As ships traverse the oceans, many of them continuously transmit their position, speed, and identity to satellites. This automatic identification system was originally developed to prevent collisions, but by training Google’s machine-learning tools on the data, the Global Fishing Watch (GFW)—a nonprofit founded by Oceana, SkyTruth, and Google—can identify different kinds of fishing vessels, and work out where they’re dropping their lines and nets.

They quantified that activity by dividing the oceans into a huge grid of around 160,000 squares. Each of these squares had sides that span half a degree of latitude, and an area of around 3,100 square kilometers. And in 2016, around 55 percent of them included some kind of fishing activity. “You’re dividing the ocean into 160,000 cells, and you’re seeing fishing activity in half of them,” says Kroodsma. “One reason this resonated with people is that however you slice it, that’s impressive.”

Actually, it’s misleading, says Amoroso. He and his colleagues had spent years trying to measure the impact of trawlers and based on their early results, the GFW’s estimates smelled fishy. The problem is that they divided the ocean into such large squares that if a single boat drops a net in an area the size of Rhode Island, that area would count as “fished” in a given year.

Fortunately, the GFW made all their data freely available, so Amoroso’s team could analyze it at finer resolutions. When they divided the ocean into smaller squares that are 0.1 degrees of latitude wide and take up around 123 square kilometers, they found that fishing activity occurs in just 27 percent of them. And when they used even smaller squares that are 0.01 degrees wide and 1.23 square kilometers in area—the size of a city block—just 4 percent of the ocean is “fished.”

Consider these two pictures, which show the proportion of the North Pacific region that’s affected by trawlers. The red striped region is a 59,000-square-kilometer area where trawling was completely banned in 2016. If you use big 0.5-degree squares, the entire space is incorrectly classified as “fished.” If you use the finer 0.01-degree squares, it’s not.

This seems like a simple matter of scale. If you look at the world at different resolutions, you’ll see different pictures. So the question becomes: Which is the right resolution to use? In a response to their critics, Kroodsma and his team argue that when it comes to fishing, there’s no right answer.

Consider trawlers. They drag large nets behind them to catch bottom-dwelling species that don’t swim very far. So you can roughly work out how much ocean they affect by looking at the area that’s swept by their nets. For that calculation, small squares seem appropriate.

How many fish are in the sea?

Longline vessels are very different. They tow fishing lines that are studded with thousands of baited hooks. These lines can extend for kilometers, drift with the currents in any direction, and intersect the paths of open-ocean fish that can themselves travel extensively. A single yellowfin tuna, for example, can roam over more than 250,000 square kilometers. These movements mean that longlines affect an area of ocean that’s far greater than the area beneath the boats—or even the hooks. And that’s why the GFW divided the ocean into such large squares.