Going Dark: When Vessels Turn Off AIS Broadcasts

Something we hear often about tracking vessels with Automatic Identification Signals (AIS) is that transponders can be turned off. Fishers that don’t want to be caught doing something illegal or questionable will simply “go dark.”

It’s true. But the darkness alone is a big red flag. AIS is a safety feature that helps captains avoid collisions with other ships, so even if they turn it off while fishing, they eventually turn it back on, especially in a busy port. Unexplained gaps in transmission are invitations for further investigation—especially when they happen near an area where fishing is prohibited.

But not all instances of AIS signals dropping out are intentional. Weak signals, spotty satellite reception and interference can all cause gaps in AIS tracks. The first step in identifying intentional “off” events is to rule out all these other possibilities. We do that by analyzing the frequency and regularity of signals before and after a gap occurs. After discarding unintentional gaps, we’re left with dropped signal events that likely warrant a closer look.

In the example below, the green line shows continuously broadcast AIS signals from the purse seiner Egalabur as it leaves port and travels to the edge of the exclusive economic zone (EEZ, an area where a coastal nation has jurisdiction over natural resources). At the border with the EEZ and the open ocean the AIS signal from Egalabur goes silent and is not received again for 24 days (red line). When the signal begins again, the vessel appears to be fishing briefly before transiting back to port.

Continuous AIS signals broadcast by the purse seiner Egalabur indicated by green line. Red line shows a 24-day gap in transmission between point A and point B.

If we just connected consecutive signals, it would appear that the vessel traveled very slowly, taking 24 days to get from point A to the next signal at point B. In this example, the green line indicates the vessel’s known track based on continuous AIS signals. The dashed line shows us the gap between AIS signals. In all probability, the vessel intentionally turned off its AIS during the 24-day gap in transmission. It may have been fishing (either inside or outside the EEZ) during that time.

The tracks from Egalabur’s AIS signals may be easy to interpret. But it gets more difficult when AIS is turned off for a shorter period of time, or when interference with other signals creates gaps in vessel tracks.

AIS drop outs, intentional or not, can be complicated to parse out. The first challenge for Global Fishing Watch is to identify and understand as many complicating factors as we can. The next step is to create an algorithm that allows a computer to do it automatically every day all around the world.

In some areas of the world, turning off AIS may be more about keeping fishing grounds secret from competitors than it is about hiding illegal activity from regulators. In Dutch Harbor, Alaska, for example, there is a clear pattern of vessels switching off their AIS when they leave port and essentially disappearing from view until returning to port a few weeks later.

Alaska fisheries are very closely tracked and monitored with air surveillance, onboard observers and fisheries management vessel monitoring systems (VMS). It is very unlikely vessels would attempt to turn off their AIS for the purpose of fishing illegally. We believe they may be trying to conceal their precise location from other fishing vessels that are competing for the same catch. Nonetheless, the widespread practice of turning off their AIS when leaving is troubling.

Our hope is that more consistent use of AIS can be achieved globally through legislation and pressure for transparency among seafood consumers. Coastal nations around the globe have an inherent stake in protecting their fishing grounds from poachers. Increasingly, more countries and intergovernmental agencies are requiring AIS use within their waters. In coming years, more fishing vessels will be legally compelled to use AIS, and it will be harder for bad actors to go dark without being noticed.

We believe that in addition to legislation, the transparency Global Fishing Watch creates will increase voluntary use by fishing vessel operators who want to demonstrate their adherence to regulations. It’s in the best interest of seafood distributors to be able to vouch for the legality of their product, and it’s in the best interest of consumers to ensure there will be seafood in the future.

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