Why the Forty Foot jellyfish could be a bad sign for the sea

ANOTHER LIFE:ANY DAY NOW, if it hasn't happened already, the stoical bliss normal to Dublin's Forty Foot bathing cove is likely to be rent with alarm as the first lion's mane jellyfish of the season is spotted underwater, its glowing mass of tentacles billowing as it travels.

Cyanea capillata, among the largest jellyfish in Irish waters, packs a powerful sting, and since a notable invasion in 2005 its presence in Dublin Bay has sometimes prompted panic more appropriate for a great white shark. Last year, the jellyfish was even more abundant in the Irish Sea than in 2009, and sightings from ferries found them from coast to coast.

Population explosions among the world’s jellyfish menace not merely human swimmers but the aquaculture industry and ocean ecosystems as a whole. Marine scientists from Cork and Swansea universities are collaborating in the urgent EcoJel study project, funded by the EU. They also joined the team, drawn from both islands and headed by Dr Christopher Lynam, that tackled the obvious question in a paper published in Global Change Biology: “Have jellyfish in the Irish Sea benefited from climate change and overfishing?” Their answer backs the global consensus: jellyfish numbers may respond to warming seas, but overfishing can give them an exponential boost.

In the Irish Sea, the decline of herring (which filter-feed on animal plankton, also the food of jellyfish) produced “a cascading regime shift” during the 1980s. Since then, the team says, “sea temperatures have increased, the fish community has altered, and jellyfish abundance has risen such that jellyfish and haddock may now play an increasingly important role in the ecosystem”.

Threats to this island’s aquaculture were shown dramatically in 2007, when great swarms of the mauve stinger, Pelagia noctiluca, a warm-water species proliferating off Spain, destroyed an entire fish farm’s stock of salmon in Co Antrim. Experiments at University College Cork’s Coastal Marine Research Centre have now shown that even the “harmless” moon jelly, Aurelia aurita, familiar on every Irish coast, can cause severe gill problems in marine-farmed fish, even when shredded through the nets.

A thorough, very readable review of the human-induced stresses on the ocean that promote the global rise of jellyfish is listed at the EcoJel website, jellyfish.ie. A team of marine scientists from Australia, South Africa, the US and the UK conjecture on a tipping point when the ocean’s normal controls on jellyfish are overwhelmed and “the hunted becomes the hunter”. The Australian Dr Anthony Richardson is lead author and gives the review (published in Trends in Ecology Evolution, Vol 24 No 6), a suitably Aussie title: “The jellyfish joyride: causes, consequences and management responses to a more gelatinous future.” The annual global removal by man of between 100 million and 120 million tonnes of marine life over the past two decades signals the most obvious trigger for what the review calls a possible “self-enhancing feedback loop” that turns the ocean food web upside down. Many species of fish not only compete with jellyfish for zooplankton but help control their numbers by eating them when jellyfish themselves are tiny plankton animals. Trawling these fish both removes the pressure of predation and gives the jellyfish more to eat. Off South Africa, in the rich Benguela upwelling, decimation of sardine stocks produced an ecosystem dominated by jellyfish such as Chrysaora, or the compass jellyfish (in my drawing), common in Irish summers.

Intensive trawling also scrapes soft seabed clean of the many other marine organisms, such as tube worms, sea cucumbers and little reef fish, that compete with jellyfish or prey on their planktonic forms, while rocky outcrops offer a safe haven for jellyfish polyps (their fixed, reproductive stage on the seabed). Translocation of jellyfish species, on ships’ hulls or in ballast water, can produce population explosions in waters denuded of their stocks of planktivorous, filter-feeding fish, as happened in the Black and Caspian seas of Europe.

Jellyfish are great survivors of disturbed marine ecosystems – even the oxygen-starved “dead zones” caused by coastal eutrophication – and the tipping point comes when the sum of human impacts allows jellyfish to outnumber the filter-feeding fish. They then eliminate their fish predators by eating their planktonic eggs and larvae. Infesting ocean currents, they travel to new seafloor habitats to colonise with reproductive polyps. A new and durable ecosystem emerges, dominated by jellyfish and reminiscent of the ancient, rudimentary Cambrian era of the ocean.

The jellyfish joyride, the team conclude, could take the ocean “back to the future”. They spell out many things the human world can do to make this less likely – all pretty obvious by now. They end by quoting Prof Daniel Pauly, the French marine biologist whose free online FishBase gets more than 30 million hits a month. He argues that entire marine ecosystems in the North Atlantic have already been altered nearly beyond recognition. Without proper precautionary action, he says, “My kids will tell their children: ‘Eat your jellyfish!’ ”

Eye on nature

On the May bank-holiday break in Castlegregory, Co Kerry, I heard two cuckoos calling near Stradbally beach. We were also treated to a sunset chorus of natterjack toads on Lough Gill. Were they mating?

Oliver Burke, Kilcullen, Co Kildare

When natterjacks are calling they are in the mating season.

Since moving here, seven years ago, we have heard “our” cuckoo every year. This year, after three weeks’ calling, its mating call got a response: another cuckoo call from a nearby field.

Tom Molloy, Mullinahone, Co Tipperary

The second call could be the same cuckoo moving about, or another male announcing his presence. The female has a low chuckling call.

I saw a group of seven hares gathered in a circle as if they were having a meeting. What is the collective name for such a group?

Martin Crotty, Blackrock, Co Louth

These spring gatherings of hares are associated with mating. Four different collective names for gatherings of hares are down, husk, mute or drove.

I opened my wheelie bin and found 30 maggots under the lid and on the sides. I left it open, and within an hour two robins had taken them off to feed their young.

Alan Jones, Greystones, Co Wicklow