On November 22, 2007, off the Glens of Antrim in Northern Ireland, a swarm of Pelagia noctiluca drifted into the cages of the province’s only salmon farm. More than 100,000 fish died, according to the company’s director quoted in the press.
The story went around the world. Since then, every summer has brought its share of headlines about “jellyfish invasions.”
We wanted to sort it out. On one side, what has actually been measured. On the other, what is still a hypothesis. And in between, the two species that really deserve the word “invasion” in the Mediterranean and the Black Sea.
Are there more jellyfish in the world than before?
Nothing proves it on a global scale. In January 2013, an international team led by Robert Condon published in PNAS an analysis of every long-term data series available on jellyfish abundance at coastal stations around the world. The authors found no solid evidence of a global increase.
The study does pick up a slight linear rise since the 1970s. But that signal falls apart when you compare the number of populations going up with the number going down: the two stay roughly equal across every period examined. The clearest pattern is a cycle. Populations rise and fall together across the planet, with a period of about twenty years. The upswing of the 1990s, which got a lot of media attention, fed the impression of a worldwide explosion.
In short, a global ocean overrun by jellyfish is still a hypothesis.
Locally, the picture is different. Some bays and enclosed seas have seen repeated, well-documented blooms. The authors call for longer monitoring to find out whether the next high phase will look like the previous ones. Nobody can settle that question today.
What causes jellyfish blooms?
Four explanations keep coming up in the scientific literature: warmer water, overfishing, eutrophication and the spread of artificial structures along the coast. None of them works alone. The review published by the FAO in 2013 for the General Fisheries Commission for the Mediterranean discusses them one by one and shows that they often act together.
| Suspected cause | How it can favor jellyfish |
|---|---|
| Warmer water | Faster growth for some species, a longer breeding season, easier settlement of tropical species arriving from the Red Sea |
| Overfishing | Fewer fish competing for the same plankton, and fewer predators of their eggs and larvae |
| Eutrophication | Nitrogen and phosphorus from rivers and wastewater feed the plankton jellyfish eat |
| Artificial structures | Ports, breakwaters and pontoons give polyps hard surfaces to attach to |
Warming and overfishing
Warmer water can speed up the growth of some species and stretch their breeding season. It also makes it easier for tropical species from the Red Sea to settle in the Mediterranean. Overfishing works differently. Remove the fish that eat the same plankton as jellyfish, and you leave more food for the jellyfish, which also swallow the eggs and larvae of their competitors. The loop closes fast.
Eutrophication and artificial structures
Nitrogen and phosphorus carried by rivers and wastewater enrich the plankton that jellyfish feed on.
Then there is concrete.
Ports, seawalls, riprap and pontoons offer hard surfaces where polyps can attach. The polyp is the fixed stage in the life cycle of many jellyfish, and on sandy or muddy bottoms it often lacks natural surfaces to settle on. Each polyp can release several small jellyfish in spring. So a heavily built coastline multiplies the possible starting points of a swarm. The link is plausible.
It has not been demonstrated everywhere. An earlier version of this page also listed ocean acidification and hormones in wastewater as established causes. We removed them because the evidence is not strong enough.
Mnemiopsis leidyi: the Black Sea invader that is not a jellyfish
Mnemiopsis leidyi is a ctenophore, a gelatinous animal that swims with rows of cilia and does not sting. It came from the Atlantic coast of the United States in ships’ ballast water and was detected in the Black Sea in 1982. Its populations then exploded.
The fallout for fishing was harsh. The ctenophore eats zooplankton, along with fish eggs and larvae. According to the FAO review, it brought the Black Sea fisheries “to their knees” by stripping the water of fish. Then another ctenophore, Beroe ovata, arrived in turn, probably from the same part of the Atlantic, and found an abundant prey waiting for it. Beroe feeds on Mnemiopsis, and its arrival softened the impact of the first intruder.
Neither introduction was intentional.
The story does not end in the Black Sea, though. In 2009, large populations were reported in Israel and Italy, a sign that the species had spread across the whole Mediterranean, according to the same review. For you as a swimmer or diver, the direct risk is zero. Ctenophores have no stinging cells and cannot hurt you.
