Jellyfish get a head start on their summer invasion of the shores in Spain, and these are the reasons why

The unusually warm spring weather has driven people to the beaches to enjoy it, but something else has had the same idea. The feared jellyfish have already started to make an appearance. The Costa Brava has registered the first major invasion of the year, but it is unlikely to be the only occurrence. As Professor Josep-Maria Gili of the Institute of Marine Sciences (CSIC) reckons, they will soon also be found along other coastal areas of Spain.

"Jellyfish arrivals can vary both in timing and location", explains Diego López Arquillo, research diver at the European University of the Canaries. There are multiple factors that have an impact on this, which in turn makes "the management and forecasting of these situations complex and dynamic". In short, it is not possible to say how many jellyfish will arrive at each location and when they will do so with any degree of absolute certainty.

However, listening to the experts, it seems inevitable to assume that this year - which has been dry and warm thus far - has created the right conditions for them to stick around.

In general, jellyfish will turn up and their presence at some point is pretty much guaranteed. Every summer we see them and those sightings are increasing. They have always been there, but this boom is "a recent phenomenon and it is growing", says Gili. "When they appear on the beaches, they appear more frequently and in swarms," he explains.

What are the reasons for this growth? First of all, overfishing of the ocean has an impact, explains the expert. "We are removing the species that eat them". Jellyfish eat plankton and there is no shortage of this right now as their primary food source. This is not the case, however, with the species that eat jellyfish - their situation is more precarious.

Secondly, their presence is also connected to changes in sea temperatures. As Gili reminds us, their appearance on beaches is a seasonal phenomenon connected to seawater temperatures and their own life cycles. That was also the reason why we used to see them only during the summer holiday months.

"They would be on the beach in summer because it coincided with their time to reproduce," he says. Now they turn up much sooner, as early as March, and leave later in September, even October, which gives them time to have more reproductive cycles. As Gili puts it, there are more adults mating, more eggs, more polyps (one of their life stages), in short, more jellyfish.

Neither is it just a question of more frequent breeding, the environmental conditions are also changing. It was common for springtimes to be rainy, with rivers and streams bringing freshwater to the coastlines. This made coastal waters different from those of the open sea and less receptive to jellyfish.

Rainfilled waters became a natural deterrent. "There were years that were dry and there were no barriers," says Gili. That's why in the past, every 10 to 12 years, there would be a sort of jellyfish invasion. Now, however, those conditions occur every year. "The coastal waters are more similar to those of the open sea than before," he explains.

López Arquillo comments on another factor: changes in marine currents, "which can transport larvae and adult jellyfish to new areas, increasing their presence on coasts where they were not common before". "The entry of an invasive species always implies an imbalance for the ecosystem they have invaded," he points out. In a new location they lack natural predators and this allows them to multiply, even pushing out native species.

In a way, you could say that jellyfish are a warning from the ocean that something is going awry. "Imagine you go to the beach and you find a bottle with a message in it, it's from the ocean and it's asking for help. That's what jellyfish are," says Gili. They are more than just a nuisance, stopping us from enjoying the beach.

"The population explosion has several negative implications for marine ecosystems and human activities beyond being a problem for bathers," says Lopez Arquillo. They can be a warning that the ecosystem is "out of balance or in decline", that it has become too polluted or that it has been overfished.

These experts also warn of economic problems. For fishing jellyfish could create future shortages in the best fishing grounds. "They don't just go anywhere, they're not stupid," says Gili. They settle in areas with shoals of fish where they will be able to eat while spawning, and this will decimate the fish population there. The problem is not so serious at the moment, but it could be.

Where they are already a current, not a future, problem is for the tourism industry; something particularly tricky for a country like Spain, whose beaches are one of its greatest attractions. Gili is not one to scaremonger, as sometimes the foreign media do with exaggerated headlines, but he does defend the importance of transparency.

"Information is a basic requirement for peace of mind", he insists. It should not be 'covered up' that jellyfish have turned up at a certain beach so as not to damage the tourist season. "You have to take precautions, as with a storm", he states. If they appear for a few days, they will end up leaving, something that bathers should know. Closing the beach may be necessary if, for example, two swarms appeared, making closure unavoidable. It is also essential to know what to do if, despite taking all precautions, a jellyfish stings you.