DUBLIN (Reuters) - An invasion of jellyfish has wiped out Northern Ireland's only salmon farm, with more than 1 million pounds' worth of stock massacred in the attack.

The jellyfish, covering an area of around 10 square miles (16 square kilometres), flooded into the Northern Salmon Company's cages a mile off the province's northeastern coast, killing 100,000 fish, the firm's chief told Reuters on Thursday.

"It was sheer devastation -- I've been 30 years in the salmon industry and I've never seen anything like it," Managing Director John Russell said.

Staff on their way to give the fish their morning feed noticed a "reddish-brown tinge" to the sea, he said, and then realised the boats were struggling to make headway through an expanse of jellyfish up to 35 feet deep.

"A few hours later all our salmon were dead, the bulk of them suffocated," Russell said.

The attack, by a type of jellyfish known as a "mauve stinger", happened late last week off the coast of County Antrim, an area popular with tourists.

The mauve stinger, noted for its purplish night-time glow, is more commonly found in warmer Mediterranean waters.

Russell said the occurrence, when jellyfish "bloom" in such quantities, only happened every decade or so and last week's appearance off the Irish coast was also due to unusual environmental factors including higher-than-normal water temperatures.

He said the 12 staff at the 20-year-old company feared for the future but that negotiations were under way with the Northern Irish government about a possible aid package.

"It's very serious for the company but hopefully we can continue," he said.