Dozens of countries just agreed to shut pirate fishermen out of their ports. Here’s why.

Indonesia has a ruthless strategy for dealing with pirates: blow up their boats.

Over the past two years, the Indonesian Navy has seized more than 100 vessels flagged from Malaysia, Thailand, Vietnam, and the Philippines — all accused of fishing illegally in Indonesian waters. Whenever this happens, authorities detain the crew, load the empty boat with explosives, and let it burn as a warning to others:

Except it doesn’t work. Or at least it’s not nearly enough. There are always more boats, more desperate fishermen looking for work. And the oceans are vast; it’s impossible to board every last pirate ship out there. Last year, New Zealand’s navy spent two weeks chasing illegal fishing vessels at sea before running out of fuel and giving up.

Worldwide, illegal fishing vessels now catch some 26 million tons of fish a year, worth $23.5 billion, or one-fifth of the entire world’s annual wild catch. It’s one of the biggest problems in marine policy. Experts say these catches undermine fishing limits that nations put in place to prevent fish stocks from getting depleted too rapidly and collapsing. Illegal fishing also undercuts the legal fishermen trying to earn a living. And many of these ships end up killing protected species, like sharks.

Since it’s tough to stop illegal fishing out on the ocean, many governments are now trying a new tactic — stopping the fishermen when they come into port. Which brings us to some news this week that didn’t get many headlines but could ultimately prove a very big deal.

On June 5, the United Nations announced that the Agreement on Port State Measures (PSMA) will go into effect after being ratified by 30 countries. This is the first legally binding international measure to stop what’s known as “illegal, unreported, and unregulated” (IUU) fishing.

The idea is pretty simple: No matter where a boat goes out on the high seas, it has to come into some port eventually to offload its wares. So countries will try to catch illegal activity at the ports. Under the PSMA, fishing boats will have to give advance notice before entering ports in these 30 countries, sending along extremely detailed information about their activities, fishing licenses, and catch totals and type. Landings can only happen at ports well-equipped for inspections.

The countries that ratified the PSMA, meanwhile, are obliged to inspect suspicious vessels or turn them away entirely. That last part is important. If an illegal fishing ship can’t actually land anywhere without hassle, it can’t make money.

“If countries suspect a vessel of illegal activity, they can take action directly against it. Or they can refuse entry,” says Tony Long, director of the Ending Illegal Fishing Project for the Pew Charitable Trusts. “When you consider what it costs to have enforcement at sea, this is a much more cost-effective way to deal with the problem.”

Now, this treaty isn’t perfect — China has yet to sign on, for instance — and it’s not sufficient in itself to stop illegal fishing. Instead, Long explains, it’s best to think of it as part of a much larger effort currently being stitched together to stop the practice.

(You can see a list of participating countries here. Thirty have officially ratified. Another 23, including Russia and Indonesia, have signed but not ratified. Of the latter group, 18, including Japan and Canada, have initiated the ratification process.)

One key aspect here is that port countries need to be able to figure out which vessels might actually be engaged in illicit activity before they can take action against them. And that’s not always so easy.

Interpol and other agencies have identified dozens of large fishing ships that regularly catch fish illegally. But these ships can evade detection by changing their names and flags before they come into port. To stop that from happening, Pew is pushing the International Maritime Organization to adopt a system that would give the world’s 185,000 large fishing vessels a permanent ID number that can’t be tampered with. (Many regional fishing management groups have recently required similar systems.)

Some experts also hope that satellite tracking will one day allow authorities to better monitor illegal fishing activity. There are already some efforts underway here: Every large ship in the ocean has an automatic identification system (AIS) that broadcasts its identity and location. A group called Global Fishing Watch developed an algorithm based on AIS to analyze vessel movements and figure out which ships are fishing ships.

In 2014, Oceana put out a report showing how this data could be used to monitor illegal fishing. It showed, for instance, that a Russian trawler had entered a protected refuge for salmon and made suspicious back-and-forth movements indicative of fishing:

Still, these satellite systems are far from perfect. For starters, smaller boats don’t use AIS (although Oceana and other groups are pushing to bring them into the system). Second, crews can manipulate their AIS to send false data. You need to gather lots of information from different sources, including the vessel monitoring system and radar, to figure out exactly what boats are doing and why. (Pew has a project called Eyes on the Sea to do just that; you can read about it here.)

“The point is that any system to stop illegal fishing will need layers and layers of information,” Long says. “You need port measures, tracking data, Interpol, a system for unique identification. But you can see how this all builds up.” The PSMA is an important part of that system, but it’s not the only one.

It’s worth mentioning at least three important weak spots in the current PSMA. First, it’s only been ratified by 30 countries. They’re big countries. Important fishing countries. But they’re far from the only countries. China hasn’t ratified it; Russia hasn’t either. Nor has Japan, although it has started the process. That still gives illegal fishing an outlet.

That said, many non-signatory countries could face growing pressure to do so. The European Union put the screws to both South Korea and Thailand after it encountered evidence that the countries weren’t properly overseeing their fleets, threatening trade sanctions. Both countries agreed to reforms and eventually ratified the PSMA.

It’s also worth mentioning that the Trans-Pacific Partnership, a trade deal between the United States and 11 other Pacific countries — including Japan, Malaysia, Mexico, New Zealand, and Vietnam — pushes signatory countries to implement the PSMA. It’s still unclear, however, if that trade agreement will get through Congress; a number of US environmental groups are opposed to other provisions in the deal.

Second, while the PSMA is legally binding, there are no penalties for failing to comply. At the same time, Long notes, governments are facing pressure from both the legal fishing industry and retailers who are seeing consumers increasingly demand clearer information about where their fish comes from.

Third, some of the poorer countries still lack the capacity to do effective inspections. Article 21 of the PSMA creates a monetary fund to help poorer countries implement these policies. And there are plenty of NGOs willing to work with these nations. Still, it’s a concern.

Like all international efforts, it’s a slow, grinding, often frustrating process. But we’re a tiny bit closer to the day when pirate fishing boats will no longer have to be blown up as a warning to others.