Suspected illegal fishing revealed by ships’ tracking data

Fishing vessels have legitimate reasons to turn off their position-tracking systems — but there are some suspicious reasons, too.Credit: Anthony Wallace/AFP/Getty

When fishing vessels hide their locations, they sometimes reveal a wealth of information. Gaps in tracking data can hint at illegal activity, finds a modelling study1.

Some ships carry automatic identification systems (AIS), which pinpoint their locations and help to prevent collisions, but can be turned off manually. Researchers studied gaps in the tracking data to identify hotspots where fishing vessels frequently disabled their devices on purpose — and to explore the possible reasons. The findings suggest that vessels hid up to 6% of their activity — more than 4.9 million hours between 2017 and 2019. Some of these gaps could mask illegal fishing, finds the study, which was published in Science Advances this month..

The study uses holes in tracking data “to tell us more about what we’re not seeing, what we’re missing”, says Juan Mayorga, a marine data scientist based in Santa Barbara, California, who is part of the National Geographic Society’s Pristine Seas project. “That is a really valuable contribution.”

Expensive problem

Illegal, unreported and unregulated fishing costs the global economy up to US$25 billion each year. It is also detrimental to marine life, and some evidence suggests that it is linked to human-rights violations such as people trafficking. Heather Welch, a spatial ecologist at the University of California, Santa Cruz, and her colleagues analysed more than 3.7 billion signals from vessels, sent over three years and recorded in the Global Fishing Watch AIS data set. The team used a model to distinguish between gaps caused by vessels intentionally turning off their AIS and those that were due to technical issues. Gaps of 12 hours or more when ships were at least 50 nautical miles from shore in areas with adequate signal reception were suspected to be intentional disabling.

Source: Ref 1.

The team found that 82% of time lost to AIS disabling happened on ships flagged from Spain, the United States, Taiwan and the Chinese mainland (see ‘Flag of origin’). Although most vessels that use AIS come from middle- and upper-income countries, so the data are biased towards those countries, the study says. “AIS is not feasible for a lot of countries globally at the moment,” says Claire Collins, a marine social scientist at the Zoological Society of London.

There are many reasons vessels intentionally turn off their AIS, says Welch, and not all of them are nefarious. For instance, crews might hide their location in areas where pirates are a threat, or might obscure their position from competitors when fishing in a bountiful area. More iniquitous reasons to hide a ship’s location include trying to mask illegal fishing or unauthorized transshipment — transfers of cargo between ships at sea — she says.

The team used another model to investigate what was behind the intentional AIS signal gaps, looking at factors such as how productive an area is for fishing, the risk of piracy and the level of transshipment activity. The results indicate locations in which the signal gaps are potentially nefarious, but they cannot definitively say whether these gaps hide illegal activity, says Welch.


The model revealed 4 hotspots for intentional AIS disabling: 16% of gaps occurred next to Argentina’s exclusive economic zone, 13% in the Northwest Pacific Ocean, 8% adjacent to the exclusive economic zones of West African nations and 3% near Alaska. Apart from Alaska, these hotspots are already regions of concern for illegal, unreported and unregulated fishing. They produce a lot of fish and have limited management, partially because of their locations in the high seas. Signal gaps near exclusive economic zones indicate that vessels could be hiding that they are crossing boundaries without authorization to fish in restricted areas, says Welch. “If they were allowed to go in that zone, why would they disable their AIS?” she says.

Drifting longlines were the fishing vessels found to disable their AIS most often, followed by tuna purse seines (see ‘Out of sight’). Intentional AIS disabling events were also common near transshipment hotspots. Offloading catch at sea helps to reduce costs, but past research has linked it to human trafficking and slipping illegal catch on to the market.

Source: Ref 1.

The research is a good way to start exploring what AIS-disabling data can expose, and could help researchers to conduct finer-scale studies in the future, says Collins. “It’s a really important study.”

Mayorga agrees that the data will aid fishery managers in understanding the magnitude and patterns of illegal fishing, helping them to zero in on specific problematic regions and improve enforcement of laws at sea.

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