Anglers are calling on whoever is tagging pike with cable ties in a Devon canal to stop.
About 26 pike have been caught with the tags since April
Fishermen at the Grand Western Canal in Tiverton say the practice may have been going on for as long as four years.
About 26 have been caught since April with ties through their dorsal fins, which has caused sores on some fish.
The Environment Agency says while it does not condone the practice it would not expect to take action unless a fishery was being seriously damaged.
The shallow canal is popular amongst tench and pike fishermen , a number of whom have reported catching fish with the cable ties pushed through their backs.
"The tags on all of the pike which have been caught have been removed," said Pete Austin, from the Pike Anglers Club.
"The general thought is that it is an amateur experiment conducted by inexperienced anglers, who are unaware of the damage they are causing."
Devon County Council owns and manages the canal. A spokesman said: "It is something that has been brought to our attention and we are very concerned about it.
"We're monitoring the situation and if someone is found doing this we will immediately inform the Environment Agency."
The agency said: "Initially, we'd recommend the owner puts up a notice banning the activity."
Mr Austin said there more humane ways of recording catches.
"Photographs of individual pike can be taken and the patterns on the scales can be compared," he said.
The club is urging members and other anglers who fish the water to report anyone they see behaving suspiciously.