The Channelsea, near West Ham, is a tidal river that flows into the River Lea, which in turn feeds into the Thames.
Steve Coates, fish scientist and principal ecological consultant at environmental consultants Ricardo, said: “The results of this survey brought a smile to everyone involved.
“Finding a 10lb sea bass was quite amazing, I’ve never found a bass that large and that far up the Tideway or into the Tidal Creeks in my 31 years of working on the Thames.”
A total of 714 fish from 12 species were captured and returned. More than 400 common roach were found alongside bream, dace, perch, carp, flounder, grey mullet and “feral goldfish”.
Thames Water opened the £700m Lee tunnel, which prevents sewage from Abbey Mills pumping station spilling into the river during downpours, in 2016.
Further improvements will come along the Thames when the Thames Tideway tunnel – which will be connected to the Lee tunnel - is completed in 2025.
The Lee tunnel connects Abbey Mills pumping station in Stratford to Beckton sewage works in Newham. At its peak the tunnel can accept 54 cubic metres per second, preventing discharges into the Channelsea.
John Sullivan, head of Tideway integration at Thames Water said: “The findings of the Channelsea River fish survey highlight the benefits of building the Lee Tunnel and upgrades to our Beckton sewage treatment works to the waterways in this region.
“We continue to have an unprecedented amount of investment directed towards safeguarding rivers and streams across London and the Thames Valley and the completion of the Thames Tideway Tunnel will also deliver a huge reduction in the discharges to the tidal River Thames in London and further improve the overall health of the river.”