The Portuguese caravel begins to spread across the Cantabrian Sea and ‘threatens’ the summer

The mass arrival of Portuguese man o’ war to the Cantabrian coast could become a common episode in the coming years. Experts point to a combination of factors such as temperature, wind, and currents due to the respective changes these phenomena are undergoing because of climate change. Faced with a situation that not only poses a risk to bathers but also to the well-being of these organisms, two scientific studies have set to work to understand the behavior of these animals and predict their movements.

Read more Ferrari is the only winner in the European motor stock market storm

The presence of the Portuguese man o’ war has spread across numerous beaches in the Basque Country this summer. The first specimen was detected in Laga, Biscay, on June 20, and in July, sightings were recorded on different sands, along with several stings. Since early August, concentrations have repeated and have produced notable episodes such as the one that occurred in San Sebastián on Tuesday, August 4, with 120 stings recorded on bathers in a single day.

However, this is not the first time these organisms have arrived in the Cantabrian Sea. “The Portuguese man o’ war has always been around,” explains Fernando Taboada, senior scientist at the Gijón Oceanographic Center of the IEO-CSIC. “The species can occasionally appear on the Cantabrian, French, and British coasts.” What is striking, however, is the repetition of mass arrival episodes in recent years. According to Taboada, a significant arrival occurred in 2008, but “in the last five or six years, they have been arriving more and more.”

In the summer of 2025, according to data collected by the Gijón Oceanographic Center of the Spanish Institute of Oceanography (IEO-CSIC), 350 sightings of Portuguese man o’ war were recorded in Asturias, with specimens ranging from 1 centimeter to 30 centimeters. They were mostly located at the eastern and western ends of the Asturian coast.

The combined pattern of currents and winds affects these organisms, points out César Bordehore, researcher and head of MedusApp. This animal cannot move on its own; it has neither muscles nor strength to counteract these phenomena, so its direction is determined by them.

Michael Sealy, biologist at Oceana, points out that one of the factors that could cause this mass arrival to the Bay of Biscay is a change in the behavior of the Azores anticyclone, which would have modified the associated wind and circulation patterns. According to Sealy, a less intense anticyclone can favor conditions of wind and currents that transport specimens to the Bay of Biscay that normally remain in open waters.

Therefore, their appearance on a Basque beach does not necessarily mean the species lives there permanently. For Fernando Taboada, ending up on the coast can even be a form of “stranding.”

Man o’ war usually inhabit the Atlantic Ocean, especially in subtropical and warm areas. According to Sealy, rising sea temperatures may allow them to find optimal conditions for longer periods to survive and reproduce in areas they previously reached more sporadically.

The increase in sea temperatures favors the metabolism of many gelatinous organisms while, at the same time, as warm water contains less oxygen, fish and crustaceans are harmed. This worrying situation for fish is compounded by fishing pressure. All this leads to a decrease in jellyfish predators, such as endangered sea turtles, which alters the balance between different species. 

The consulted experts are cautious and explain that it cannot be stated with certainty that this phenomenon is driven by climate change, although they acknowledge that “everything points” to it, as the episodes are concentrated mainly in recent years. 

Read more Consumer credit before the holidays reaches a 20-year high

Climate change affects in various ways, so the result is not a simple cause-and-effect relationship, the researchers point out, but a combination of factors (wind, currents, temperature, and ocean conditions) that can lead certain organisms to appear in new places.

The Iberian Peninsula is in a transition zone between temperate and subtropical conditions. Fernando Taboada points to a progressive “subtropicalization” of certain environmental conditions, such as heat waves or winds coming from the Sahara, which cause the appearance of (tropical) species in areas where they were previously less frequent. 

The mass arrival of Portuguese man o’ war has not gone unnoticed, as it is also an episode that “will surely happen again,” says Michael Sealy of Oceana. In response, two scientific investigations are already underway to determine the magnitude of the phenomenon and predict the behavior of these marine organisms.

On one hand, the ECOAQUA University Institute, through the Marine Organisms Ecophysiology group (EOMAR), and the University Institute of Intelligent Systems and Numerical Applications (SIANI), of the University of Las Palmas de Gran Canaria (ULPGC), began a new observation and monitoring campaign of the Portuguese man o’ war in the waters of Gran Canaria in March. The goal, according to marine biology graduate May Gómez, is to “improve predictions” of their arrival on the coast.

The collected specimens have been placed in culture for study and observation for a month to learn about their physiology and biological mechanisms. In 2025, this group of scientists managed to keep them alive in culture for a total of 45 days, an unprecedented achievement in the research of this marine species. “We aim to provide scientific knowledge to the study of a species about which little is still known,” summarizes Gómez.

Meanwhile, the “Physalia Asturias” campaign, led by staff from the Gijón Oceanographic Center of the Spanish Institute of Oceanography (IEO-CSIC), is responsible for locating, recovering, and analyzing different specimens. The captured specimens are kept in controlled conditions for several days to conduct experiments that allow studying their population structure and movement patterns. For this, the project has the collaboration and advice of the BIOPARC Aquarium of Gijón, with leading facilities and experience in maintaining marine organisms.

This fieldwork is complemented by the construction of biomimetic replicas launched into the sea. That is, scientists are designing a 3D prototype of a man o’ war that mimics the shape, weight, and density and houses an electronic board inside that records all movements. Once the model is finished, buoyancy and electronics tests will be carried out, and 80 copies will be printed. The intention is to launch them into the sea in Gibraltar, Bermuda, Hawaii, and Rapa Nui to understand navigation flows.

Experts also lead to logic: many more people observe the sea during summer, so the increase in the number of bathers increases the chances of encounters and, therefore, stings.

Swimming has been restricted on some beaches, and on others, the yellow flag has been raised to request “caution.” Authorities recommend not touching the man o’ war, not even when they appear dead on the sand, and to go to the first aid station in case of a sting.

Read more Count Trash Can Face as an indicator