speed of the water flow. Plus, beaver dams shift as the river conditions change. “Crucially, unlike concrete human structures, beaver dams are temporary, leaky barriers that are constantly changing,” he says.
Although the salmon and trout can jump over obstacles, they don’t love doing it and prefer to swim through if they can. “Leaping is abnormal, risky, and energy-inefficient behaviour for an individual fish,” he says, “but in certain situations… a fish may judge that jumping is necessary.”
But the inconvenience of having to take the leap might be worth it when looking at the bigger picture. “We believe, based on the weight of scientific evidence, that beavers and their activities can help restore rivers and streams more quickly and more effectively than we can through human intervention alone,” he adds, “creating the diverse range of habitats that trout and salmon need at every stage of their life cycle before they go out to sea.”
