According to the Marine Conservation Society (MCS) the number of jellyfish around the UK coastline is increasing. It is believed that the increase is due to a combination of low water quality, rising sea temperature and unstable marine ecosystems. The depleted fish stocks result in an abundance of zooplankton which is helping to support larger numbers of jellyfish. The jellyfish, in turn, are contributing to the fish population problem by feeding on the fish eggs and larvae in addition to the zooplankton.
The Barrel, Moon, Compass and Lion's Mane jellyfish seem to be most rapidly increasing species but other species of jellyfish seen around the UK include the Blue, Mauve, By the Wind Sailor and the Portuguese Man of War. Early sightings of jellyfish are uncommon as their numbers usually only increase in the summer months but the first reported sightings of the barrel jellyfish off the coast of North Wales were back in January with huge numbers spotted in the Irish sea and elsewhere ever since. The MCS needs everyone's help to report any sightings of jellyfish using their jellyfish identifier and survey submission site. It is hoped that the information gathered will help to shed further light on the population growth and also ties in with sightings and the population of the critically endangered Leatherback Turtle which are a seasonal visitor to UK waters and are known to feed on a number of UK jellyfish. The data collected so far has identified possible links between jellyfish numbers and the North Atlantic Oscillation, a phenomenon which affects warm air flow between the tropics and northern regions. The jellyfish have already started to cause problems with modern technology and facilities. Torness nuclear power plant in East Lothian was recently shut down for two days when swarms of moon jellyfish blocked water cooling filters. The shut down was a precautionary measure and there was never any danger to the public.