Restrictions on eel netting are being imposed in Lincolnshire to protect depleting stocks.
The Environment Agency is warning poachers that staff will be patrolling waters to make sure the ban is being observed.
The decline in eels is being caused by climatic change, commercial over-fishing and an eel parasite.
Roger Ferguson, from the Environment Agency, says the changes are necessary to protect the species.
"Normally eel-fishing areas would be leased out to eel fishermen, but we have decided not to lease them out as a conservation measure," he said.
Regular enforcement patrols will ensure that no poaching takes place in waters controlled by the agency.
Anyone ignoring the ban will have their equipment seized and may end up in court, a statement from the agency said.
The ban will be in place along 200 miles of agency-owned riverbank throughout Lincolnshire and parts of Cambridgeshire.