OCEAN · FIELD GUIDE
What Fishing-Vessel Density Shows — How AIS Reveals Where Boats Work
A fishing-fleet map looks like it is tracking boats. It is not. It is something subtler and, in its own way, more useful: a portrait of where fishing effort piles up, built from millions of position reports added together. Here is how that picture is made, what it can tell you, and what it cannot.
A fishing-fleet map is one of those things that looks simpler than it is. The bright streaks and hot patches feel like they are showing you boats. What they are really showing you is time — the accumulated hours that fishing vessels have spent in each square of sea.
From a safety signal to a map of effort
Every commercial ship of any size carries an AIS transponder. It exists for a mundane, life-saving reason: so ships can see each other in fog, at night, and around busy headlands. Several times a minute, each vessel broadcasts its position, its identity, and — usefully for us — its type: cargo, tanker, passenger, fishing, and so on.
None of that was designed to make maps. But if you collect every one of those little broadcasts over a month, keep only the ones from boats that declared themselves as fishing vessels, and tally them into a fine grid, a landscape appears. The grounds that fleets work hardest light up. The lanes they run to reach those grounds thread between them. And the water they rarely bother with stays dark.
What the colours mean
On this map, the brightness of each one-kilometre cell is hours of fishing-vessel presence per square kilometre, over a month. Brighter means more boat-hours in that cell. It is deliberately a measure of presence, not of catch — nobody is reporting tonnes of fish into this grid. Presence is a good proxy for where effort goes, but it is a proxy, and it is worth holding that distinction lightly in your head as you read the map.
Where it is reliable — and where it isn't
The honest edges of this picture matter as much as the picture itself:
- It sees AIS-carrying vessels. That is most of the commercial fleet, but not every small inshore boat. The smallest craft can be under-represented.
- Presence is effort, not catch. A bright cell tells you boats spent time there. It does not tell you what — or how much — they brought up.
- It is monthly, not live. This is a settled, published record of a past month, not a real-time tracker. It is built for seeing the pattern, not for chasing a particular boat.
- It is regional. This layer covers European seas and their neighbours. It is the openly-licensed, commercial-friendly record for that region, which is exactly why it can be shown here.
Read with those limits in mind, a fishing-vessel density map is a quietly powerful thing: a way to see the human footprint on the sea not as a headline but as a shape — the real geography of where a continent goes fishing.
Frequently asked questions
What is fishing-vessel density?
Fishing-vessel density is a measure of how much time fishing boats spend in each small patch of sea over a period — usually a month. Instead of plotting individual boats, it divides the sea into a grid (here, one-kilometre cells) and counts up the hours of fishing-vessel presence in each cell. Cells where boats spend a lot of time glow bright; cells they rarely enter stay dark. It is a map of effort and presence, not of any single vessel.
How is it built from AIS?
AIS — the Automatic Identification System — is a radio signal that ships broadcast to avoid colliding with each other, giving their position, identity and type several times a minute. Coastal antennas and satellites pick these signals up. To build a density map, every position report from vessels classed as 'fishing' is collected over a month and summed into the grid. Because the boats report their own type, the map can isolate fishing vessels from cargo ships, tankers and ferries.
Is this a live, real-time map of fishing boats?
No — and that is an honest distinction worth keeping. This layer shows a month of accumulated presence, published after the fact. It is not a live tracker following individual boats minute by minute. That makes it excellent for seeing the pattern of where fleets work — the grounds, the lanes, the quiet water — but it will not tell you which boat is where right now.
Does it cover the whole world?
The layer here, from EMODnet Human Activities, covers European seas and neighbouring waters — not the entire planet. Europe maintains an unusually complete, openly-licensed record of vessel activity, which is why a commercial-friendly map exists for this region. Global fishing-effort data exists too, but the best-known global source is licensed for non-commercial use only, so it is not used here.
What can't fishing-vessel density tell you?
Three things to keep in mind. First, it sees vessels that carry and transmit AIS — most commercial boats, but not every small craft. Second, presence is a proxy for effort, not a direct measure of how many fish are caught. Third, a bright cell means boats spent time there, which usually but not always means fishing — a vessel can be steaming through, sheltering, or waiting. Read it as 'where the fleet is active', not 'how much was caught here'.
SEE IT LIVE
Everything in this guide is on the live ocean map.