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Jolly Jellies

Jellyfish, members of the Phylum Cnidaria (silent “c”), are an integral natural part of the cycle within marine ecosystems.  However, with impacts such as overfishing, coastal pollution and climate change, they have been on the increase, with very large blooms having been reported since the start of the Millennium.

The reason for these blooms has been subject to much discussion, with overfishing identified as a major culprit  as it removes fish from the ecosystem that once gone, have their space taken by the opportunistic jellyfish.

The Lion's Mane Jellyfish (Cyanea capillata), one of the species that poses most danger to humans (Photo: Bernard Picton)
The Lion's Mane Jellyfish Cyanea capillata, one of the species that poses most danger to humans
(Photo: Bernard Picton)

Jellyfish are about 95% water, and as such, they are not identified in the analysis of stomach contents of marine predators.  They mainly feed on zooplankton (animal plankton), which is also the main diet of small forage fish such as anchovies, herring and sardines and as such, jellyfish compete with these fish for food sources.

Natural predators of jellyfish, include the Sunfish Mola mola, and the Leatherback Turtle Dermochelys coriacea, which feed on jellyfish as a major part of their diet.  Tuna will also eat jellyfish, as will anemones and crabs at the end of the season when many are dying and sink to the bottom, bringing them within reach.

A Sunfish, a natural predator of jellyfish (Photo: Bernard Picton)
A Sunfish, a natural predator of jellyfish
(Photo: Bernard Picton)

There are not many birds that feed on jellyfish, although Northern Fulmars Fulmarus glacialis are known to eat large jellyfish, and have been seen feeding on the oil-rich gonads of jellyfish.

Fulmars are one of the few Irish bird species known to feed on jellyfish (Photo: Shay Connolly)
Fulmars are one of the few Irish bird species known to feed on jellyfish
(Photo: Shay Connolly)

Marine litter is a major cause for concern as, for example, a floating plastic bag looks exactly like a jellyfish to a hungry turtle!  The bags do not biodegrade but break into tiny microscopic pieces that accumulate high levels of toxic pollutants which are then eaten by plankton, fish and then us!  So please help the sea and all the life in it by using reusable bags!

With jellyfish blooming, it is important that people keep away from the stings that are around at this time or year, particularly from large orange jellyfish called Lion's Mane Cyanea capillata, purple cousins of jellyfish (hydroids) called Portuguese Man O’War Physalia physalis and others that sting to a lesser degree such as the Blue Jellyfish Cyanea lamarckii, and the Compass Jellyfish Chrysaora hysoscella.

Blue Jellyfish (Cyanea lamarckii) (Photo: Melanie Gomes)
Blue Jellyfish Cyanea lamarckii
(Photo: Melanie Gomes)

Keeping the seas around Ireland healthy and productive for marine wildlife and for us is essential.  The management of fish stocks and fishing methods, through an effective implementation of the Common Fisheries Policy, by eliminating pollution and by completing an ecologically connected network of Marine Protected Areas is vital to produce healthier seas around Ireland.  We cannot afford to ignore the signs, unless we want to replace our ‘fish & chips’ with jellyfish burgers!

Compass Jellyfish (Chrysaora hysoscella) (Photo: Melanie Gomes)
Compass Jellyfish Chrysaora hysoscella
(Photo: Melanie Gomes)

Want to write to your TD about fishing? Email me for a template letter: mgomes@birdwatchireland.ie

Melanie Gomes, Marine and Fisheries Policy Officer

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