A war is being waged in the rivers of north-east England.
The American signal crayfish is more aggressive
The battle for supremacy is between two species of crayfish in rivers in Northumberland.
Environmentalists say the northern stronghold of native British crayfish, in the rivers Wansbeck and Aln, could be under threat from the spread of the aggressive signal crayfish, from the United States.
Environment Agency officials have warned the American species has been spotted for the first time in the North east.
Now angling clubs and conservation groups have been warned that North American signal crayfish have been discovered in the River Derwent near Blaydon, Tyneside.
As a result special measures have been put in place to try to halt their spread.
The first report came in October when agency officers were informed by staff in Gateshead that they had found two signal crayfish.
Further surveys revealed another female signal crayfish, which was removed from the river.
Agency conservation officer, Anne Lewis, said: "At the moment, the signal crayfish seem to be present in the Derwent in low numbers, but it is essential that we take every step to prevent their spread.
Native crayfish populations are threatened in the North East
"The American species of crayfish is bigger, more aggressive and out-competes our native crayfish.
"Most importantly it also carries a fungal disease known as crayfish plague that has wiped out our native crayfish from most rivers in the south of England."
Anglers are now being urged to guard against the spread of the signal crayfish by disinfecting equipment if fishing in more than one river.
An agency spokesman added that it is illegal to keep or release to the wild any crayfish without a special licence.
Signal crayfish already dominate in the south and east of the UK.
In the last decade, native crayfish strongholds have shrunk towards the north.