Just 3 per cent of rivers in the UK flow unobstructed today, according to a European Union project. Centuries of industrialisation have reshaped these waterways and artificial structures such as weirs now hinder their flow. This has ramifications for river ecosystems and the fish that live in them. “All fish migrate, so they move either for shelter or reproduction or for feeding,” says Charles Crundwell, a fisheries specialist at the UK Environment Agency. “If you have a barrier across the river, like a weir, it acts like a brick wall.”
Now, a project on the river Severn, the UK’s longest river, crossing England and Wales, is changing that. The Unlocking the Severn initiative is designed to get twaite shad, a fish whose numbers in British rivers have dropped over recent decades, back to its historic spawning grounds.
To do this, a series of fish passes have been built. These special channels allow shad, which are unable to leap like salmon, and other fish to overcome weirs on a stretch of the Severn. “Ultimately, what we want from these passes is to make the weirs invisible,” says Jamie Dodd at the University of Hull, UK, who is involved in monitoring the movement of fish through the passes. “What we’re doing is actually reconnecting both sets of habitat both above and beneath the weir and creating what is essentially one river.”
Dodd and his colleagues from the University of Hull, Bournemouth University, the Canal & River Trust, Natural England and the UK Environment Agency have been tagging, monitoring and collecting data on numerous fish species to understand what effects the passes are having. Already, the researchers have identified 26 different species passing through Diglis fish pass, one of the lowest in the system.
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“We have a saying on this project,” says Crundwell. “If you can design a pass that twaite shad will use, any fish will use it.”
Analysis of environmental DNA – released by animals through faeces, for example – found in the water confirms the presence of shad upstream of the passes. Ongoing tagging and monitoring will confirm whether fish return year after year.
However, for the team, fish passes are a last resort. “It’s always our preference to remove a weir over building a fish pass,” says Crundwell. “Fish don’t want to just live in a swimming pool.”
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