Barbel breed in clear water
The Environment Agency has restocked the River Thames in Oxfordshire with more than 1,000 fish.
The batch of barbel was released in three areas along the river as part of a project to give the declining population a boost.
The fish have been marked with a small orange spot in the translucent skin adjacent to the eye.
The marks will help the Environment Agency keep track of how the population is developing in the coming years.
Anglers are being asked to do their bit by reporting any catches of barbel which have the distinctive orange spots.
They were released between Buscot and Newbridge.
Environment Agency fisheries officer Lizzie Rhymes said: "Our aim is to see these barbel flourish in areas of the Thames where numbers have declined.
"The stocking is part of the Upper Thames Barbel Project, which we started in 2005.
She added: "The project identified areas of the Upper Thames where barbel populations are threatened.
"We have also begun a programme of habitat restoration to improve and create new spawning grounds."
The Environment Agency will keep track of how the population develops
Barbel, which can live up to 25 years, indicate high quality river habitat.
However, the species faces many pressures which threaten populations in the River Thames, including a lack of suitable spawning grounds.
To spawn, barbel require shallow gravel areas with fast flows.
In rivers without suitable spawning grounds nearby, adult barbel will migrate over 30 km to spawn.