Fishermen in Somerset are being warned to stay within the law as they try to land lucrative catches of baby eels.
The Environment Agency said illegal methods were being used to net elvers, lowering stocks in the River Parrett.
It is legal to fish by hand for elvers - a delicacy in the Far East which can fetch up to �600 per kilo.
Day and night patrols have been introduced along the riverbank to stop fishermen using oversize nets with ropes and floats to boost their catch.
Elver stocks are down across Europe and there is concern that they are being over-fished.