Indonesia's strict crackdown on illegal foreign fishing boats is paying off, according to new research.
Kicking out interlopers has relieved pressure on the country's overtaxed fisheries at no cost to its domestic industry, the study says, and may point the way for other countries to make their fisheries more sustainable.
About a third of the world's commercial fish populations are overfished, according to the U.N. Food and Agriculture Organization.
One study estimated that restoring depleted fisheries would ultimately generate $53 billion in additional annual profits.
But reducing overfishing usually means putting unpopular restrictions on local fishers to allow populations to recover.
"Telling fishers to stop fishing for a few months or years would be something that's not that realistic," said study lead author Ren Cabral at the University of California, Santa Barbara.
Violators will be sunk
But in Indonesia, as in many developing countries, locals are only part of the equation. Many foreign vessels fished the country's waters, often illegally.
The study notes that the country lost an estimated $4 billion per year to illegal fishing before 2014, when the government banned foreign fishing vessels in its waters.
Since then, more than 300 ships found violating the ban were evacuated and sunk.
Cabral and colleagues wanted to see what the impact had been.
Using government registries, vessel tracking data and satellite imagery, they saw a drop of more than 90 percent in the time foreign vessels spent in Indonesian waters. That meant at least a quarter less fishing activity overall.
"That's huge," Cabral said.
The study is published in the journal Nature Ecology & Evolution.
"You have a large benefit, but the cost to local people is zero," said marine biologist Boris Worm at Dalhousie University, who was not involved with this research.
Do this first
"This paper argues, I think convincingly, that this is the first thing you should do: if you want to fix fisheries in your country, first, kick out the fishers that don't need to be there," he added.
Worm notes that the study could only account for large vessels that are required to carry tracking equipment. It could not assess what smaller vessels are doing.
"You're really only seeing the tip of the iceberg," he said. "The tip of the iceberg is getting smaller, which is good in this case. But there are a whole lot of problems below."
With foreign fishing boats out of the way, local fishers are filling in the gap. If not managed properly, they could undo the benefits of fighting illegal fishing, Cabral said.
If Indonesia continues to ban illegal fishing and also manages local fishing sustainably, the study estimates profits would be 12 percent higher in 2035 compared to today.
On the other hand, if local fishing remains unchanged, 2035 profits would drop by half as fish populations declined.
"The next step would be Indonesia managing their local fishing effort," Cabral added. "If they do that, they can definitely get the benefit from their policies."