Indonesia, one of the world’s leading producers of tuna, decided several years ago it had had enough of illegal foreign fishing boats entering its exclusive economic zone and taking an average of $4 billion a year in fisheries profits. In 2014 the Southeast Asian nation—a vast archipelago of more than 13,000 islands—imposed a one-year moratorium on fishing vessels built abroad, in order to evaluate their impact.

During the moratorium officials discovered boats disguising foreign ownership under local names, falsifying Indonesian fishing permits or using the same permit for multiple boats. This evidence augmented other reports of foreign vessels underreporting the sizes of their boats, avoiding taxes and intruding in waters reserved for local small-scale fishers.

To address the problem, Indonesia created a task force consisting of the country’s navy, marine police, coast guard and attorney general’s office. Task force members started out by aggressively capturing illegal foreign boats and deporting their crews. Then, to drive the point home, they cut, torched or dynamited holes in bottoms of the boats—sending hundreds of vessels to the seafloor to join the fishes they had sought. According to Mas Achmad Santosa, special advisor to the Indonesia Presidential Task Force to Combatting Illegal Fishing, “We sank 363 illegal fishing vessels to send a signal of strong enforcement and to create a deterrent effect.”

This immediately drew international media attention, and Indonesian minister of Maritime Affairs and Fisheries Susi Pudjiastuti traveled the world and appeared in YouTube videos defending her country’s aggressive strategy.

Yet has all this drama done any good when it comes to protecting the world’s troubled fish stocks? A group of fisheries experts from Indonesia and the U.S. has now analyzed catch results and fisheries models in Indonesian waters (its exclusive economic zone). The study, published in April in Nature Ecology & Evolution, found total commercial fishing shrunk by 25 percent following Indonesia’s explosive enforcement efforts against foreign boats. This followed an earlier finding, reported in 2016 in Proceedings of the National Academy of Sciences, which showed a 15 percent reduction in catch could put fisheries on a trajectory toward sustainability. “Instead of putting the pressure on local legal fishers to recover fisheries, the Indonesian government put their attention on foreign illegal, unreported and unregulated fishing, which is a huge problem in Indonesia," says postdoctoral researcher Reniel Cabral at the University of California, Santa Barbara, lead author of the new study. Applying this strategy in other countries could generate similar long-lasting results in many other regions of the world, the researchers concluded.

The earlier PNAS study had postulated an initial short-term reduction in fishing pressure could return fisheries to sustainable yields. But countries have been reluctant to impose such restrictions on normal catches for local fishing communities; the study defined “short-term” as an average of 10 years of catching far fewer fishes—a highly unpopular prospect for local fishermen and their families. Cabral refers to such measures as the “valley of death” for politicians and community leaders.

Determining the length of quarantine necessary to produce sustainability in future fish stocks had been the goal of the PNAS paper, a massive study of 4,713 fisheries representing 78 percent of the global reported fish catch. From the results, scientists projected the percentages by which fishers would have to reduce their catches of targeted species in order for the fishery to become sustainable. The science behind those reductions was also applied in a March 2018 Sciencepaper that examined 20 marine mammal, bird and sea turtle populations. That study found that if fishing was curtailed in close proximity to those animals—again for an average of 10 years—not only would the fisheries recover, but so could these rarer species, which were mostly caught accidentally.

To determine if Indonesia’s enforcement efforts had worked, Cabral's recent study looked at the skipjack tuna fishery—the largest in Indonesia. These fishers work at night using special lights to lure their catch, and researchers analyzed data from the Suomi National Polar-orbiting Partnership satellite to identify their boats. Team members correlated this information with automatic identification system (AIS) data (processed via an online monitoring project called Global Fishing Watch) from onboard signals that identify and track positions of ships at sea to avoid collisions. The team also examined boat identification data from the country's vessel monitoring system (VMS), which licensed boats are required to use in Indonesian waters. Models were then used to project future growth. The results showed the total of all commercial fishing in Indonesia—legal and illegal—shrunk by 25 percent after the country started sinking boats

The Indonesian enforcement effort looked not only at the number of illegal fishing boats, but also their sizes. Cabral’s team recorded a 40 percent reduction in fishing effort by boat weight. In other words, it was the really big foreign-built fishing boats (greater than 100 metric tons) that did the most damage to fish stocks. The Indonesian government is currently planning to build more than 3,300 new boats for its local fishers. These boats will have a much smaller size distribution than the foreign boats they replace; thus the total fishing effort should decline despite the addition of new boats. But Cabral cautions the domestic fisheries need to be contained as well, lest they dim prospects for sustainability. “There’s room for expansion but not uncontrolled growth,” he says. “Fisheries expansion requires that management be in place before sustainable limits are reached.” In the U.S. the Magnuson–Stevens Act in 1976 virtually wiped out all illegal fishing activity by foreign vessels. Overinvestment in U.S. domestic fishing capacity, however, prevented many stocks from recovering. Subsequent revisions have set most U.S. fisheries on a trajectory toward long-term growth without compromising the needs of future generations.

Since Indonesia implemented its boat-sinking policy, its ranking in the list of (legally and illegally) most-fished nations by foreign boats dropped from 13th in 2014 to below 80th in 2015 and 2016. This reduction has persisted despite the increase in distant water fishing (fishing in international waters or other countries' exclusive economic zones) that is currently happening across most of the world. Meanwhile Indonesia's seafood export volume in 2017 increased by 7 percent compared with 2016, and export value increased 17 percent during the same period. “The fish are getting closer to the shore and can be harvested by our fishermen to sustain their livelihoods,” Santosa says.

Some researchers remain skeptical of the attention given to Indonesia’s rather extreme methods, however. “So you go into Indonesia and you sink a bunch of boats, and you send all these people home,” says James Estes, a professor of ecology and evolutionary biology at the University of California, Santa Cruz. “What are they going to do? Is that really going to solve the problem or are they [illicit fishers] just going to go somewhere else?”

It is likely they will indeed seek out areas where enforcement is weak. But Cabral’s study shows a country does not have to starve the local fishing community to achieve sustainability.