An investigation was under way yesterday by the Shannon Regional Fisheries Board into the first major fish kill to occur in the midwest region this year.
According to a spokeswoman for the Shannon Regional Fisheries Board, the fish population of a one kilometre stretch of the River Graney in north-east Clare was wiped out as a result of an algal bloom from nearby Lough Graney.
It is first fish kill to occur on the River Graney and after visiting the scene, the Fisheries Board spokeswoman said: "One of the dead fish we found was a 7lb trout, which is very alarming. Normally a fish of that size can tolerate more hardship than smaller fish and it suggests that any fish under that weight perished in the fish kill."
The spokeswoman said she was not able to give an estimate of the numbers of fish killed; however, she said along with fish being wiped out in the one kilometre stretch of the river, dead fish had been found along other stretches, but not in the same numbers.
However, a local angler, Mr Michael Waterstone, who alerted the Fisheries Board to the fish kill, said yesterday there were dead fish over a five-mile stretch of the river and that the number of dead fish "runs into the thousands".
As a result of the presence of the algal bloom, Clare County Council yesterday imposed a swimming ban in Lough Graney and also advised farmers to keep cattle away from the shoreline to prevent stock from drinking water.
Lough Graney is the public water source for the Flagmount area; a council spokesman said the water is treated and is safe to drink, adding that the council is monitoring the situation.
The occurrence of the algal bloom occurred due to excess phosphates in Lough Graney which, along with sunshine and a very gentle breeze over the weekend, increased the algae.
The Fisheries Board spokeswoman said that the algal bloom moved from Lough Graney to absorb the oxygen in the water from the early stretches of the River Graney.
The absence of oxygen there killed the fish. The spokeswoman said the water in the river would have resembled "pea soup".
Pointing out that Lough Graney lies in a hollow, the Fisheries Board spokeswoman said that the excessive phosphates in the lake more than likely came about through a number of factors.
She said that these included farming methods, forestry in the area and general private use.
She added that after the Fisheries Board completes its report into the fish kill, it will call a public meeting in the area.