A massive pollution leak affecting the Ballinderry River could wipe out unhatched fish and fish food stock, anglers have warned.
With the fishing season due to open this Saturday, angling clubs in the Cookstown area have warned that the slurry pollution could not have happened at a worst time.
Last night the Northern Ireland Environment Agency (NIEA) said it was investigating a report of an extensive spillage of slurry into a tributary of the Ballinderry River earlier yesterday.
In a statement it described the environmental impact as "high".
It added that a "significant volume of agricultural slurry", believed to be more than 400,000 gallons, had reached the Ballinderry River potentially affecting over four miles of the river which flows into Lough Neagh.
NIEA has collected water samples with a view to any necessary enforcement action once the investigation is complete.
Environment Minister Mark H Durkan said: "I am concerned that such a large amount of slurry has polluted the local environment here. The important thing now is for the problem to be addressed quickly."
DUP MLA Ian McCrea called for the pollution to be investigated and cleaned up as soon as possible, adding: "I hope that this was an accident and not intentional and the damage to the fish stocks is minimal."
Stanley Aspinall of Kingsbridge Angling Club, Cookstown said last night that the impact may not be as bad as feared as water levels were currently high and the slurry would be diluted.
However, Noel Morton of neighbouring Killress Angling Club described how the pollution could impact on this season's new fish.
"This sounds pretty bad. It will impact on fish eggs that have been laid along the river and then eventually will impact on the fly food which the fish feed on," he added.