What's behind the huge blooms of jellyfish round our coasts?

It’s a bumper, extravangza, blobtastic summer for jellyfish round the coasts of the British Isles. You may have seen for yourself the enormous blooms. The population explosion has even made newspaper headlines.

Most dramatically, a vast jellyfish bloom on the east coast of Scotland caused the Torness nuclear power station to shut down in June. The numbers were so great that they were clogging up the cooling water filters.

I saw a similar invasion when I was in Loch Spelve on the east side of Mull in Scotland a couple of weeks ago. We were on a mooring at the north-west corner of the loch when, at about 2230, a silent and dense wave of purple-ringed jellyfish completely surrounded us.


The jellyfish were the common or garden translucent types with four purple rings at their centre. They stretched in every direction as far as we could see, so dense that they seemed to fill the water around us and were feet deep beneath.

What was going on with the alien attack?

According to Richard Harrington at the Marine Conservation Society, these are moon jellyfish. They bloom in large numbers round the UK every summer. “But,” he adds, “there seem to be very large volumes this year.”

Moon jellyfish lead a mysterious life. They are some of the oldest organisms on earth, thought to have been around for 500 million years.

They emerge from polyps lying on the sea bed and, says Harrington, bud off into “effectively clones”, live for a couple of weeks, spawn the next generation and die – or wash up or get eaten.

As for what causes such large bloom, or even how numerous they are, no-one seems to be certain. It may be triggered by rising sea water temperatures. But jellyfish do also bud in the depths of winter, and Richard Harrington tells me of a massive bloom of mauve stingers off Scotland and Ireland a few years ago that emerged in December.

Unlike moon jellyfish, which consume zooplankton, mauve stingers prey on fish and they decimated farmed salmon in Scotland. They even gulped their way through the entire £1 million stock of a salmon farm in Northern Ireland.

Where do they all come from? Loch Spelve, where we were, is keyhole shaped and almost enclosed, so would be odd if they’d found their way in from far afield. Richard Harrington agrees: “They could have been quite local,” he says.

Once they’ve budded, jellyfish go where the wind and tide go. “They can move a little bit in the water column, but basically they are at the mercy of tides and current,” he says.

I wanted to know if I’d been foolish enough to jump into the seething mass of moon jellyfish, would I have been badly stung?

Not necessarily, says Harrington. “Only if you were allergic to the toxins,” he says. “They do have a mechanism for firing a poison dart, but most people can touch them without feeling a thing.”

It’s a different matter with other jellyfish, notably the Lion’s Mane, which with its long trailing brown tentacles is like a galactic force of the deep. Their sting can affect the nervous system.

The moon jellyfish may not be much of a predator, but it is a tasty snack for other marine creatures, such as leatherback turtles, which you see in Scottish waters, and basking sharks. Jellyfish are also the staple diet of the equally mysterious sunfish.

Large fish, such as cod suggests Harrington, will also gobble up a jellyfish.

And who knows you may see it on the menu yourself one of these days. Jellyfish are eaten as a delicacy in Japan, and the Marine Conservation Society were once contacted by ‘molecular gastronomist’ Heston Blumenthal, who wanted to know how to get hold of the bulky, non-stinging barrel jellyfish to cook at his restaurants. His big idea was to make jellyfish and chips.

How on earth would you cook them, I wonder? “You’d have to dry them,” suggests Harrington.

That would make them look, I suppose, like the sandy gelatinous pustules you see washed up on beaches. Not that appetising.

Maybe I would try one cooked, but it would have to be served with a lot of tomato sauce.