The white-clawed crayfish is a protected species
An aggressive American crayfish is threatening the existence of a protected native crayfish in a river.
The American signal crayfish has entered part of the Afon Llwyd in Torfaen and is taking over the habitat of the white-clawed crayfish.
Native crayfish have been wiped out in some rivers by the much larger signals, with whom they compete for food.
The American signal also carries crayfish plague, to which it is largely resistant, unlike its British cousin.
Anglers can help reduce the risk of spreading the disease by cleaning nets and boots before moving between rivers.
Torfaen had been a hot-spot for white-clawed crayfish, a protected species, and previously free from the American variety.
Ecologist Kris Roberts said: "This is a tragedy for one of our rare, native species.
"It highlights the importance of protecting native crayfish populations and preventing the spread of diseases between river catchments."
The disease can be spread by the introduction of signal crayfish into previously uncontaminated waters, or on people's wet footwear and equipment.
Crayfish plague spores can survive for up to two weeks in water, but can be killed by drying or disinfecting.
Anglers are advised to dry or disinfect boots or nets before moving between rivers, or avoid fishing different rivers on the same day.
"We may never know how the signals were introduced into the Afon Llwyd but there are measures that people can take to help prevent crayfish plague spreading to other important white-clawed crayfish sites," said Mr Roberts.
A spokesperson for Environment Agency Wales said American signal crayfish were present in a number of rivers in Wales, including a tributary of the River Usk near Abergavenny and a tributary of the Upper Wye in Erwood, Powys.
"The Environment Agency is concerned about any deliberate or inadvertent introduction of this large, non-native crayfish which has the potential to seriously threaten the smaller native species," he said.
Kill on sight
Signal crayfish were introduced to waters in Wales and England through fish farms about 20 years ago.
In Scotland this summer anglers were told to kill signal crayfish on sight to help stop them spreading.
They have been blamed for eating young fish and destroying their natural habitat.
In recent weeks, crayfish plague was blamed for wiping out a colony of white-clawed crayfish in Staffordshire and killing hundreds of crayfish in Colchester.