Staff on their way to give the fish their morning feed noticed a "reddish-brown tinge" to the sea and then realized the boats were struggling to make headway through an expanse of jellyfish up to 35 feet deep, Russell said.
"A few hours later all our salmon were dead, the bulk of them suffocated."
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.