Inland Fisheries Ireland, which along with the Environmental Protection Agency has launched an investigation into the matter, say the substance which was discharged into the River Kiltha was toxic as its effects were quick acting.
According to IFI, many young, juvenile, and adult fish died, with brown trout, salmon, brook lamprey, stickleback, and stone loach killed over a 5.5km stretch of the river from Mogeely Bridge to where it meets with the River Dower.
IFI is awaiting test results from water samples taken earlier this week. However, it is believed the results may not be made public, as legal action may yet be taken.
The Womanagh Angling and Development Association has also completed tests on the river to see how its insect population has suffered.
The invertebrates, which often hide in the riverbed, are fed upon by the fish.
On Monday, Dairygold admitted it may be responsible for the pollution incident as contaminant from a nearby grain storage site that was being cleaned, could have entered the river.
The EPA confirmed that it completed an investigation of the Dairygold site on Monday. A spokeswoman said they are “actively pursuing” their investigation.
Consultant freshwater ecologist Thomas Drinan said it could take two or three years for the river to recover stocks, as a three-year life cycle of some species could have been killed. “According to Inland Fisheries, fry [very young], parr [up to two years old], and adult fish were killed. To replace fish of such varying ages in the life cycle could take two to three years,” said Mr Drinan.
He also pointed out that the salmon fry and parr are protected species.
The IFI yesterday said: “This fish kill is particularly disappointing as the Womanagh River, which is currently closed to salmon angling, had been responding well to management and has seen its fish populations increase over the past several years.”
A few months ago, anglers on the river expressed fears that salmon and trout stocks could be wiped out by discharges into the river which they say are emanating from a speciality cheese plant and a nearby waste treatment plant.
The dead fish were first discovered by members of the public walking around Castlemartyr village on Saturday evening.
* Inland Fisheries Ireland operates a confidential 24-hour hotline. Suspected illegal fishing or pollution can be reported to 1890 347424.