In the high-stakes war to end illegal fishing, big data is increasingly becoming a potent weapon against the poachers and pirates of the high seas.

Case in point: A recent research study by SkyTruth and Global Fishing Watch that analyzed more than 21 billion satellite signals collected from hundreds of refrigerated cargo vessels (known as ‘reefers’) that roam the world’s oceans.

The satellite signals revealed more than 90,000 at-sea rendezvous behaviors consistent with a practice known as ‘transshipment’ – the transferring one vessel’s cargo to another.

Transshipment is common in the fishing industry, and sometimes a necessity. Large fishing vessels that are frequently at sea for months, or years, at a time need to meet up with cargo vessels to transfer fish catches and resupply fuel and food.

The problem is that many of these transshipments occur outside any nation’s exclusive economic zone (EEZ), away from the prying eyes of regulators and free from any monitoring for criminal activity.

That creates an environment ripe for abuse. It allows fishing vessels to catch multiple loads of fish without ever returning to port for inspection, making it impossible to track their catch and determine if the ships are fishing illegally.

Moreover, unregulated transshipments can facilitate modern slavery. Nefarious operators can prevent their crew, often immigrants from poor nations such as Burma, from ever leaving ship.

“A fishing vessel that never goes into port never has to endure any sort of oversight,” says Paul Woods, chief technology officer at SkyTruth, which operates Global Fishing Watch through a partnership with Oceana and Google.

“We found a surprising number of vessels that would spend a year at sea, never coming into port. The transshipment is what facilitates that kind of extreme activity. It’s hard to imagine what conditions on a vessel like that are like.”

The good news is that technology is shining a light into the dark corners of global transshipments, providing countries and regional fisheries management organization with useful data that can help them crack down on bad actors.

While large-scale commercial fishing has traditionally been managed at port, satellite technology and computer modeling is providing regulators with a much clearer picture of what’s happening over the horizon.

Working with an online data program co-developed with Google, SkyTruth and Global Fishing Watch were able to create maps from signals sent by cargo vessels’ automatic identification systems (AIS) – which track their movements at sea.

They were able to identify nearly 800 reefers, about 90% of the refrigerated cargo vessels used in fishing that were active worldwide as of 2010. Analysis of the data found 86,000 instances in which the ships’ speed slowed to less than 2 knots from more than 8 hours, a pattern consistent with transshipments.

It found another 5,000 instances of ‘likely transshipments,’ in which cargo vessels encountered a fishing vessel that was also sending AIS signals, at distances of less than 500 meters apart for longer than three hours.

The data do not provide definitive proof of wrongdoing, but the patterns of global transshipment appear to be associated with patterns of illegal, unreported or unregulated fishing. Relatively fewer transshipments were discovered, for example, around North America and Europe, jurisdictions with stronger fishing management regimes. Far more potential transshipments were recorded off the coasts of Peru and Argentina and around Africa.

“The scale of the transshipment activity we found is kind of jaw dropping. It is a pervasive global problem,” Paul says. “We were not surprised where the hotspots were, because we knew where these kinds of things happen.

More than 40% of these potential transshipments happen in the high seas, outside the 200-mile exclusive economic zone.

In some areas – such as Palau and Indonesia – fishing occurs inside a nation’s EEZ, but vessels move outside the zone to offload their catches and avoid detection and inspection. These vessels skirt the law by shifting their transshipments just outside the line of national jurisdiction.

The threat to fish populations from illegal fishing masked by unregulated transshipments is, of course, significant. Migratory species such as tuna, in particular, are threatened.

“Transshipment facilitates the overfishing of tuna everywhere,” Paul says. “Tuna are a migratory species, so they move across the entire ocean. And it’s the economies that are created by transshipment – and the opacity in terms of regulation – that facilitate the overfishing of tuna.”

SkyTruth and Global Fishing Watch have made this dataset publicly available, providing conservation groups with new insights to shape their advocacy, such as trying to persuade buyers to stop purchasing from providers with transshipments in their supply chain.

Governments in nations like Peru and Indonesia can use the data to better understand fishing and shipping patterns inside and outside their territory, and build better policy to combat illegal fishing.

“That is the kind of outcome we are looking to support,” Paul says.

The Walton Family Foundation provided funding to support SkyTruth’s research.