Smoked salmon could be off the menu this Christmas if a plague of killer jellyfish reaches the Scottish coast.

The mauve stingers have already destroyed Northern Ireland's only salmon farm at Glenarm Bay, Co Antrim, killing more than 100,000 fish. They numbered in the billions and formed a 10 sq mile drift 35 ft deep. Now, millions of the carnivorous animals are heading for Scotland on the current.

"It would be very bad news if they got to Scotland," said a Scottish Salmon Producers' Organisation spokesman. "In that sort of quantity, there's nothing much that can be done to protect the fish; the usual tactic of pumping bubbles from beneath pens won't work."

Marine Harvest, which runs 26 farms on Scotland's west coast and in the Western Isles producing 35,000 tonnes of fish, said it was very concerned.

"We've already had one site slightly affected by the jellyfish, and they have been sighted near two other farms in Uist," said technical development manager Steve Bracken. "At any time it's bad news, but it's even worse before Christmas."

The salmon industry is gearing up for the usual festive binge on smoked salmon, which accounts for as much as 70% of annual sales, SSPO said.

Northern Salmon Company, which owned the two destroyed fish farms in Northern Ireland, will not be able to supply any fish for at least a year as it rebuilds stocks, and is claiming lost sales of £1m. The attack has also come as a blow to Young's Bluecrest, which sourced Glenarm organic salmon from the farm for its Spey Valley Smokehouse luxury smoked salmon line. At the launch, Young's claimed the fish were the best available in the UK, because of the farm's position in unpolluted waters on the west coast of Northern Ireland.

"We have enough supplies in stock for that line to last for Christmas," said a Young's spokeswoman. "But after that, we'll have to find an alternative source of fish. However, Young's does have other lines of smoked salmon that will be on the shelves for Christmas."