Tens of thousands of mauve stinger jellyfish have made their way into the State’s only saltwater lake, Lough Hyne in West Cork.
University College Cork (UCC) scientists confirmed the influx late last week and advised swimmers to stay out of the water.
Lough Hyne is one of the most important marine habitats in Europe and was made Ireland's first marine nature conservation reserve.
Mauve stingers, Pelagia noctiluca, are small jellyfish which are bluey purple or mauve in colour with a globe shaped “umbrella” or “bell” and long tentacles, and can emit a painful sting.
More common in the Mediterranean, they have been seen with increasing frequency in Irish and British waters due to marine heatwaves – most recently from June 2023 in the Atlantic.
However, UCC jellyfish expert Dr Tom Doyle, who intends to undertake research into the jellyfish in Lough Hyne, told RTÉ Radio 1 Countrywide that it may be due to a change in a coastal current 500 metres deep in the Atlantic which has acted as a barrier to oceanic jellyfish.
In the years when Pelagic noctiluca were more frequent, this coastal current was weaker, he said.
That shelf current has been weaker this year and it has allowed some of this oceanic water to reach our shallow seas, he said.
“It could take decades of research to really show that,”Dr Doyle said.
He estimated there were tens of thousands of jellyfish in Lough Hyne, and said some may be offspring which spawned four or five weeks ago.
“This jellyfish is a real survivor …it can lose up to 70-80 per cent of body weight if there is not enough food,”he said.
These are the “Mauve Stinger” jellyfish that have inundated Lough Hyne in West Cork, that we were just discussing on #RTÉCountrywide. I’ll post a link to the report later. pic.twitter.com/cFKONBuiV3
— Philip Boucher-Hayes (@boucherhayes) November 9, 2024
“They don’t have much say over where they go. They are in the lough now and the only way they are going to come out is through the narrow rapids,”he said.
The jellyfish are predators and will consume fish larvae, and may also affect “sea sparkle” or Noctiluca scintillans, a marine species of dinoflagellate that can cause bioluminescence and which was very abundant until recently in Lough Hyne, he said.
“It is really hard to know the impact the jellyfish will have as this is such a small space,”Dr Doyle said.
At just a kilometre long and three to four kilometres wide, Lough Hyne is connected to the Atlantic Ocean via Barloge Creek, by a narrow tidal channel, known as the Rapids.
Tidal flows from the Atlantic fill Lough Hyne twice a day, running over the Rapids at up to 16km per hour, and create in the lake a unique habitat of warm oxygenated seawater, which sustains a huge variety of marine plants and animals including 72 species of fish.
