Drop the anchor in a clear Riviera cove and look down: light patches are sand, dark green patches are usually posidonia. Where you put the anchor matters more than most people think.
What posidonia is
Posidonia oceanica is a seagrass that grows only in the Mediterranean. It forms wide underwater meadows from the surface down to 30 or 40 metres. Along the French Mediterranean coast these meadows cover more than 800 square kilometres. They shelter fish, keep the water clear and protect the beaches: the dead leaves you see piled on the sand in winter break the waves and hold the beach in place.
The problem is that posidonia grows very slowly. About 15 percent of the meadows have disappeared in the last hundred years, and an anchor dragged through them or a chain swinging over them tears up years of growth in minutes.
What the rules say
Since 2021 the maritime prefect has banned large vessels, from about 20 to 24 metres depending on the area, from anchoring in posidonia meadows along a protected strip of the coast. Most boats listed on Goon Boat are smaller than that, and on the larger yachts the skipper takes care of it. Other local rules apply to everyone: some coves have no anchoring zones marked in the beach plan, and the Port-Cros National Park around Porquerolles and Port-Cros has its own rules and mooring buoys. Your operator explains them in the briefing.
How to anchor the right way
- Look for sand. In clear water you can see it from the boat; a pair of sunglasses with polarised lenses helps.
- Use a mooring buoy when there is one. It is the easiest option and it protects the seabed.
- Let out enough chain for the depth, and no more than you need.
- When you leave, motor slowly over the anchor and lift it straight up instead of dragging it out.
Free apps such as Donia show the seabed, the depth and the local rules on a map, and the official SHOM charts mark the authorised anchoring areas.
Where to try it
The islands are the best place to see posidonia with a mask and snorkel. From Hyères you can reach Porquerolles and Port-Cros, and from Cannes the Lérins Islands are a short ride away. Anchor in the sand, jump in and look at the meadow from above.
Figures and rules from the French Ministry of the Sea and the maritime prefect for the Mediterranean.