Experts are trying to allay fears that salmon and fish in the South are under threat from a foreign fish species.
The gudgeon can spread a parasite deadly to native species
A new report claims that the Asian Topmouth Gudgeon is spreading an infectious parasite, directly threatening native fish in the region.
The Environment Agency said the fish had been in the Test river since 1983 but had not had any measurable effect.
The agency said it was monitoring rivers in Hampshire and Dorset but had not found any evidence of the parasite.
The silver topmouth gudgeon, which grows up to 10cm (4in) in length, is one of the most invasive fish in Europe.
A study by the Centre for Ecology & Hydrology in Dorset claims that it poses a major threat to native fish.
Lead author Adrian Pinder said: "This new work has identified 25 populations of topmouth gudgeon in England and Wales.
"The threatened rivers contain many highly valuable coarse and game fisheries and are also linked to the extensive canal network which will provide further means for topmouth gudgeon to disperse across the country."
Andy Thomas, Environment Agency fisheries officer for Hampshire, told the BBC News website: "Topmouth gudgeon have been in a tributary of the Test since 1983 but have had no measurable effect on the environment of the river."
Mr Thomas said that it appeared the species were more suited to still water and did not thrive in rivers.
He said it was also likely that the gudgeon would be "out-competed" by the larger salmon and trout.
Andy Martin, fisheries officer for Dorset, said that the introduction of non-native fish was a concern, but no evidence of the species or parasite had yet turned up in the county.