On the six-month, 10,500-mile journey an albacore tuna typically makes from where it is caught in the southern Indian Ocean to a can on a U.S. grocery store shelf, it passes among countless hands—and from fishing boats on the high seas to larger vessels that ferry it to port in a process called transshipping. Now, for the first time, a series of satellite- and data-based analyses of the fishing supply chain shows how common this bucket brigade–like process is in the fishing industry, and could illuminate how it helps hide illegal fishing.

Transshipping “is something that we didn’t have a scientific handle on. Now we have eyes in the sky that can see some of this,” says Boris Worm, a marine ecologist at Dalhousie University in Nova Scotia who co-authored two new studies on the topic, published this week in Frontiers in Marine Science and Science Advances.

Transshipping is a decades-, if not centuries-old, practice employed for efficiency: It means fishing boats have to make fewer trips back to port, thus saving fuel, and can harvest fishes at sea for months or even years at a time. About half of all fishing that occurs in the so-called high seas (waters 200 nautical miles or more from shore) may involve transshipping, according to the Frontiers in Marine Science study. But there is a well-established lack of oversight or monitoring of these transfers, and this can muddy the origins of catches when authorities are trying to enforce regulations. The vast quantities of fishes caught illegally each year—about one in every five fishes that reaches the U.S. market, according to a robust worldwide study published in 2014—can easily be mixed with legal catch. Illegal fishing can deplete fish stocks, impede the recovery of fish populations and ecosystems, and costs the industry between $10 billion and $23.5 billion per year, a 2009 study showed. International organizations, including the United Nations Office on Drugs and Crime, have also investigated and found (pdf) ties between transshipping and the smuggling of weapons and drugs—and even human trafficking.

Douglas McCauley, a marine scientist at the University of California, Santa Barbara, who was not involved in the new work, likens transshipping to a person standing alone in a dark alleyway, waiting to meet someone else. “It’s not illegal, but it’s a behavior that’s often characteristic of something illegal,” he says. This new research, he adds, shines a headlight into the alleyway.

The dots on this map show transshipment-related behavior, with red denoting when two boats travel closely to one another for extended periods of time, and black denoting when a boat slows in one area for a long amount of time. Credit: Nathan A. Miller, Aaron Roan, Timothy Hochberg, John Amos and David A. Kroodsma, and Frontiers in Marine Science 

The group behind this work tracked fishing craft worldwide using global, publicly available data from systems vessels employ to communicate their locations with one another. Whereas the data has its limitations—it only tracks craft of a certain size, and boats can turn off their communications—it suggests when transshipping is happening based on the time vessels spend slowly drifting near a fishing boat or loitering in one place and possibly waiting for another craft. The marine biologists partnered with a team from Global Fishing Watch, a nonprofit consortium partly funded by Google, Bloomberg Philanthropies and the Leonardo DiCaprio Foundation, that aims to make the global fish supply more transparent. The researchers also worked with SkyTruth (a nonprofit outfit focused on using satellite technology to monitor changes on Earth’s surface) to map these interactions. The resulting map shows where transshipping most commonly takes place: Nearly 40 percent of all encounters occurred within 200 nautical miles of Russia’s northern coastline, with other concentrations off the coasts of Argentina, Peru and west Africa. Through vessel identification networks, the research identified the types of ships most often used in probable transshipments.

Worm says this work is just beginning to clarify the role of transshipping in the fishing industry, and that the team plans to next look at specific regions where the practice was found to be particularly high to identify areas that could benefit from more oversight. Along with recent cases of illegal fishing that involved transshipments—such as the thousands of protected sharks found dead in a Chinese vessel in the Galápagos Islands last summer—research is spurring people in the fish supply chain to track the process more widely, says Mark Young, a project director at The Pew Charitable Trusts who was not involved in the research. Retailers and consumers know the source of the grapes that comprise their wine or the beans that make up their coffee, he notes—but they rarely know the origin of their fish: “Transshipping is one of the last key pieces of the seafood supply chain that is not well understood by the general public,” Young says. His team launched electronic tracking of Atlantic bluefin tuna in 2010, allowing for more accurate record-keeping.

Such technology helps, but the complexities of maritime regulations and a lack of political will still leave the high seas relatively lawless—what study co-author Nate Miller, who works with SkyTruth, calls the “wild wet.” People working in this field believe scientific research is a crucial antidote. “We create our own political will by showing how many players [there are] and how pervasive [transshipping] is,” McCauley says. “These papers are pushing that forward.”