Fishing interests on the River Blackwater in Co Cork claim last week's fish kill could be in excess of 46,000 fish and have called it an "ecological disaster".
Inland Fisheries Ireland (IFI), who are investigating the fish kill, yesterday confirmed that between 8,000 and 10,000 fish mainly wild brown trout have died in the River Blackwater, and its tributaries the Rivers Clyde and Awbeg near Mallow.
However, fishery operators and angling clubs estimate the death toll along a 30 kilometre section of the River Blackwater between Lombardstown to Killavullen is much higher and has affected every species present in that section of the river including stickleback, trout and salmon parr, adult trout, dace, roche, salmon and fresh water eels, which are an endangered species.
The Marine Institute took samples from the river on 14 August and the IFI has said it will be 20 days before they get the results.
Conor Arnold, who is chairman of Killavullen Angling Club and runs a fisheries business on the river which has been forced to temporarily close, said they are looking for written directions from IFI as to how the river should be managed until such time as the "root cause of the fish kill is found".
He and others are also calling for the test results to be published when they are available.
"Transparency in the investigation is paramount," Mr Arnold said.
John Ruby, who is chairman of the Mallow Trout Angling Club and whose members first spotted the dead fish in the water, said they are anxious that the source of the infection is found as soon as possible.
"But we just have to wait for the results to come through," he said.
In a statement yesterday evening, IFI said there is no evidence of "a polluting or harmful discharge" being detected, and said "disease outbreaks can occur in wild fish populations and are influenced by number of factors including environment change".
Separately, the Environmental Protection Agency said it inspected EPA-licensed industrial sites and urban water discharges in the area and while investigations are ongoing, "preliminary sample results indicate no causal link between the fish kill and discharges from EPA-licensed sites".
The statement went on to say that river water samples taken by the EPA and Cork County Council indicate "normal water quality in the area".
'Great concern' following fish kill in Cork, says Dooley
Minister of State for Fisheries and the Marine Timmy Dooley said the fish kill in Cork's River Blackwater has led to "great concern" and "upset" among fisherman.
Speaking on RTÉ's Today with Claire Byrne, Mr Dooley said: "We met with the fishermen… they are extremely concerned and upset. These are custodians of the River Blackwater for generations.
"They are really concerned at the scale of what has happened here."
Mr Dooley said there is a level of frustration as there has not been an immediate set of answers.
He said he has been in touch with the IFI, whose senior environmental officer has been carrying out an investigation along with the EPA and Cork County Council.
"All the preliminary data capture would indicate that there isn't evidence of pollution or harmful discharges," he said.
He added that the anglers told him that there have been events before where there were discharges in the water that led to "everything being wiped out in a short area".
He stressed that the information flow to the anglers in the area from the agencies is something that he has been "trying to improve upon".
Mr Dooley said sampling must go through a particular process, leading to preliminary results from samples taken from tissues, organs and smear samples from fish.
"It will take an additional two weeks to go through the scientific process that will lead to more definitive results, but by 28 August we will have some clear indication," he said.
"The IFI have identified their potential concerns around this fungal infection, that where fish are generally in a stressed state, it may come from the low water levels or increased temperatures…"
The minister said that when temperatures go to 22C or 23C, then that "effectively leads to the death of trout".
"It could be the rising temperature; it could also be potentially reduced oxygen levels in the water as a result of the enrichment of nutrients in the water over a prolonged period of time.
"All of these are potential reasons," Mr Dooley added.
