Helping Fish Find Their Flow

The European eel embarks on an extraordinary journey, traveling all the way from Bermuda to mature in freshwater rivers in Europe before returning to the Sargasso Sea to reproduce and complete their lifecycle. 

However, dams in one of those freshwater rivers, the River Trent in the U.K., hampered that journey and contributed to the European eel becoming a critically endangered species.

“The decline of migratory fish in the Trent catchment dates back to the Industrial Revolution, when large weirs [dams] were first built to open up the river for trade, which had a negative impact on water quality,” says Matt Buck, fisheries technical specialist at the U.K.’s Environment Agency.

Now, thanks to the completion of the Colwick Fish Pass, the largest structure of its kind in the U.K., the European eel and many other migratory fish species are once again able to pass safely through the River Trent.

The Colwick Fish Pass was constructed around the Holme Sluices in Nottingham. A huge concrete structure built to protect the city from flooding, the sluices consequently acted as one of the largest barriers to fish migration in central England from the time it was built in the 1950s.

Opened in November 2024, the fish pass is one big step toward restoring the River Trent and its tributaries to their pre-industrial ecological glory, allowing salmon and the voyaging European eel to thrive.

“Salmon and other migratory species such as lamprey and eels used to be plentiful in the River Trent, and they used the river to swim up and down en route to their spawning grounds,” Buck says.

A fish pass around the Holme Sluices had been a vision as far back as the 1970s, Buck explains. It will help more than 35 miles (60 kilometers) of the U.K.’s third-largest river to reach “good” status for fish, a classification based on water levels, flow, and habitat condition that supports a diverse range of fish — and one that only 43% of England’s rivers meet. The pass is also set to create about 35 miles of new spawning habitat immediately upstream of the barrier.

The fish pass was meticulously planned to benefit all species of fish, including enabling the mighty Atlantic salmon to travel upstream to access the gravel riverbeds they need to lay their eggs in autumn and winter. The pass is nearly 700 feet (210 meters) long, 20 feet (6 meters) deep, and 21 feet (6.5 meters) wide, divided into 20 ascending chambers where fish can rest before their onward journey. It also includes an eel pass to help eels wriggle upstream.

It won’t be long before the Environment Agency finds out just how successful the pass has been. “We plan to undertake short-term monitoring using underwater camera systems during autumn 2025 and spring 2026 to assess the range of species utilizing the fish pass,” Buck says. 

But they’re confident in the design. As Buck explains, “The deep vertical slot fish pass is well used across Europe, with evidence of them being successful for a range of species.”