A member of the public called the agency after spotting suspicious nets in Jurys Gap sewer, near Camber, East Sussex.
Jon Whitmore, fisheries team leader, said officers found two unlicensed fyke nets, traditionally used to catch eels, at the site on Wednesday.
The illegal equipment was seized using powers under the Salmon and Freshwater Fisheries Act.
The agency had also intervened in four other cases of illegal fishing in Kent in the past two months after receiving alerts from the public, he added.
He said: “The information we receive from the public is invaluable.
“The intelligence they provide helps us protect endangered fish populations, like the eel, from further decline.”
According to the agency, eels are still “a popular fish for the table”.