Scientists are investigating how tens of thousands of native crayfish died in Northumberland.
There have been similar pollution incidents in north rivers
Government environment officials discovered the rare crustaceans had been wiped out following a mystery pollution incident.
Environment Agency officers were called in after the fish were found dead at Hartburn near Morpeth.
The River Wansbeck site was home to one of the largest populations of native crayfish in the UK.
The site is also one of the few areas where the larger American signal crayfish have not threatened the native population.
Officers were called on Friday night after a resident reported seeing dead crayfish.
They have surveyed a three-mile stretch of affected river to assess the damage.
A spokeswoman for the Environment Agency said: "This is one of the largest populations of native crayfish in the UK and for them to be wiped out in this way is absolutely tragic.
"This is one of the few remaining places where American signal crayfish haven't threatened the native population, so this is a massive blow."
Experts estimate it could be up to three years before crayfish repopulate the area.
A number of other crustaceans in the river, including freshwater shrimps, were killed.