Work has been carried out in Horsham and Wisborough Green to stop fish and eels being trapped and isolated away from their breeding grounds and habitats.
The scheme to improve fish passage on three rivers in West Sussex is part of the Arun and Rother Connections (ARC) project by the Environment Agency.
Fish and eel populations are put at risk by the obstructions and changes to river flow caused by weirs on the main Arun and Rother rivers and their tributaries.
Unable to get over the weirs, they were swept down or upstream away from their habitats.
This has caused the loss of fish and eel populations on sections of the Arun and Rother, and affected the river environment.
Removing a weir at Wisborough Green on the River Kird, a tributary of the River Rother, has allowed that section of the river to flow more naturally, improving habitat and fish passage.
Allowing fish to pass upstream of weirs opens up many more kilometres of habitat and breeding grounds.
A series of baffles made of recycled plastic have been added onto the weir on the Rother, which means that fish can now more easily pass over it.
A baffle is a series of steps which helps the fish to climb over the weir, and it should also give the fish somewhere protected to rest.
A pass for eels consisting of a pipe filled with brush material that allows eels and elvers to navigate the weir has also been added on the Rother.
On the River Arun near Horsham, a type of fish pass called a Larrinier also enables fish and eels to freely move upstream.
“There are hundreds of weirs on our rivers in the South East which are having negative effects on our wildlife and the ecological status of our rivers and streams,” said Damon Block from the Fish and Biodiversity team at the Environment Agency. “The Arun and Rother are priority water bodies for improving fish passage in the South East River Basin District.”