Anglers in Cornwall are worried a pollution incident in a tributary of the River Camel could have wiped out countless salmon and trout spawn.
Anglers said the extent of the damage will be difficult to quantify
China clay waste from a disused pit at Stannon on the edge of Bodmin Moor poured into the stream after a dam was breached on Tuesday.
China clay company Imerys said it was co-operating fully with the Environment Agency over the incident.
The firm said temporary measures had been taken to plug the gap.
But the River Camel Fisheries Association said the pollution could not have happened at a worse time.
Chairman Tyson Jackson said: "The poor unborn fish in the gravel will be the hidden dead and that's what worries us.
"It's going to be desperately difficult to quantify how many eggs have been killed."