An operation to protect fish stocks against criminals in Northamptonshire and Cambridgeshire has been launched by the Environment Agency.
Operation Tapestry aims to tackle illegal fishing, such as netting and the use of long-lines - a fixed unattended line with one or more hooks.
The move also aims to protect eels, whose numbers have fallen by 95% since the 1980s.
Intelligence will be gathered to deploy resources to catch those responsible.
Environmental crime officer Mark Rumble said: "We need to reel in the perpetrators of these crimes.
"Some of these fish are worth quite a lot of money. A big specimen carp can be worth up to £5,000 and a lot of carp are illegally imported from Europe where they grow bigger.
"One diseased or parasite-carrying fish introduced into another fishery can have a massive impact on that fishery."
He added: "This type of crime threatens the livelihoods of law-abiding fisheries owners, the future of angling and the environment."