Work to build an £8.5m 'fish elevator' to allow them to swim freely up the River Trent has commenced.
Holme Sluices is a huge flood management structure that spans the full width of the River Trent - the third-longest river in the country - and runs to the south of Nottingham.
It was built in the 1950s as part of a large-scale flood defence scheme and protects the city from flooding.
However such a structure has prevented fish, such as salmon, from naturally migrating up and down the river.
The Colwick (Holme Sluices) Fish Pass will be the largest of its kind in the country, costing £8.5m. to build, and will serve as a 'ladder' or 'elevator' to allow fish to hop up and downstream.
The direct environmental benefits of the fish pass will be £18.6m, the Environment Agency says.
This work will include 60km of river improvements from poor to good status for fish, as well as 60km of new spawning habitat immediately upstream of the barrier.
Clearance work has started to create additional space for construction of the Colwick (Holme Sluices) Fish Pass and a groundbreaking took place on Friday, January 28.
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Colwick Country Park, which is owned and run by Nottingham City Council, will become the hub for the Trent Gateway.
Future plans, the Environment Agency says, could include a visitor centre telling the story of the Trent, its history, ecology and how it has shaped communities along its length for centuries.
Simon Ward, fisheries technical specialist at the Environment Agency, said: "We are excited to share our plans for what will be the largest fish pass in the country.
"Our priority is to open up the River Trent for all fish species through the ambitions of the Trent Gateway Partnership, starting with the Colwick (Holme Sluices) fish pass.
"There are a number of barriers to fish migration within the River Trent catchment, including Holme Sluices, which is the largest barrier to the natural migration of fish in the Midlands.
"By installing fish passage, it will become easier for salmon and other fish to reach their spawning and feeding grounds."