Rugulopteryx okamurae , known as Asian or Japanese seaweed, is relentlessly plaguing the Spanish coast. Originally from the South China Sea, it has been established in Andalusia for over a decade and, since last summer, has surged along the Cantabrian coast. What began as a localized problem in the Strait of Gibraltar has become an unstoppable invasion affecting ecosystems, the economy, and tourism in several autonomous communities.
In the Basque Country, the algae was first detected in 2024 in the Port of Bilbao, but this summer it has colonized entire seabeds on beaches such as Meñakoz (Barrika), the port of Mutriku, and the dock of Castro Urdiales. According to Jon Hidalgo of the Lurgaia Foundation, in just one year it has gone from being almost invisible to completely covering the rocks at depths of five meters and below, displacing native algae such as Gelidium and leaving the seabed practically barren.
Ecological and economic impact in the Cantabrian region
Researchers at the Azti research center have confirmed the presence of the species on the Gipuzkoa coast, between Orio and Hondarribia, and also in Muskiz and near the Nervión River. Although it does not yet form a continuous carpet as in the Mediterranean, its rapid expansion is a cause for concern . Iñigo Muxika, a benthic ecologist at Azti, explains that the decline of red algae meadows due to rising sea temperatures and increased sunlight is facilitating the invasive species' spread.
The consequences are serious: benthic fish (such as wrasse, groupers, and sea bass) that feed on native algae are disappearing, along with the predators that depend on them. Furthermore, the Gelidium mats provided refuge for fry, so habitat loss reduces biodiversity. In the Port of Bilbao, the variety of algae has plummeted from 40-49 species to just 12-16. Andalusian fishermen have already reported the drastic reduction in fishing grounds and the damage to their nets, which tear easily when caught in the algae mats.
Massive strandings and cleaning costs
In Andalusia, the situation is particularly critical in the province of Cádiz. Beaches in Rota, Chipiona, El Puerto de Santa MarÃa, and the city of Cádiz itself have had to remove thousands of tons of biomass this season. In Rota alone, the City Council has collected 3.407 tons, and on June 1st, a record 217.620 kilos were removed in a single day from Chorrillo Beach. The Cádiz Provincial Council has allocated €605.000 in aid to the affected municipalities, but the local councils are demanding greater involvement from the regional and national governments.
The stench of decay as seaweed decomposes under the summer sun drives away bathers and damages the area's tourism image. In towns like Mutriku, residents warn that the odor will become unbearable if the seaweed isn't removed soon. Eradication is impossible with current methods, according to experts, so efforts are focused on managing the seaweed washes.
From waste to resource: ongoing research
Faced with this threat, science is seeking alternatives. A team from the University of Malaga has demonstrated that Rugulopteryx okamurae can be transformed into feed for farmed fish. After treatment to eliminate its toxicity, the biomass improves the nutritional profile of mullet: it increases omega-3 fatty acids, reduces total fat, and strengthens the animals' immune system. This finding, published in the Journal of Applied Phycology , opens the door to a circular economy model that reduces dependence on fishmeal.
On the other hand, the University of Cádiz has launched the RUGULOMICS project , funded by European funds, which uses molecular tools to detect the algae early and study its biology. The initiative, coordinated by Almudena Escobar-Niño, includes genome sequencing and the search for proteins of biotechnological interest. The use of the biomass to manufacture bioplastics is also being investigated, as noted by Azti.
The spread of the invasive algae shows no signs of abating. While scientists are working on solutions, the municipalities along the Basque and Andalusian coasts continue to wage a daily battle against the tons of algae washing up on their beaches, facing an uncertain future for marine ecosystems and local economies that depend on the sea.