Barbel marked with a dye are being released into the River Great Ouse in Bedfordshire to monitor their habits.
The Environment Agency is marking the bellies of 750 fish so they can be monitored as they move around the river system.
Angling clubs will record information such as size and location, when one of the barbel is caught.
The data will be put towards a three-year Environment Agency study into the future of barbel in local rivers.
Barbel tracking
Environment Agency fisheries officer James Hooker said: "This is the first time we have used dye to track barbel on the Great Ouse.
"The fish suffer no ill effects and, with the support of angling clubs, we expect to get a fuller picture of barbel habits and behaviour."
The agency is asking anyone catching a barbel with a blue spot on its belly to photograph it and send in the photo with the time, date and location of the catch.
The aim of the three-year study is to learn more about the breeding habits and life of the barbel, which is regarded as a secretive fish.
In total, 4,000 fish of different species are being released into the upper reaches of the Great Ouse at Radwell to restock the river.
The fish, which include chub, dace and roach, have been reared at the Environment Agency's national fish farm at Calverton in Nottinghamshire.
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