At the fish stall in the village of Aspra, near the Sicilian capital of Palermo, the day’s catch is garnished in green. But far from being an ingredient to be cooked up as a side dish, it is the worst enemy of local fishermen. Known as Rugulopteryx okamurae, this has become the bane of their lives. For the past two years they have spent days at a time untangling it from their nets, watching their income suffer as a result.
The problem is well known among their fellow fishers in Spanish waters. “We’ve had the algae here for 10 years,” says Gregorio Linde, skipper of a small-scale fishing boat in Tarifa, on the Gibraltar Strait. “The seabed is a carpet of it, and the nets don’t catch anything.”
His family has lived from the fruits of the sea for generations, but now his days often consist of hauling seaweed — just to dump it back. And he is not alone. Losses to such small-scale in exceed €3 million ($3.48 million) annually.
And it is also harming coastal tourism in countries like Spain and where vacationers wrinkle their noses as they tiptoe through foul-smelling mats of algae on once white-sand beaches. Despite the effort and cost of carting it off to landfill, new tides refill the beaches.
The greatest damage, however, is out of sight. Underwater, the seaweed is smothering vital seagrass meadows, harming sea urchins and occupying fish shelters.
“Socioeconomic impacts can be offset with money, but this unprecedented ecological impact cannot,” warns Maria Altamirano, a researcher at the University of Malaga, who first identified the in Spain. The “invasion “is like a wildfire in a national park: Everything is wiped out, and only one species remains,” she says.
The Mediterranean: A hotspot for invasions of non-native species
Though it covers less than 1% of the area, hosts nearly a quarter of global maritime traffic. And that, according to the European Environment Agency, is responsible for introducing half of all non-native species into the sea since 1970.
More than half of all non-native species found in European waters dwell in this semi-enclosed sea, according to Luca Castriota, an invasive species expert at Palermo’s National Institute for Environmental Protection and Research. He says one in 10 turns invasive, displacing native marine life and reshaping .
Rugulopteryx okamurae is a major offender. It’s native to the Pacific, where it lives in balance with ecosystems in Japan, China, Taiwan, Korea, and the Philippines. The species was first identified as invasive in 2015 in Ceuta, a Spanish enclave on the North African coast, along the Strait of Gibraltar.
Encouraged by — which weaken ecosystems — as well as and the absence of natural predators, its aggressive advance didn’t stop there. It has since spread to different parts of the Mediterranean and reached Atlantic archipelagos such as the Canary Islands, the Azores, and Madeira. In 2022, it became the first algae on the EU’s list of concerning alien species.
How ballast water balances ships and unbalances seas
Scientists believe the seaweed reached the Mediterranean via ballast water discharged by cargo ships traveling between Asia and Europe. Before leaving harbor, vessels pump water into their internal tanks to stabilize cargo. They then expel it at their destination — along with any hidden spores, larvae, or eggs.
“Today the world is interconnected. We buy products in for instance, that are distributed across the planet. But along with the goods we want, we are transporting species from one place to another,” explains Maria Garcia of the Spanish National Research Council (CSIC). She has been monitoring invasive species in Mediterranean coastal waters for 30 years and says trade is “completely altering” its fauna and flora.
In 2004, the International Maritime Organization (IMO) adopted a convention requiring ships to treat ballast water — though it only took effect in 2017. And from September last year, ships flying a signatory country’s flag, or discharging ballast in its ports, must install onboard treatment systems preventing anything but tiny particles — 10 microns — from being discharged.
Experts call it a step forward, but enforcement remains weak. “Efforts have been ongoing for years, but compliance is difficult because of the costs involved. It often depends on the goodwill of shipowners and crews,” Garcia admits.
Checks rarely rank above other port priorities such as vessel safety. In countries like Spain, authorities are underfunded. Others like Italy have not ratified the IMO convention, which means foreign ships can legally discharge untreated ballast water in the country’s ports.
“We cannot directly intervene with private individuals, because this would conflict with economic activities,” Castriota says. “Until the IMO convention is ratified, our hands are tied.”
Citizen science: An ally against the tide
Once a species is installed, slowing the spread becomes essential. “We know the algae can thrive in a much larger area than it currently occupies. We must prevent it from getting there,” Altamirano stresses.
Stronger ballast water controls and cooperation with fishers and recreational boat owners are key, since nets and hulls often carry the algae further. But civil participation also matters.
It was a citizen science platform that first flagged the Asian algae’s arrival in Murcia, southeastern Spain. And Garcia’s team collaborates with more than 5,000 volunteers in the northern Catalonia region, which is also affected. They upload photos of suspected invasive species to the Observadores del Mar platform. “We scientists work in our labs, but there are huge areas of the sea we cannot cover,” Garcia says.
But preventing future invasions will take more than dawn sails and lab experiments.
It requires international coordination, dialogue between science and policymakers and affected sectors, and active citizen participation, Altamirano says.
“Rugulopteryx okamurae is just one of many invasive species,” she adds. “We need to have the machinery running smoothly for the next ones that are sure to come.”
Edited by: Tamsin Walker
This reporting in this story was supported by Journalismfund Europe.
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