The Department of the Marine says that 233,000 salmon have died in a massive fish kill in Inver Bay in Co Donegal.
Three fish farms are affected, and the owner of the largest one, Ocean Farm, says that a freak sequence of events, including dredging for prawns, appears to have caused the mortalities.
But the Department of Marine says there is no evidence that dredging for prawns caused the problem.
Its initial scientific tests suggest the deaths were possibly caused by a water born irritant, which may be biological or chemical, but which would appear to have disappeared from the water.
Its investigations are continuing. All the dead fish are being taken to a rendering plant in Germany for disposal.
