Pollution levels in rivers in Sussex have halved in the past 10 years, tests on eels have revealed.
It is hoped improved water quality will increase eel populations
The Environment Agency said muscle samples had shown reduced levels of metals such as mercury, cadmium and copper as well as pesticides.
Scientists now hope eel populations, which have dwindled by up to 90% in the past five years, will start to recover.
"It bodes well for eels, and for bigger fish, herons and otters that rely on them," said ecologist John Barnes.
"Eels are quite an important part of the food chain.
"We expect that over time, as the eel population improves, we will see more predators coming back as well."
Mr Barnes said eels were good indicators of pollution levels in water because they were long-lived and accumulated large levels of contaminants in their bodies.
Poor water quality over the past 30 years had put populations at risk, along with factors including loss of habitat and over fishing.
"A lot of compounds in our study were banned in the mid-80s and 90s and there has been improved sewerage. so we are seeing better water quality in general," said Mr Barnes.