The Department of the Marine is at sea over the cause of death of thousands of fish at Inver Bay in Co Donegal.
The fish kill, described by a Department spokesman as being "very significant", was first noticed last week.
Investigators have been working since then to establish the probable cause of death but so far no definite cause has been identified.
Initial reports suggest the kill was caused by a waterborne irritant, either biological or chemical, that has since disappeared from the water.
Divers from the Department are removing the fish by pumping them into bins for disposal of the remains.
An examination of the seabed in the area has found it to be in good condition.
The Northern Regional Fisheries Board has received no report of a fish kill in any of the rivers entering the bay.
