City spokeswoman Varvara Trunova said: "The fish has been confiscated from the fisherman and sent for examination."
She added: "According to the preliminary findings the Pacu fish is 20 centimetres long and weighs 240 grams."
Experts believe the fish may have been an exotic pet that was abandoned in the lake when it grew too large, sparking fears of a pair may have been dumped together and started breeding.