In 1950, about 1.7 million fishing vessels of all shapes and sizes plied the world’s oceans, but just 20 percent of them had motors, limiting their range and the amount of fish they could collect. Now, 65 years later, the number of boats has jumped to 3.7 million fishing vessels, 68 percent of which are motorized in some form, an increase that is putting more and more pressure on the world’s oceans, according to a new study published in the journal PNAS.

The boom in fishing vessels doesn’t mean there’s plenty of fish in the sea. In fact, it’s quite the opposite. To understand the health of fisheries, ecologists calculate “catch per unit of effort (CPUE)” or the amount of effort and resources it takes to catch a fish. Fishing boats today only catch about 20 percent of the fish for the same amount of effort as boats in 1950, reports Erik Stokstad at Science.

“[D]espite its advanced technology and increased numbers, the modern motorized fleet is having to work much harder to catch fewer fish,” says the study’s lead author Yannick Rousseau, a graduate student studying fisheries ecology at the University of Tasmania, in a press release.

To gain a comprehensive view of the global fishing fleet, Rousseau studied national registries, scientific papers and local reports to find out about fishing in 100 countries, according to Science’s Stokstad. He then analyzed data about three classes of fishing vessels: industrial boats, motorized small-scale boats and unmotorized, and artisanal, small boats.

The increase in fishing vessels, however, did not happen evenly across the globe. While fish stocks in most of North America, Europe and Australia have stabilized in recent decades due to more stringent regulations, that’s not the case in much of the rest of the world. Fleets in Asia, for example, have increased by 400 percent over the same time period, reports Jen Christensen at CNN. And the number of fishing boats hitting the water won’t decrease anytime soon—the study estimates that by 2050, another 1 million fishing boats will be chasing fish.

“CPUE reflects how many fish are caught for the amount of effort expended, such as during a day’s fishing, and this measure paints a dark picture of the state of the ocean’s resources,” Rousseau says. “In recent years, a sharp drop in CPUE in Southeast Asia, Latin America and the Southern Mediterranean indicates their fisheries expanded at a much faster rate than fish stocks could support.”

Things are likely to get worse before—or if—they get better. Study co-author Reg Watson, a fisheries ecologist at University of Tasmania, tells Stokstad that the world has not seen peak fishing yet. In coming years, boats with larger motors will begin moving more and more out of territorial waters and into the high seas, exploiting the remaining fish stocks.

And there’s not that much left to go around. According to the United Nations Conference on Trade and Development, 90 percent of the world’s fish stocks are either currently overused or completely exhausted.

The new study, however, might help change things. Watson says he hopes fisheries managers around the globe could use the data on the fishing fleets to design fishing regulations and keep track of illegal fishing vessels.