Up to 5,000 farmed salmon may have escaped from a site in Glenarm, County Antrim, according to the Department of Agriculture, Environment and Rural Affairs (Daera).
It said a “rip in the net of a farmed salmon cage” had been found on Sunday evening.
A local angling group said it feared the escaped salmon could pose a threat to the long-term health of the wild salmon population.
A Daera spokesperson said its staff were “currently assessing the situation and any actions that need to be taken”.
Gary Houston, chairman of the Ulster Angling Federation, told BBC News NI that there was a danger that the escaped salmon will reproduce with the wild population.
He said that could reduce the fitness of the offspring, resulting in a lower survival rate.
Mr Houston said that salmon populations in Northern Ireland were “under a severe threat of extinction from farm escapes, pollution and poor in-river and sea survival”.
“Farmed salmon are treated with a variety of antibiotics and anti-sea lice chemicals”, said Mr Houston.
“While these antibiotics and chemicals are usually withdrawn before the fish are slaughtered, the escaped salmon will not have gone through this withdrawal period, and thus may be unfit to eat,” he said.
A spokesperson for Daera added that the salmon farming business had repaired the cage and divers were now servicing and inspecting the remainder of cages in both Glenarm and Red Bay.
