It was a stunning operation and came as something of a shock to the top mouth gudgeon. The alien fish, which Environment Agency experts reckon could dominate Devon's waterways given half a chance, had to be stopped in its tracks.

And electrocution was the means.

To date, the Asiatic invader has spread from an ornamental pond in Bicton College and into a stream linking it with a lake in Bicton Gardens.

The fear was that from there it could take its promiscuous ways and a parasite harmful to other fish into other lakes and rivers.

The Bicton Stream, goes into the Colaton Raleigh Stream and ultimately the River Otter where the parasite could affect trout and salmon.

The top mouth gudgeon's early sexual maturity and habit of breeding up to four times a year mean it can quickly squeeze out native species by sheer weight of numbers.

Aware of the threat posed by the fish, which is three inches long at most, the Environment Agency had already installed a gravel filter at the Bicton College pond so water could pass through, but not the top mouth gudgeon.

Freak rains during a thunderstorm meant water, and thousands of the fish, swept over the filter and into the Bicton Stream.

More than 6,000 fish have so far been collected but the agency think there could be up to 100,000 of the aliens still left.

So a drastic culling operation has got under way. It involves Environment Agency staff putting an electric paddle into the water which stuns fish so they float to the surface.

Then it is a question of netting the top mouth gudgeon and leaving other species to get their senses back and return to the depths.

A spokeswoman for Bicton College said the agency had been working with their head gardener to deal with the problem.

Environment Agency spokesman Dave Brogden said: "At the moment, Bicton is thought to be the only site in Devon with a population of top mouth gudgeon. We are keen to eradicate them.''

The fish mature after a year, compared with roach and rudd which take two to three years to mature sexually.

Yesterday, more than 600 yards of the stream at Bicton was given the electronic paddle treatment.

The top mouth gudgeon will be sent to the agency's national fisheries experts for analysis and then disposal.