Once a common sight in Britain’s waterways, the twaite shad, a small silvery fish belonging to the herring family, had gradually become a rare find.

“I used to live by the Severn estuary and I had never heard of it before – that is a sad state of affairs for a fish that used to travel up this river in its hundreds of thousands every year,” says Richard Harrison, programme manager for Unlocking the Severn (UTS).

But in a remarkable project completed in 2022, these once-abundant fish have been given the opportunity to return to their natural spawning grounds in the River Severn after an absence of more than 180 years.

The project involved the construction of four fish passes, which were integrated into navigational weirs that had been blocking access to over 150 miles of the river. This initiative has breathed new life into the Severn’s beleaguered fish species.

Charles Crundwell, a senior technical specialist at the Environment Agency, has been involved with this project since 2009, when the Severn was designated a special area of conservation. At that time, the twaite shad, scientifically known as Alosa fallax, was considered to be in unfavourable condition primarily due to the obstruction of their historical spawning grounds.

“We don’t know what the run size of shad was on the Severn back in the 1840s, but we do know it supported a very healthy, local and international fishery,” says Crundwell. “They were sold locally for food or they were pickled, put in barrels and sent abroad. They ran in their hundreds of thousands; we know that because there is an equivalent species called the American shad and they still have runs in the multimillions.”

Recent studies have shown a staggering 76% decline in freshwater fish populations worldwide between 1970 and 2016, with Europe experiencing an average decline of 93%. Barriers such as weirs and dams that obstruct watercourses have contributed significantly to this decline.

“If you think about it, putting weirs along a river means you are making it into separate linear lakes; they’re swimming pools effectively,” says Crundwell.

The situation in the River Severn is unique due to the presence of four navigational weirs within a 10-mile stretch between Worcester and Stourport. These “choke points” effectively severed 83% of the Severn from fish access. This isolation led to increased competition, heightened predation, and significantly reduced survival rates for many fish species.

“It is only a really small section for the shad to get through before they are back to a river which is effectively the same as it always was,” says Crundwell. “It’s like having a perfect habitat, but they just can’t get to it.”

In July 2016, funding was secured from the EU Life program to construct the four fish passes, enabling the twaite shad and other species to bypass the weirs and continue their journey upstream. Additional funding from the national lottery allowed work to commence in 2019.

The project aimed to create three “deep vertical slot” passes at Diglis, Holt, and Linkum, resembling a horizontal ladder of deep pools that fish could use to navigate past the weirs. At Bevere, where the Canal & River Trust led the project, more space was available, enabling the construction of a “natural bypass channel” – a gently sloping ramp that allowed fish to safely navigate the weir.

Because twaite shad primarily reside at sea and only enter the river to spawn between April and May, the team had to wait until the following year to track their progress using acoustic trackers and environmental DNA techniques.

“We have been able to prove that during the migration season last spring, the shad had been able to pass through all four structures that we built. The fish are using them; it is amazing,” says Crundwell.

The project has actively encouraged public engagement with the river and its ecosystem.

“We have had about 7,000 visitors now,” says Harrison. An underwater viewing gallery built alongside the fish pass at Diglis offers backlit views of all of the species that swim past the two giant windows on their journey up the river.

Volunteer Caroline Attwood-Reusser believes that the project is fostering a new sense of excitement and pride around the river. “The power of the viewing gallery is that it gives people the chance to actually see what’s beneath the surface. There is always excitement in the pass if we see a fish – yesterday we had the tiniest minnow and people were so excited about it.”

An underwater viewing gallery situated beside the fish pass at Diglis offers illuminated views of various fish species swimming past, creating an engaging experience for visitors. With approximately 7,000 visitors and the active involvement of local children in projects like contributing designs to a shad flag, a newfound sense of excitement and pride around the river is taking root.

“The warmth the project has received has been incredible. There are a lot of problems with rivers at the moment, but what is really important is being able to connect people with the fish that live in the river. People are understanding that rivers are important and they are worth protecting,” she says.

In 2022, over 700 twaite shad successfully passed through the constructed fish passes, alongside 24 other species. While the numbers were slightly lower in the following year, scientists remain optimistic, emphasising that the inter-year variation is within expected levels. The hope is that with each passing season, the fish will venture further upstream, reclaiming all of their previously blocked spawning grounds.

The project team is hopeful that their work on the River Severn could serve as a model for success in other locations, providing inspiration and insights into reviving and conserving fish populations and their natural habitats.

The return of the twaite shad to the Severn stands as a testament to the positive impact of concerted conservation efforts, setting a promising precedent for the restoration of biodiversity and ecosystem health in waterways worldwide.

“This is the biggest project of its type in Europe. The Severn is the UK’s longest river, so it raises the profile of the whole issue. It’s a global problem that needs addressing. It’s not a cheap solution – this was a beast of a project – but we’ve shown it can be done,” says Harrison.