More than 10,000 fish have been killed by blue green algae in a fishery and reservoir in South Yorkshire.
Algae occurs naturally and can happen in any still water
Fishing has been suspended at Scout Dike Reservoir and Nether Mill pond, Penistone. The Environment Agency has also found dead trout in the River Don.
Samples of the algae have been taken to the agency's laboratory for tests.
Spokesman Neil Trudgill said: "Unfortunately nature has conspired to create these conditions and it looks like it has killed most of the fish."
Scout Dike is a compensation reservoir that constantly releases a flow of water into the River Don and it is thought this could have spread the toxins that are killing the fish.
However, fisheries officers do not expect the rest of the river to be affected.
Mr Trudgill said: "This is a tragic incident that has all but wiped out a successful commercial fishery.
"We will continue to work with Yorkshire Water and the fishery owner to monitor the situation in case anything else can be done to mitigate the effects."