John Fitzpatrick and Anthony Senior - both were countryside wardens - were jailed yesterday for eight months and banned from holding fishing licences for two and a half years after admitting conspiracy to poison.
Mold Crown Court heard that anyone swimming in the Rivers Elwy and Aled, at near Llanfair and Abergele, Clwyd, at the time would have been at risk from the poison which killed any living creature.
Senior, 46, of Weybourne Drive, Bradbury, Stockport, and Fitzpatrick, 30, of Compstall Road, Marple Bridge, Stockport, had used large quantities of cyanide in their four-year campaign.
They did not make any profit from it but either ate the fish or gave it away. But they did not use enough poison to cause serious pollution.
Judge David Hale said they were not poachers - they were entitled to fish in the rivers - but he found their poisoning campaign inexplicable.
He told them: "The whole of your background must be directed towards maintaining fish and animal life in the countryside not setting indiscriminate poison in the water."Reuse content