The Shannon Regional Fisheries Board has warned that the unauthorised introduction of a British species of coarse fish is now threatening the trout population in the Shannon system.
The board says it has discovered large quantities of a fish known as chub in the River Inny in Co Westmeath where it has already wiped out trout numbers on one of the most popular angling locations in the country.
Fisheries inspectors are blaming English anglers for bringing chub into the country.
They say the species was only to be found up to now in UK waters and is attacking trout near hatcheries and lake systems here.
Irish angling groups say the chub are wiping out stocks of trout introduced to the Shannon system through hatcheries at a cost of more than €10,000.
The fisheries board says the introduction of a non-Irish species of fish will become illegal next month when new Department of Marine legislation is brought into effect to deal with the situation.