The elvers will be ready to breed in 10 to 15 years
About 50,000 European eels are being released in the washlands of the Dearne Valley in South Yorkshire to address a decline in the numbers of the fish.
The Royal Society for the Protection of Birds (RSPB) said the once common European eel had declined by 95% and was becoming an endangered species.
The Don Rivers Catchment Trust received £92,000 from Defra for the project.
Baby eels, known as elvers, will be released at reserves in Old Moor, Bolton Ings and Edderthorpe Flash.
'Ready to breed'
The RSPB said eels used to be in the Dearne in huge numbers before the Don and Dearne became highly polluted last century.
The elvers will arrive in temperature-controlled vehicles on 14 May.
RSPB project manager Pete Wall said staff and volunteers would work quickly to disperse the eels around the site to settle in the reed beds.
"When the eels are finally ready to breed, in about 10 to 15 years, they will make their way into the Dearne to start the long migration back to the ocean and their spawning grounds in the Sargasso Sea, halfway round the world.
"Barnsley and the Dearne Valley are gaining a growing reputation for great wildlife.
"One of our big messages is that people and wildlife need each other, and it's great that the river Dearne and RSPB Old Moor were selected as the site for this project."
Eel passes have been installed to allow the eels up the river and past the weirs to breed.
The society said the eels were an important part of the food chain and would be eaten by birds and other wildlife such as otters.