A total of 14,749 fish have been killed in a massive incident on the Ollatrim River outside Nenagh, Inland Fisheries Ireland have confirmed.
The river is a tributary of the Nenagh River.
Fisheries Officers attended the site at Ballinahemery Bridge near Ballymackey, Nenagh, on Monday, July 9, after receiving a report.
Dead fish observed over a 5km stretch of the river.
The species affected included brown trout (1,400), lamprey (10,500), stoneloach (805), minnow (1,820), salmon (70), crayfish (70) and stickleback (84).
Inland Fisheries Ireland immediately commenced an investigation following the discovery of the fish mortality.
Indications are that the fish kill occurred on July 8, and locals have reported observing one or two dead fish on the Saturday evening.
This is the largest fish kill of lamprey, a protected species, in recent years and it is anticipated that recovery will take several years.
The investigation to identify the source of the fish kill is continuing this week. The cause appears to have been a chemical agent, possibly a herbicide or pesticide, which has now passed through the system.
Inland Fisheries Ireland is reminding the public and the farming community that if they are using spraying equipment to be aware that these herbicide and pesticide chemicals, even when diluted with water, are liable to be extremely toxic to all aquatic species and fish in particular. Any mixing must be done far from natural watercourses, especially in the current conditions when diluting waters are in short supply therefore increasing the toxicity of the chemical.
If mixing chemicals, washing or using spraying equipment for any purpose, particular care must be taken to ensure that the rinsing of equipment does not take place near any water body or watercourse including small drains. Any washing must be carried out in a manner that will not pollute the waters.
Inland Fisheries Ireland has a confidential hotline number to enable members of the public to report incidents by telephone 1890 34 74 24 or 1890 FISH 24. The phone line is designed to encourage the reporting of incidents of illegal fishing, water pollution and invasive species.