Four men have been arrested on suspicion of illegally removing fish from Norfolk's River Wissey.
Police swooped on Sunday, May 25, on a stretch of river at Roxham Fen, between Hilgay and West Dereham.
The four men - believed to be Eastern Europeans - were questioned and are currently on police bail awaiting decisions on prosecution by the Environment Agency.
Det Sgt Chris Curtis, of Downham Market CID, said: "Persons caught committing this type of offence face not only the prospect of arrest and prosecution for offences of unlawfully taking or destroying fish, they also face the prospect of all their fishing equipment being confiscated and forfeited.
"The maximum punishment that can be awarded for these offences is three months imprisonment and or a fine."
Richard Ingham, Fisheries Enforcement Team Leader at the Environment Agency's Brampton office, said: "We are working more and more closely with the police and local angling clubs to crack down on any such crime. We take all angling offences extremely seriously and are convinced that this kind of co-operation pays dividends."
The arrests come after an Crimestoppers-style hotline was launched in the Fens to gather information on incidents and enable patrols to be targeted, after talkes between the EA, local angling clubs and the Pike Anglers Club.
The PAC understands a number of dead fish were seized, along with live tench and pike restrained on stringers, while monfilament gill nets and boats were confiscated.
Anyone with information should call 01480 483023.