The Environment Agency is investigating after a large illegal fishing net, which could have killed hundreds of fish, was found hanging across a waterway in Tydd Gote near King's Lynn.
The net, which was about 75ft x 6ft, was found in the waters of the North Level Drain.
Anglers say the net would have been put in by poachers looking to catch fish.
David Parlett, secretary and treasurer of the Tydd Gote Angling Club, said: "A net like this would have caught and killed hundreds of fish of all types.
"The holes are only 75mm sq and would have held fast any fish just behind its gills. Once trapped the fish would have died."
The find comes in the wake of a catalogue of complaints made by anglers over the last few years about poaching of fish in the waterways in and around the area.