Invasive Japanese algae colonize the Marseille


Invasive algae of Japanese origin, which create a foul odor when rotting, colonize the rocks and seabed of Calaque de Marseille, we learned from supporting sources.

Seaweed rugulopteryx okamurae, green in color, “are not poisonous or annoying, but pose a health risk because when stranded, they give off gas, hydrogen sulfide, potentially deadly in large doses,” a Calanques National Park spokesman told AFP

Since its appearance near Île Maïre, south of Marseille, in 2018, algae have spread to the de Marseilleveyre, and have even been found in the city, in the Vallon des Auffes. She also won the Côte Bleue, west of Marseille.

Seaweed “with a proven invasive character” according to Calanques Park probably arrived in Marseille “because someone ate sea urchins from the Thau lagoon, where it has been present since 2008, and threw them back into the sea,” explains Thierry Thibault, a Mediterranean researcher. Institute of Oceanology.

At the end of town, in the small port of Callelongue, “everything is swarming and stinking, the smell is nauseating,” resident Guy Coulet told AFP. Underwater, its presence is most striking, he assures us, adding that algae “greatly annoy fishermen caught in nets”.

An invasive alga rugulopteryx okamurae that attacks invasions of Marseille streams. Photo of June 25, 2021 (AFP – Christophe SIMON)

Seized in Calanques Park, the city of Marseille on Friday assured AFP that “the situation is under control and that analyzes are underway”.

“There is no danger to people at this stage, but if the gas rate becomes too high, the beaches should be closed,” Hervé Menchon, deputy mayor of Marseille, told Marine Biodiversity.

– 150 invasive algae in the Mediterranean –

“Bathing children in the harbor, in principle prohibited, is a problem,” admitted Mr Menchon, who added that the city had “strengthened the means of deterrence and human presence for surveillance”.

The metropolis of Aix-Marseille-Provence, responsible for the port of Callelongue, claims it wants to “suck up the algae that accumulate at the bottom of the water in several deep places on the coast”. But she recognizes that “the question of treating these algae after the evacuation remains unknown for now.”

Calanque de Callelongue, where invasive algae rugulopteryx okamurae breed.  Photo of June 25, 2021 (AFP - Bertrand LANGLOIS)

Calanque de Callelongue, where invasive algae rugulopteryx okamurae breed. Photo of June 25, 2021 (AFP – Bertrand LANGLOIS)

Scientists are currently assessing the impact of algae surrounding rocks and the seabed on local flora and fauna.

In southern Spain, where it has multiplied, the alga rugulopteryx okamurae costs “two million euros a year to the authorities, which must be disposed of,” according to Mr Thibault.

For the researcher, “the problem of imported species, the sputum (larvae) of oysters coming from Japan, must be called into question.” In the Mediterranean, the researcher recalls, 150 invasive algae were listed.

“These imported species are another cause of declining biodiversity in the world, we really need preventative measures,” he said, citing the example of Australia, where legislation is drastic in this area.



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