Salmon have been introduced to the River Thames after experts declared the water clean enough for the fish to breed - after almost 200 years.
The 2cm (0.8in) long salmon have been released into the river
The young salmon, were released into the Thames tributary, Lambourne river, at Welford, near Newbury, Berks.
Thames salmon died out in the 1830s, with salmon from other sources, which do not breed there, present from 1974, the Environment Agency (EA) said.
It is hoped a salmon population will be back in the River Thames in 5-10 years.
An EA spokesperson said the new salmon should stay in the river for a year before heading downstream through London, and up to Greenland before coming back to breed.
Darryl Cilfton-Dey, of the Environment Agency, told BBC news: "People do fish for salmon on the Thames but the population is so small at the moment that there's not a great deal of chance of catching one.
"Hopefully if these come back, and if they breed and if the young from those come back, then in a few year's time there'll be quite a few salmon around."
Salmon eggs, about 5,000, were incubated and 2cm (0.8in) long baby salmon introduced to the river.