Young eels released into Blagdon Lake
Some 25,000 eels have been released into a North Somerset reservoir as part of a national project to boost numbers.
Eventually 100,000 eels will be released in UK waterways, including in Shropshire, east of England and Wales.
Eel numbers are falling as they cannot reach their breeding habitats due to man-made barriers in their way.
Andrew Kerr, from the Sustainable Eels Group, said: "You want to see the eel doing well and prospering as it tells us how well we are managing our water."
Mr Kerr added: "What we've had to do is catch them in hand nets and then put them in tanks.
"Sometimes we immediately move them on past the barriers but on this occasion we've taken 100,000 and we've grown them on from two inches long, to four, five, six inches long."
UK Glass Eels used licensed fishermen who caught the baby eels in hand nets during the spring tides along the River Severn earlier this year.
The first batch was released into Blagdon Lake on Monday.
This site was chosen as it has shallow water, lots of reeds for protection from predators like birds, and plentiful food.
The young eels will then grow and mature over the next 15 to 20 years as they live low down in the water.
Once mature they will return to the Sargasso Sea in the Atlantic to breed and spawn.
A total of £45,000 is being spent on the project organised by the Bristol and Avon Rivers Trust, the Sustainable Eel Group and UK Glass Eels.