The VIIRS satellite sensor can identify bright lights at night. This is the same sensor that is used to make the maps of lights at night of the world that you may have seen. In the dark ocean, vessels using bright lights stand out to this sensor and can be tracked. Some forms of fishing, such as squid jigging, use bright lights at night and squid fleets can be easily seen in the VIIRS data. The night light vessel detections activity layer on the map is a heatmap of likely vessels – both fishing and otherwise – detected by VIIRS.
To further explore the activity of vessels at night, there is a radiance filter on the night light detections layer, which can indicate different activities.
There is also the experimental ability to filter detections based on AIS matching and subsequent AIS records on the flag and vessel type. Global Fishing Watch has developed a sophisticated system to match available AIS data to respective night light vessel detections. This matching is done using a probabilistic model that determines AIS-message/VIIRS-detection pairs based on all available AIS records right before and right after the time the satellite VIIRS image was taken, as well as the probability of pairing a specific AIS message to any of the vessels appearing on that image.
A few caveats, though, are necessary for understanding these detections. One is that not all are actually fishing vessels – some may be other large vessels. Another caveat is that many vessels fish without lights, or with dim lights and would not show up in this dataset. Finally, there are “false positives” – places where the algorithm detects a light, but which result from errors or noise. There are also some cases of the algorithm picking up airplanes on international flights, although these are rare.
Nonetheless, the night light detections (VIIRS) data shows some clear fishing patterns that are not visible in other Global Fishing Watch data layers, helping to illuminate human activity around the ocean.