The dreaded American crayfish has infiltrated the River Aire, threatening to bring with it crayfish plague, a fungal disease which poisons our native species.
One of the foreign species was found, dead, at Silsden Bridge last week.
The Environment Agency confirmed today it was the first time the species had been found in the River Aire.
"This is not good news," a spokesman said. "There are various problems associated with them mixing with native crayfish. There is a question mark over whether it has been used as an angler's bait or not, it is difficult to know how it has got in there."
He said that the agency would be monitoring the river to check for more evidence of American crayfish and would take action to restrict its movements if necessary.
The River Wharfe between Burnsall and Conistone has already seen an invasion by the American signal crayfish, driving the native species from its habitat there.
Meanwhile more welcome wildlife has been attracted back to the River Aire, thanks to an improvement programme by the Environment Agency and Yorkshire Water.
There have been sightings, which are so far unconfirmed, of otters at Shipley and Keighley.
Kevin Sunderland, chairman of the Aire and Calder Rivers Group, said a worker had seen an otter swimming in the river downstream of Baildon Bridge, Shipley.
He said: "From his description, there is no doubt it was an otter.
"They've been seen in the centre of Leeds, and at various other points on the Aire but this, to my knowledge, is the first report between Shipley and Rodley."
The Environment Agency spokesman added that £60,000 had been spent restocking the river with chubb and dace. The agency is currently working to improve the habitats for fish in the Skipton area.
He said that if these creatures were otters, they would be a welcome sight, but he warned that it was difficult to distinguish between an otter and a mink.
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