The trout are considered an "unexpected bonus"
Conservationists have welcomed evidence that brown trout are spawning in the Upper River Thames for the first time in more than 20 years.
The trout were found near Eynsham in Oxfordshire, in an area recently restored by the Environment Agency.
"Brown trout are very sensitive to the amount of oxygen in the water, and to the concentration of pollutants," said fisheries Officer Lizzie Rhymes.
"Discovering brown trout in this area is a brilliant result for the river."
The 300m section of river known as the Eynsham Loop was restored in 2007 at a cost of £13,000.
It was funded by the sale of anglers' rod licences.
While the aim of the project was to boost species such as chub, barbel, gudgeon and dace, it has also improved habitats for brown trout.
"It's an unexpected but a very welcome bonus of the project," said Ms Rhymes.
"We'll be keeping a close eye on their progress in the future."