A unique study reveals that commercial fishing operations now cover 55 percent of Earth's waters – four times the area devoted to terrestrial agriculture. The research finally quantifies the footprint of the largely unsustainable industry, a difficult task given its massive scale and the size of the world’s oceans.

To generate a map of fishing boat movements, a collaborative team of marine scientists and Google experts turned to a new data source: satellite navigation records. Their study, published in Science, analyzed 22 billion automated identification system (AIS) messages from more than 70,000 fishing vessels sent between 2012 and 2016. Developed to help ships avoid collisions, AIS automatically tracks the ship’s position, speed, and heading every few seconds.

Unsurprisingly, known coastal fishing hotspots off of Europe, South America, East Asia, the Pacific Northwest, and Africa were identified as the most heavily trafficked regions.

Vast swathes of the open ocean also hosted a good deal of fishing activity. Fleets from China, Spain, Taiwan, Japan, and South Korea accounted for more than 85 percent of the total high seas fishing documented. Determining which areas are currently targeted for trawling, netting, and other forms of harvesting, and which areas remain unaffected, is crucial information for ongoing and future conservation efforts.

The satellite data overlaid on a map, showing the areas that host the most fishing vessels. Kroodsma et al./Science, 2018

According to the satellite data, some areas in the open ocean appear to have no significant fishing activity, yet the authors note that this finding is likely caused by poor AIS connectivity. Using information from nearby regions that do have satellite coverage, the team calculated that the true extent of industrial fishing is around 73 percent.

As jarring as this number may seem, it’s actually less than a previous estimate of 95 percent. This suggests that large areas could be designated as protected marine reserves, in which depleted seafood stocks could recover, without lowering the current supply. (Although it may be too little, too late for some regions.)

Another interesting observation from the study is that fishing activity often corresponds more to the fishermen’s schedules than it does to changes in pricing or availability.

“We find that global patterns of fishing have surprisingly low sensitivity to short-term economic and environmental variation and a strong response to cultural and political events such as holidays and closures,” the authors wrote.

But encouragingly, a sizable reduction in fishing occurs during China’s annual fishing moratorium. The nation’s fishing fleet is the largest in the world and harvests extensively from marine areas around the globe.