Illegal fishing in some of Lincolnshire's waterways is depleting stocks of carp as poachers try to cash in on the fish's value abroad.
The Environment Agency's environmental crime team has asked anglers in the county to keep an eye out for suspicious behaviour.
Mark Rumble, one of the agency's crime investigators, said nets or multi-hook lines were used to catch fish.
He said: "These people don't use a rod and line, but nets or spears."
Mr Rumble added: "We need to stop 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."
As well as targeting people who illegally remove fish from lakes and rivers the agency's purge on illegal fishing hopes to protect eels, whose numbers it says have dropped dramatically.