Freshwater Jellyfish in Lough Derg – Surely Not!
Angler Pat Joyce from Caslteconnell in Limerick nearly fell off his fishing stand when, about three weeks ago (early August), he spotted what looked like a single small jellyfish pulsing in the water in front of him. The jellyfish disappeared and Pat thought that perhaps he was mistaken as, everyone knows, there is no such thing as a freshwater jellyfish. Two weeks later, again fishing for bream in the tranquil surrounds of Scarriff Harbour, just off Lough Derg, Pat noticed not one but hundreds of tiny jellyfish moving on and below the surface. Pat immediately contacted the Environment Protection Agency (EPA) to seek clarity, and they put him on to Inland Fisheries Ireland (IFI). Staff from the Clare region of IFI’s Shannon River Basin District sped to the scene and collected up to 20 live specimens. They really did exist!
IFI’s Colum Walsh and Dave Germaine contacted Dr Joe Caffrey, invasive species specialist with IFI, who immediately organised a site visit to Lough Derg with a team of experts. These included Dr Tom Doyle, a jellyfish expert from the Coastal & Marine Research Centre in UCC, and Dr Dan Minchin from the Lough Derg Science Group. The survey revealed small numbers of the jellyfish at Scarriff Harbour, but specimens were also recorded from two other locations within the lake – at Rossmore Harbour and at Dromineer.
So what is this jellyfish, where did it come from and why was it never spotted in Ireland before? As soon as specimens were collected IFI forwarded them to Dr Tom Doyle (UCC), who identification them as the free-swimming life stage of a species called Craspedacusta sowerbii. This is the first official record for this species in Ireland. This freshwater jellyfish hails from the Yangtze River Valley in China but currently has a worldwide distribution. It was initially discovered in exotic aquatic plant tanks in Regent’s Park, London in 1880 but has since spread to widely throughout the globe. The jellyfish is about the size of a euro coin and broadly resemble their marine cousins. It is more or less transparent with a distinctive white/greenish cross and a white/cream circular outline. It possesses in the region of 250 – 300 small tentacles. These jellyfish have two distinct life stages; one is a tiny attached polyp and the second is what we know as the jellyfish or medusa stage. The polyp buds off medusa under warm water conditions, generally when water temperature reaches 25 degrees Centigrade. This species is known to occur in single sexed populations and Dr Doyle confirmed that all of the specimens he examined from Lough Derg were female.
It is probable that the discovery of this jellyfish relates to the wonderfully warm summer that we experienced in Ireland this year, when water temperatures in many watercourses exceeded 25 degrees C for prolonged periods. This probably stimulated the budding off of the medusae or jellyfish, which pulsed in the warm water in search of plankton prey. It is noteworthy that jellyfish were also reported from Lough Erne in recent days. Experience in other countries suggests that blooms of such freshwater jellyfish occur only sporadically and that they last, in any one year, for only a few weeks. So it is possible that we may not see such a sight again for many years.
It is important to state that the freshwater jellyfish is not harmful to humans and that, while they do capture their tiny prey by stinging, the stinging cells are not sufficiently powerful to harm humans. The jellyfish do not appear to have any significant effect on the biology or ecology of the waters they are recorded in, probably due to their sporadic occurrence and the short period that the jellyfish blooms are in any water body.
' Our inland fisheries resource must be protected' stated Minister Fergus O' Dowd, 'anglers are the eyes and ears on our rivers and lakes. I ask all anglers to continue to assist in the protection and conservation of this resource, reporting any invasive species they come accross to the IFI Hotline immediately. '
One serious cause for concern relates to the pathway whereby this jellyfish (and many other non-native and potentially harmful or invasive species) was introduced to Ireland. The fact that the two watercourses from which the jellyfish was recently recorded (loughs Derg and Erne) are both internationally renowned navigation waterways suggests that boating and perhaps ballast water from newly introduced craft may represent an important causative agent. Boats and cruisers are commonly imported from abroad and are introduced into our waters without having to prove that they were cleaned and disinfected before leaving their country of origin. According to Dr Joe Caffrey (IFI) “This practice is unacceptable and poses a significant threat to biodiversity in our waters and to their functionality, be it as recreational, amenity or municipal waters. It is imperative that boats being imported into this country carry certificates of disinfection prior to being granted entry if we are to stop the ever-increasing spread of harmful invasive species.”
Media enquiries: Suzanne Campion ,Head of Business Development, Inland Fisheries Ireland,
Inland Fisheries Ireland is a statutory body operating under the aegis of the Department of Communications, Energy and Natural Resources and was established under the Fisheries Act on 1st July 2010. Its principal function is the protection and conservation of the inland fisheries resource. IFI promotes, supports, facilitates and advises the Minister on, the conservation, protection, management, development and improvement of inland fisheries, including sea angling. It also develops and advises the Minister on policy and national strategies relating to inland fisheries and sea angling. www.fisheriesireland.ie
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