Swarms of stinger jellyfish have wiped out large stocks of salmon after invading four fish farms along the west coast of Ireland in recent weeks.

It is estimated 80 per cent of the salmon stock at Killary Harbour in Co Mayo were killed after sustaining fatal jellyfish stings. Tens of thousands of fish were also killed at Kilkieran Bay and Outer Bertraghboy Bay in Connemara, Co Galway, and in Bantry Bay, Co Cork.

The Marine Institute has confirmed two varieties of jellyfish were responsible: the “mauve stinger” (Pelagia noctiluca) and Muggiaea atlantica, both of which are naturally occurring. A large phytoplankton “bloom” is believed to have contributed to fish deaths at one location.

The jellyfish can drift in from open ocean waters, and do not pose a public health risk, although the mauve stinger can cause skin irritation in humans, according to the Marine Institute in Galway, which is leading the fish-kill investigation.

The level of “mortalities” is believed to be the worst of farmed fish on the island of Ireland since 2008 when some 250,000 salmon were killed in Northern Ireland. In November 2007, a 10-square-mile swarm of mauve stingers wiped out a salmon farm with 100,000 fish also in the North, causing £1 million worth of damage. Up to 20,000 farmed salmon were lost due to a jellyfish “bloom” off Clare island, Co Mayo, in 2013.

The exact number of fish killed will not be known for several weeks, until all dead fish are removed from cages, the Institute added. The dead fish “are being disposed of at authorised rendering plants in compliance with the animal by-products regulations”. Its staff had visited the four sites to ensure proper disposal to avoid any further environmental damage.

Galway Bay Against Salmon Cages chairman Billy Smyth said they feared some hundreds of thousands of fish may have died, and inland fisheries were threatened due to a large number of escaped salmon that had occurred as a consequence. “We have reports from fishery managers on the Delphi Fishery, Erriff and Kylemore rivers of large amounts of Atlantic farmed salmon entering their rivers over the past three weeks,” he added.

“We are worried that, as the salmon angling season has now finished, a large amount of these ‘escapees’ will not now be caught and will inter-breed in coming winter months with wild salmon populations in these rivers and severely damage the genetic integrity of the wild fish.”

Some anglers believed farmed salmon now entering the rivers were remnant survivors of the jellyfish attack on salmon farms, he said, and were released into the wild as there were not enough of them to make them commercially viable to harvest. “Of course, this theory cannot be proven as farmed salmon are not micro-tagged, so it cannot be proven as to where they came from.”

An Irish Salmon Growers Association (ISGA) spokesperson said, “The jellyfish swarm” along the west coast was more severe in some areas than others. “The salmon farmers, as per the terms of their licences, informed the appropriate authorities in the fish health unit of the Marine Institute of the mortalities.”

Procedures required under law, which are necessary for environmental protection, were immediately implemented. This was documented and information made available to the authorities, the association stated. “At no stage were any fish released. Salmon farmers wish to protect each and every fish alive so that they may on-grow them for the market,” the ISGA spokesperson added.

The mauve stinger is usually found offshore in warmer waters, but swarms affected parts of the Irish coastline during 2016 and 2017. Its colour varies from pink to mauve, and it glows in the dark. Muggiaea atlantica has a bell-shape and is about two centimetres long with a trailing stem. Its onshore presence in greater numbers is believed to be an indicator of the incursion of warmer waters on the continental shelf off the west coast of Ireland.