Experts are carrying out a survey of fish stocks along the River Lea in east London.
This week Environment Agency officers have been checking eel populations on a 7km (4 mile) stretch from Stonebridge south to Lea Bridge Weir in Hackney.
They have been using an electro-fishing boat to count the eels.
Electro-fishing involves passing an electric current through the river, which attracts the eels and allows them to be caught, monitored and returned.
Eels migrate from further upstream, into the River Thames and across to the Bermuda Triangle in the Sargasso Sea.
The information will provide a picture of the river's fish population and help experts understand how factors such as water quality, flows and habitat, influence the "health" of the river.
Historically the Lower Lea catchment was written off as an area that would be too difficult to clean up due to its shallow water channel and obstructions.
But in 2009 it was identified as an area that could and should be cleaned up.
Last year a £2m dredging programme was completed with silt removed from the 7km (4 mile) stretch from Tottenham Lock to Old Ford Lock.
The Environment Agency's Pete Rudd said: "The environment of this isolated section of the Lower Lea is of poor quality and so it may come as a surprise to some that fish can survive there at all.
"Fish surveys allow us to monitor and map how well the ecology of river is recovering after decades of neglect."
They are also working with local businesses to reduce pollution and improve water quality.