An investigation is under way after almost 15,000 fish were killed in a river near Nenagh in Co Tipperary last week.
Officers from Inland Fisheries Ireland were alerted to the fish kill on the Ollatrim River, a tributary of the Nenagh River, and attended the scene near Ballymackey, Nenagh on Monday 9 July.
14,749 fish were believed to have been killed, including an estimated 10,500 lamprey, which is a protected species.
They say it is the largest kill of lamprey in recent years and believe it may take several years for numbers to recover.
An investigation to identify the cause of the kill is continuing, with the likely cause a herbicide or pesticide that has now passed through the system.
In a statement the fishery body reminded people to exercise caution when using herbicides and pesticides, particularly near watercourses.
Other species affected in the kill included brown trout (1,400), stoneloach (805), minnow (1,820), salmon (70), crayfish (70) and stickleback (84).
