Pike in Devon are being tagged illegally by having cable ties pushed through their backs.
Anglers on the Grand Western Canal, which stretches from Tiverton to the Somerset border, have caught numbers of small fish festooned with plastic fastenings.
Mystery surrounds why anyone would wish to mutilate fish in this way.
"I've been hearing about this for a while," said Exeter-based lure angler Steve Moore.
"I've only recently started fishing the canal again and we've probably already had eight of them."
Some fish have grown since having the tag inserted an one photograph taken by Steve shows a fish with festering sore.
Steve said the biggest tagged fish he had seen was around the 3lbs mark. The shallow canal, which has suffered water loss through leakage in some sections in recent seasons, is not regarded among the South-west's best pike fisheries.
"My own personal view is that someone's trying to determine what they've caught before," he said. "It's really getting our goat down here."
Tonight the Pike Anglers Club urged anglers who fish the water to remain vigilant and report anyone they see behaving suspiciously. The Environment Agency has been informed.
PAC chairman Colin Goodge said: "This is clearly amateurish and we would urge who ever is doing it to contact us.
"Pike anglers elsewhere identify fish and spot repeat captures by photographing them, since it is widely acknowleged that they all have unique and individual markings, much like human fingerprints.
"This could be a school or college project which has gone wrong for all we know, but whatever the reason we would urge those responsible to stop."
