Earth’s oceans, which cover two thirds of the planet, are so immense and under-explored that it can be hard to imagine that they are a finite habitat and resource. But the dramatic drops in many fish stocks coinciding with the modern rise of industrial fisheries has made it abundantly clear that vessels must be closely monitored and policed in order to preserve ocean life, as well as the human livelihoods that depend on it.

Global Fishing Watch, an online platform developed by Google and the nonprofits SkyTruth and Oceana, was launched in 2016 to provide a public big data platform for this effort. The tool displays a near-real time map of fishing vessels across the globe, generated by billions of data points from automatic identification system (AIS) trackers onboard ships. These trackers, which were originally designed to prevent ship collisions, broadcast vessel locations through satellite networks. Since this information is public, Global Fishing Watch can literally put these vessels on the map for everyone to see.

That is, of course, assuming these ships want to be seen. An expansive new report released Monday by Oceana isolates several case studies of fishing vessels “going dark” by switching off their AIS trackers. While there may be legitimate security reasons to do this—evading pirates, for instance—the report suggests that these tactics could also be used to evade detection while illegally fishing in marine protected areas, or abusing other vulnerable fishing grounds.

“Until now, there was really no public platform for monitoring commercial fishing,” Lacey Malarky, an illegal fishing expert at Oceana, told me over the phone. “Before [Global Fishing Watch], that really wasn’t possible, so there would not have been a way for us to do this investigation. It would have taken forever—you would have to manually go through each individual event, whereas now we can use big data analytics, write out a code, and see where these events are occurring.”

To that point, Malarky and Oceana senior campaign director Beth Lowell spotlighted four cases of fishing vessels going dark over particularly suspicious timeframes and regions. In October 2014, for example, the Panamanian purse seiner Tiuna switched off its tracker on the western side of the Galápagos Marine Reserve, and turned it back on again 15 days later on the eastern end.

Tiuna AIS record. Image: Oceana

From 2015 to 2016, the Australian commercial longliner Corinthian Bay reportedly turned off its AIS ten times as it passed by the Heard Island and McDonald Islands (HIMI EEZ) Marine Reserve. Two Spanish vessels, Releixo and Egaluze, went dark multiple times in the waters off the West African coast.

It’s important to emphasize that no wrongdoing has been proved in these cases, but the report does show that AIS avoidance is a very common phenomenon on the high seas. “We wanted to highlight situations that were more suspicious, that were occurring near really vulnerable protected habitats that should be monitored and have effective enforcement,” Malarky said. “These are the four case studies that stood out to us, but there are millions more around the world in similar types of situations.”

When reached for comment by Malarky and Lowell, a representative of the Australian Corinthian Bay said that “the reason for the AIS turning off once inside the HIMI EEZ as you are seeing is to protect the commercially sensitive nature of our IP [intellectual property].”

Austral Fisheries, the company that owns Corinthian Bay, said that the IP being protected in this case is "the location of fishing grounds within the fishery," according to operations and policy officer Rhys Arangio.

"We had a history of illegal fishing in this fishery from the mid-90s to mid-2000s, and so our concern is the display of public AIS information in real time, or even to be able to analyze our historical positional information, as there is potential for unscrupulous operators to take advantage of that information," Arangio told me over email. "Additionally, it is in our commercial interests to not openly display our fishing positions to other licensed fishing vessels in this fishery."

On Wednesday, the Australian Fisheries Management Authority (AFMA) responded to coverage of the Oceana report with a statement emphasizing that Australian fishing vessels must carry two AFMA fisheries officers onboard as observers, and that their trips are monitored with Proprietary Vessel Monitoring System (VMS) signals. Unlike AIS, VMS data is typically only available to governments.

Meanwhile, the AIS avoidance of the Spanish vessels is currently being investigated by the European Commission and Spain’s government. That’s especially significant because these vessels went dark off the coasts of developing countries like Senegal and the Gambia, which are vulnerable to illegal fishing because they don’t have ample resources to spare in defense of their ocean resources.

“This is an unprecedented step,” Malarky told me. “The [European Union] has never started an investigation about a vessel turning off its AIS, so our report has resulted in the first investigation into AIS non-compliance by the European Commission and the Spanish government.”

“We hope that this report brings to light this important issue and that it’s really up to the governments to require all commercial fishing vessels to continually transmit AIS, and that they need to enforce those requirements,” she added. “It really is up to them.”

Oceana is also working on integrating other data sources into Global Fishing Watch, such as VMS signals. Indonesia and Peru have been persuaded to release their VMS information into Global Fishing Watch, and Malarky hopes to see more make that choice in the future.

“There should never be a question of whether fishing vessels are following the rules because they’re going dark to public tracking systems,” she said. “As more vessels are required to use [AIS], Global Fishing Watch will become a more comprehensive view of what’s going on, and researchers, NGOs, and scientists can continue to use it to help fight against illegal fishing and bring these issues to light.”

Update: This article has been updated with comments from Austral Fisheries and a statement from the AFMA.