Why it matters
Governing: Decreto 25/2018 (BOIB 93), preamble; RD 191/2026 (BOE-A-2026-5877), preamble. Builds on: nothing. Start here.
The plant under your anchor
Posidonia oceanica is not a seaweed, whatever the locals and the beach bars call it. It is a flowering plant (a seagrass) found only in the Mediterranean. It grows as dense meadows (praderas) on sand and rock, and it grows slowly. RD 191/2026 describes it as a slow-growing climax species. Contrast Cymodocea nodosa, the other protected seagrass in the state decree, which is a fast-growing pioneer that often colonises bare sand first.
The Balearic decree gives the local picture:
| Point | What the preamble of Decreto 25/2018 says |
|---|---|
| Area round the islands | more than 650 km² of meadow |
| Depth | mostly 0–35 m, down to 43 m in Cabrera National Park |
| Share of Spain's inventoried meadows | about 50 % |
| Inside Natura 2000 | about 75 % |
| World Heritage | meadows between Ibiza and Formentera, UNESCO 1999 |
| Oxygen | up to 20 litres of O₂ per m² per day |
A 2026 press report quotes a lower figure (591 km²) from an NGO report. Different surveys, different dates; use the decree figure and do not argue about it on the aft deck.
What a meadow does
- Nursery and habitat. It is the main biodiversity hotspot in Balearic waters, including the habitat of the fan mussel Pinna nobilis, now endangered.
- Beach protection. Dead leaves wash up as banks (arribazones) that absorb wave energy and hold sand on the beach in winter. The shells of creatures living on the leaves become beach sand.
- Carbon store. RD 191/2026 cites an IUCN estimate that Posidonia meadows may account for 40 % of the carbon stored each year by coastal vegetation. Even dead meadow (mata muerta: rhizomes, roots and sediment with no leaves) still stores carbon and is the base for any future restoration.
- Water clarity. The meadow traps sediment and nutrients. The turquoise water your guests photograph over the sand patch exists partly because of the dark meadow next to it.
Why anchors are the problem
Both decrees list mechanical damage from anchoring among the main pressures, alongside dredging, poor sewage treatment, trawling and warming. An anchor dropped on a meadow tears out rhizomes; the chain then sweeps an arc every time the wind shifts. Because the plant grows so slowly, a scar cut in one afternoon does not close in a season. Multiply that by the number of yachts in a popular cala in August and you have the reason the law now reaches the chain, not just the anchor.
CAUTION — Legal status Posidonia is a priority habitat (1120) under the Habitats Directive 92/43/EEC and is on Spain's list of wild species under special protection (RD 139/2011). That is why damaging it is an offence under Ley 42/2007, not just bad manners. Lesson L2.3 covers the fines.
What this means on deck
You will anchor over sand, pick up a buoy, or go somewhere else. Everything in this course follows from that sentence. The rules in Module 2 tell you what is forbidden; Modules 3 and 4 tell you how to comply without ruining the guests' swim.
TIP — Reading the bottom Meadows look dark from above; sand looks pale turquoise. In good light and clear water that is usually enough to start with, but depth, glare and cloud shadows fool the eye. Confirm with the official maps (L3.1) and your own eyes on the bottom before you let go.