Some of the UK's main coarse and game fisheries are at risk from an invading species which spreads an infectious parasite deadly to salmon and trout.
The gudgeon spreads a parasite deadly to trout and salmon
The Asian Topmouth Gudgeon is now directly threatening native fish in the Trent, Severn and Yorkshire Ouse, says the Centre of Ecology and Hydrology.
A silver fish, which grows up to 10cm (4in) in length, the gudgeon is one of the most invasive fish in Europe.
It also eats of eggs of native species and competes for other food sources.
According to scientists it is thriving in lakes and ponds and is a major threat to rivers.
Earlier in the year, the Environment Agency removed topmouth gudgeon from a lake - the Ratherheath Tarn near Kendal, in Cumbria - by trapping the native species, poisoning the lake and then slowly reintroducing the indigenous fish.
An agency spokeswoman said they may have to take similar action in other areas to stop the fish spreading.
In Hampshire, the fish has already invaded the River Test, which is regarded as one of the most famous chalk streams in the world.
Originally from eastern Asia, the Asian Topmouth Gudgeon has been accidentally introduced to waters across Europe causing widespread damage.