Rhopilema nomadica: the nomad jellyfish of the eastern Mediterranean
Rhopilema nomadica is a large jellyfish that came from the Red Sea through the Suez Canal. First reported off Israel in 1977, it was described as a new species in 1990 from specimens collected in the Levantine Basin. Its summer swarms now reach the entire eastern Mediterranean.
The low biodiversity of that part of the sea left it plenty of room, and its populations quickly reached huge densities along the Israeli coast. The teams that monitor it describe it as the largest and most venomous jellyfish in the region. It has spread to Lebanon, Syria, Turkey, Greece, Egypt and Malta. More recently, it has been reported in Tunisia, Sardinia and Sicily. How far west will it go?
Nobody knows yet.
It is not the only species to take that route. Cassiopea andromeda, the upside-down jellyfish that lives resting on the seabed, also came from the Red Sea. We have a dedicated page on Cassiopea, the upside-down jellyfish if you want to know more about it.
How do blooms affect fishing, power plants and tourism?
The damage is real and dated. A bloom can clog nets, kill farmed fish, block the cooling water intakes of power plants and desalination plants, or empty a beach. The best-documented examples remain local and short-lived.
On June 28, 2011, the Torness nuclear power station in Scotland shut down both its reactors as a precaution. Jellyfish were clogging the screens that filter incoming seawater. The operator, EDF Energy, stated that there was never any danger to the public. Along the Israeli coast, Rhopilema nomadica clogs fishing nets and the intakes of power and desalination plants, then stings swimmers at the height of summer. That means hours of downtime.
For aquaculture, the Antrim salmon farm is still the most cited example.
For swimmers, waves of Pelagia noctiluca in the Mediterranean have a long history. The FAO review mentions records going back to 1775 and heavy blooms in the Adriatic in the early 1980s. In most cases, a sting can be handled with simple steps, and our article on what to do after a jellyfish sting covers the ones that hold up, including rinsing with seawater.
What should you do if you meet a swarm while diving or snorkeling?
Report what you see and keep your hands to yourself. A swarm photographed with the date and place feeds scientific monitoring. The FAO review cites the Jellywatch campaign, run from 2009 to 2011, which gathered thousands of observations from the public.
A photo, a precise location, a date and a rough count are enough. Add the water temperature if your dive computer shows it, along with the depth where you met the swarm. Those details help researchers link a swarm to currents and season. They are exactly what most viral videos leave out, since those clips rarely say where or when they were filmed. Even an ordinary sighting counts.
If a swarm drifts across the planned site, the call is made at the briefing, with the guide who sees the water that day and can delay the trip or switch spots. Accepting a delay costs you half a day. A sting on the face can cost you the next few dives.
FAQ
Are there more jellyfish than before in the Mediterranean?
On a global scale, the Condon study published in PNAS in 2013 found no demonstrated increase. It describes cycles of about twenty years. In the Mediterranean, local blooms are well documented and species from the Red Sea are settling in the east. The long-term trend is still debated.
Does Mnemiopsis leidyi sting?
No. Mnemiopsis is a ctenophore with no stinging cells, and it cannot hurt a swimmer. Its impact is ecological: it eats zooplankton as well as fish eggs and larvae.
Why do jellyfish shut down power plants?
Coastal power plants pump seawater for cooling. A swarm can clog the filter screens and reduce the flow. In June 2011, the Torness power station in Scotland shut down both its reactors as a precaution for this reason.
Is overfishing the cause of jellyfish invasions?
It is one of the explanations, along with warming, eutrophication and artificial coastal structures. By removing competing fish, overfishing leaves more plankton for jellyfish. None of these causes is enough on its own to explain a bloom.
Sources
- Recurrent jellyfish blooms are a consequence of global oscillations (Condon et al., PNAS, 2013)
- Review of jellyfish blooms in the Mediterranean and Black Sea (FAO, GFCM Studies and Reviews No. 92, 2013)
Photo: tata_aka_T, Wikimedia Commons, CC BY 2.0
