Thousands of Jellyfish Wash Ashore on Belgian Coast Beaches

Thousands of moon jellyfish have washed ashore on the beaches of Knokke-Heist and Wenduine along the Belgian coast, creating a striking natural spectacle for beachgoers during a major heatwave. Marine biologists have confirmed that the jellyfish, known as moon jellyfish or Aurelia aurita, are completely harmless to humans.

A Seasonal Phenomenon

According to VRT NWS, the mass stranding is not caused by the current heatwave sweeping across Western Europe. Jan Seys, a marine biologist and spokesperson for the Flanders Marine Institute (VLIZ), explained that this is a typical seasonal occurrence around this time of year.

“It’s a typical phenomenon around this period,” Seys told VRT NWS. “In June there is a lot of small plankton in the water. That is the result of the approaching summer every year. That is the main food source of the jellyfish.”

The phenomenon is driven by a combination of high plankton levels in June and offshore wind conditions. When winds blow away from the coast, surface water is pushed out to sea while the undercurrent brings jellyfish toward the shore. This convergence of factors creates the mass presence now being observed along the Belgian East Coast, from De Haan to Knokke-Heist.

No Cause for Alarm

Seys was emphatic in reassuring the public that the jellyfish pose no danger. Speaking to HLN, he explained: “It concerns the moon jellyfish. You recognize them by their white rings. They actually cannot penetrate human skin. They do sting, but you don’t feel it.”

His advice to anyone hesitating to swim? “Just do it. I wouldn’t worry about it. It’s harmless. Only people with sensitive skin and children should be careful. And even then.”

The moon jellyfish (Aurelia aurita) is one of the most common jellyfish species in the North Sea and along the Belgian coast. It is easily recognizable by its translucent bell with four distinctive white rings.

Scale of the Event

The SeaWatch-B monitoring network, VLIZ’s citizen science beach observation program, received numerous reports of the jellyfish from the public. According to the SeaWatch-B network, which tracks marine life along the Belgian coast, the scale is significant.

“In the photos you can see one jellyfish per meter in some places,” Seys told HLN. “If you calculate that our coast is 30,000 meters long, there must be at least thousands of jellyfish washed ashore.”

Not All Jellyfish Are Harmless

While the moon jellyfish is harmless, Seys warned that other species may appear later in the summer. “After the moon jellyfish, other jellyfish may wash ashore soon,” he cautioned. “Toward the height of summer, compass jellyfish arrive, which do sting. They are easily recognizable by their brown stripes and brown rim. If those are present, I would not go swimming.”

Compass jellyfish and blue jellyfish can cause skin irritation and are distinctly different from the harmless moon jellyfish currently blanketing the beaches.

Climate Change and Jellyfish: A Complex Picture

Seys cautioned against drawing simplistic conclusions linking this event to climate change. In an interview with HLN, he noted that warmer winters could actually reduce moon jellyfish populations, as their polyp stage requires temperatures around 6°C to properly segment and release juvenile jellyfish.

“You certainly cannot say that all jellyfish, and certainly not the moon jellyfish, will increase with a warmer climate,” he said. “Science currently does not agree on which direction that will go.”

However, he acknowledged that other jellyfish species with different life cycles may increase with warming waters, and new species may arrive with changing currents and temperatures.

Heatwave Context

The jellyfish event coincides with a major heatwave in Belgium. June 26, 2026, is reported as the hottest day of the year so far, with temperatures reaching extreme levels across Western Europe. The heatwave has caused widespread disruptions, including the cancellation of the Defqon.1 festival in the Netherlands. Many Belgians are flocking to the coast to escape the heat, making the jellyfish presence particularly noticeable.

What to Watch For

The phenomenon is expected to be temporary, typically passing within days to weeks depending on wind conditions. The HLN liveblog covering coastal news noted that police in Knokke-Heist and Oostende have deployed extra personnel for the expected crowds heading to the beach this weekend.

For now, beachgoers can enjoy the sea without worry. As Seys put it simply: “It’s harmless.”