Skip to content
posidonia-watch

About this map

posidonia-watch maps where boats anchor on protected Posidonia oceanica seagrass meadows, across the whole Mediterranean and month by month, using only free and open satellite data.

Why Posidonia matters

Posidonia oceanica is a seagrass found only in the Mediterranean. Its meadows shelter juvenile fish, keep beaches from eroding, clear the water and store large amounts of carbon in the sediment beneath them. It is also extremely slow: a meadow spreads by a few centimetres a year, so a scar left by an anchor and chain can take decades to centuries to recover.

Anchoring is one of the main local pressures on these meadows, and it is concentrated in popular summer bays. Several countries now restrict it – France, for example, bans anchoring on Posidonia for vessels of 24 m and longer in many areas. Existing studies of anchoring pressure usually cover a single bay. This project tries to give a consistent, Mediterranean-wide picture.

Data sources

Exact inputs of the current release: GFW Sentinel-2 vessel detections (CC0-1.0); EMODnet Seabed Habitats – Seagrass cover (EOV), version 2025 (CC-BY-4.0); Flanders Marine Institute – Maritime Boundaries Geodatabase (EEZ), version 12 (CC-BY-4.0).

Method

  1. Detect. Every vessel Global Fishing Watch found in a Sentinel-2 image, with its estimated length and speed, is the starting point. Low-confidence detections, fixed infrastructure, ice and cloudy scenes are filtered out.
  2. Anchored. A detection counts as anchored when its inferred speed is below 1 kn.
  3. On Posidonia. An anchored detection counts as “on Posidonia” when it lies inside a mapped meadow, buffered by 20 m to allow for position and pixel uncertainty.
  4. Large. Vessels with an estimated length of 24 m or more.
  5. Aggregate. Detections are counted per H3 hexagon at resolution 7 (~5 km² per cell), per month and per country (EEZ).
  6. Normalise. A place that is often cloudy is seen less often. So the map's density divides the number of boats on Posidonia by the number of clear satellite overpasses (less than 20 % cloud) of that cell. It reads as “how many boats are anchored on Posidonia here, on average, at the moment a clear image is taken”.

Monthly totals on the Trends page are raw counts and are not normalised for cloud cover.

AIS matches

Larger boats broadcast their position, speed and identity over AIS. Global Fishing Watch matches each Sentinel-2 detection against these broadcasts, and the match ships with its open data. On the map, boats with a confident match (confidence ≥ 0.9) have a dark ring, and their popup shows their flag state and the length and speed they reported. The Boats panel recounts the hexagons, the monthly bars and the single boats for AIS match (confident matches only) or AIS: anchored(matches whose own AIS speed is below 1 kn, the only boats whose anchoring is confirmed).

AIS also checks the method: a boat counted as anchored should report almost no speed. Of the 21,900 confidently matched boats counted as anchored on Posidonia in this release, 11,670 (53 %) reported less than 1 kn. 9,822 (45 %) reported 1 kn or more: they were under way when the image was taken, and the image-based speed misses that. For the other 408, AIS gives no speed. The popup marks the boats AIS reports as moving. The counts on this site still include them, and boats without AIS cannot be checked.

No vessel names, MMSI numbers or other identifiers are published. A flag is shared by thousands of boats. Matched boats are mostly larger and more often commercial than the average boat here, so they are not a representative sample.

Limitations

This release

Open source and licences

Code (MIT), pipeline, data contract and issue tracker are on GitHub. Corrections and better seagrass data are very welcome.