EA scientists have ruled out the dreaded prymnesium as the cause of recent pike deaths on Norfolk's River Thurne.
Tests were carried out at Hickling, Horsey and Martham after a number of fish to 17lbs were found dead.
But EA spokesman Steve Lane said prymnesium cell readings were well below the danger levels when the algae can explode into a massive bloom to produce a lethal toxin when it dies off.
“We have the results of the latest water tests back from the laboratory and the prymnesium cell count in the River Thurne was 1,400 per ml and only between 200 and 300 per ml in Hickling Broad.
"These numbers are well below the threshold accepted as danger levels but we would urge all anglers to keep their eyes peeled for any more distressed, dying or dead fish and ring our emergency line 0800 80 70 60 immediately if they spot any trouble."