Beached jellyfish are a surprising sight but not dangerous

For anyone understandably heading to the coast this weekend, don’t worry if you see masses of jellyfish washed ashore. It’s normal for this time of year.

On the beaches of Knokke-Heist and De Haan, there are currently a great number of beached moon jellyfish (Aurelia aurita). Non-poisonous to humans, they tend to wash ashore in great numbers every June – at least if the wind is right.

Plankton is present in abundance in the shallow waters of the North Sea this time of year, and the jellyfish gather to eat them up. When offshore winds push the surface water away from the coast, a subsurface return current moves toward the shore, carrying the jellyfish with it.

Being weak swimmers, the jellyfish wind up on the beach and can’t get back in the water. “The intersection of these circumstances means there are masses of them on the beach now,” explains Jan Seys of the Flemish Sea Institute. “We’ll have to wait and see what the coming days bring.”

Visitors to the beach can recognise the moon jellyfish by its white, horseshoe-shaped rings. “They cannot penetrate human skin,” says Seys. “They sting, but you don’t even feel it.”

Anyone inclined to try to save a moon jellyfish that appears to still be alive should do so with the aid of a spade and bucket. It needs to be taken fairly far out, though, if it has any chance of not washing back to shore.

Seys says to “not hesitate” to go swimming, though swimmers should put some distance between themselves and jellyfish in the North Sea. If it’s obviously a moon jellyfish, no problem, but “it’s a different situation with the blue or the compass jellyfish,” says Seys. “Their sting can be quite painful.”

While those species of jellyfish are not present in the numbers of the moon jellyfish, they can be present. So swimmers take heed.