Any animal that eats the fish ends up suffering a 'lingering miserable death'.
John Anderson, of the Burton Trent Partnership, was made aware of the illegal anglers and after trying to get in touch with nearby fishing clubs, he went out to see what was happening for himself.
When he got there, he found two men who he described as 'drunk and inebriated', who had a bag of dead fish next to them on the riverbank.
He told them it was illegal to fish with the line they had, but he says they became abusive and only left when he threatened to call the police.
Mr Anderson also informed the Environment Agency hotline about what he had witnessed.
He said: "They were very verbally abusive, but when I threatened to call the police, they went and took everything with them apart from the line as it got caught in the undergrowth.
"The swallow rig is used for poaching. A fish (pike) swallows this and the bent nails wedge on the stomach. Of course since it’s baited with a dead fish, any animal that eats fish could be caught and left to have a lingering miserable death.
"It is disgraceful and cruel and it makes me really angry. This is why we are trying to do something about it.
"We've had a lot of people getting in touch asking how they can help as I think the cruelty aspects shocks people when they see it.
"I have to tell them that they are often not very pleasant people and the best thing to do is call the Environment Agency hotline."
The Burton Trent Partnership was set up to stop illegal and dangerous activity along the River Trent in and around Burton.
It has carried out four sting operations so far, one of which led to a recent prosecution.
An angler caught coarse fishing in the closed season was fined £435 for illegal fishing on the River Trent, having been caught by the partnership back in March.