As the mercury begins to rise steadily, people are dusting off their beach umbrellas, pulling lilos out of storage and checking the expiry dates on last year’s bottles of sunscreen.

But the question on the tip of each beachgoer’s tongue this summer is whether jellyfish will spoil an otherwise delightful day at the beach.

Last summer, Malta was treated to a jellyfish invasion that had swimmers peering into the depths, awaiting a break in the dense purple underwater clouds, diving in and getting out again at breakneck speed lest one is brushed by a tentacle.

And this summer appears to be no different, with reports of jellyfish amassing near the coast becoming an almost daily occurrence.

But where are they coming from, why are they here, will they stay for the rest of summer and what can be done to rid the waters of the menace?

According to marine biologist Professor Patrick J. Schembri, the explanation to their dense presence around Malta is twofold: jellyfish are being driven to the islands by sea currents generated by temporary wind patterns, while the increased numbers being seen can be explained by a particularly good breeding season catalysed by a plentiful food supply.

Nor is there any way to predict their arrival, their length of stay, or to limit their entry into popular swimming spots.

Speaking to The Malta Independent on Sunday, Prof. Schembri explained that jellyfish infestations are anything but a recent phenomenon and have been taking place for at least a couple centuries since records started being kept. Many will recall, for example, how in the early 1980s Malta had played host to a much larger infestation, when numbers as large as 50 per square metre were recorded.

The species in question is the Mauve Stinger (pelagia noctiluca), a name derived from its distinctive colour. Although they can be seen along the Maltese coast in great numbers, the species is of the pelagic, offshore variety and individuals do not even live together in their natural habitat. The species is exclusive to the Mediterranean and are very rarely found outside the region.

A number of explanations behind the recent infestation have been floated, such as the depletion in turtle populations, shipping movements, pollution and climate change but, Prof. Schembri explains, “The reason they are being found together in numbers around Malta is the currents – full stop, there is no doubt about that.”

The mauve stingers are, in fact, being driven toward Malta by water currents generated by temporary wind patterns. With its very poor powers of propulsion, the mauve stinger is not able to swim sideways, let alone swim against the current. The winds blow them into coastal areas and bays, where circular currents trap them.

When the wind changes, they move on, which explains why on a given day a particular beach would be infested while the following day the same beach might very well be clear of jellyfish.

“This is an offshore species that is just coming in with the currents. There has not been any change in the long-term set sea currents, but it’s the surface, temporary wind-generated currents leading them to aggregate around Malta,” Prof. Schembri explained.

The recent, unusually high numbers of mauve stingers, which grow to a maximum size of 10 centimetres, meanwhile, could be linked to an especially good breeding season spurred by a bountiful food supply. Being plankton feeders that breed all the year round and mauve stingers respond to an increased food supply by breeding quickly, usually at the end of winter or the beginning of spring, at the same time that plankton blooms.

This, according to Prof. Schembri, explains the increased numbers currently in the Mediterranean, while wind and sea current patterns account for them being blown from around the Mediterranean to Malta and being trapped along the shore and coast.