Anglers are being encouraged to get in touch with the Environment Agency if they catch a barbel with blue dye on its stomach.
A total of 1,412 barbel have been added to the River Ivel at Biggleswade Mill and Blunham gauging weir – and 12 of them were given the marks.
This has been done so that the Environment Agency can identify some of the fish that have been stocked and evaluate how successful the stocking has been.
Anglers are encouraged to take a picture of these marked fish, weigh them, and get in touch.
Kye Jerrom, a fisheries specialist, said: “Our aim is to create a river that can manage natural predators and support angling.
“Every penny made from fishing licence income is spent on making improvements to rivers to benefit anglers."
Follow the agency on Twitter @EnvAgencyAnglia