Environment experts say they have been left puzzled after dozens of fish were found dead in a stream.
Trout were the main victims of the incident
A farmer contacted the Environment Agency after seeing the fish in the South Low, near Haggerston, Northumberland, on Sunday.
An officer found between 40 and 100 dead fish. They were mainly trout, but some perch were affected too.
An Agency specialist said whatever had killed them had also damaged their fins - something he had not seen before.
Fisheries technical specialist Phil Rippon said: "There are a number of things that could cause this phenomenon, however I am surprised by the amount of damage to the fins as a result of this incident.
"Hopefully something as unique as this will help us narrow down the cause."
The South Low is important as a wild trout stream and the Agency says the fish it sustains are likely to be "genetically distinct" because the stream is not joined by any other watercourse and flows straight into the sea near Holy Island.