What’s killing the world’s biggest fish?

The largest fish on Earth is a shark. Capable of reaching a length of up to 60 feet — roughly the height of a four-story building — whale sharks, named for their size, are so large that they make great whites look like minnows.

But even giants can disappear. Over the last several decades, more than half of all whale sharks have vanished from the ocean. Some populations have fallen by more than 60 percent.

This decline has been something of a scientific mystery. It can’t be explained by known threats like overfishing. And because whale sharks sink when they die, there aren’t bodies washing up on shore that researchers can study.

Now a new clue has emerged, and it’s a big one. A study in the journal Proceedings of the National Academy of Sciences reveals that cargo ships are likely a leading cause of whale shark deaths. Often, where you find high densities of these endangered fish, you also find shipping traffic, the authors found, and ships are already known to strike and kill these animals.

Our lives as consumers connect us to this seemingly far-away problem. More than 80 percent of all internationally traded products are carried by cargo vessels, such as TVs and cookware and whatever device you’re reading on now. And the world’s marine fleet is rapidly multiplying, growing from 1,771 large vessels in 1995 to more than 94,000 in 2020. The ocean is now full of highways packed with ships.

Whale sharks are not the only roadkill. Vast cargo vessels harm many species of marine giants, such as the endangered North Atlantic right whales, and some smaller creatures, like sea turtles. Ships also emit loud noises that disrupt marine life and spew planet-warming carbon dioxide into the atmosphere.

“Shipping is a serious problem for giants of the sea,” said Robert Harcourt, a marine ecologist at Macquarie University in Australia who was not affiliated with the study. “We have an economy that’s derived from moving things around the world in a way that’s not taking into account the cost to the environment.”

Ships strike at least 75 different kinds of marine animals

Last fall, a large tanker painted red, white, and blue pulled into a harbor in southern Japan. Draped limply across its bow was a dead, 39-foot whale.

A dead Bryde’s whale seen hanging over the bow of a ship.Mizushima Coast Guard Office

Tragic photos that document whale strikes are rare, but the strikes themselves aren’t. They affect at least 75 different species of marine animals, according to a recent review, and likely kill thousands of them each year. Creatures that tend to hang out near the ocean’s surface are especially vulnerable, such as whale sharks and sea turtles.

A good step toward decreasing collisions is figuring out where animals are most at risk, and that’s where this new whale shark study comes in. Large ships are required to report their locations, and the authors compared those points to the movement of hundreds of whale sharks, which they had previously tagged with satellite trackers. (This is no easy feat: “You’ve gotta have some nice long fins, a good pair of lungs, and sprint after it underwater,” said David Sims, a marine ecologist at the University of Southampton and a study co-author.)

The results revealed just how vulnerable these fish are: More than 90 percent of the ocean’s surface area that whale sharks use overlaps with the routes of tankers, passenger ships, and fishing vessels. Whale sharks tend to congregate near the coast, where shipping is especially busy, according to Freya Womersley, a doctoral student at the University of Southhampton and the study’s lead author.

She also discovered that many of the sharks’ tracking devices stopped working when the animals entered busy shipping lanes, perhaps because they were killed by ships. (Some trackers even showed sharks swimming into dense shipping routes and then sinking slowly to the seafloor — “the smoking gun for a lethal ship strike,” as Womersley and Sims wrote in The Conversation.)

Sharks cruising in the Gulf of Mexico, the Arabian Gulf, and the Red Sea were at the greatest risk of collision, according to Womersley. “Not only are they spending a lot of time at the surface where they may be vulnerable to being struck, but they’re also occupying the same places that some of these ships are moving through,” she told Vox.

North Atlantic right whales near Duxbury Beach, Massachusetts.David L. Ryan/The Boston Globe via Getty Images

While scientists don’t know exactly how many sharks boats have killed, they do have this information for other marine giants including the North Atlantic right whales. Ships killed at least a third of the right whales that died in the last few years, and they injured more. Today, only about 360 remain. (“Right whales are notoriously bad at not being run over by ships,” Harcourt said.)

Other kinds of whales, mackerel sharks, otters, manatees, and a whole host of other creatures are vulnerable, too, according to the review. But physical strikes are only part of the problem.

Noisy ships mess with animals’ senses

While most humans perceive the world through sight and dogs see the world through smell, many whales and dolphins rely on sound. For them, sound is everything: It’s how they map their environment, find prey, and talk to each other, often through hundreds or thousands of miles of ocean.

Shipping throws a big, clanging wrench in this strategy. Over the last 50 years, there has been a 32-fold increase in low frequencies of sound along the world’s major shipping routes, largely caused by giant propellers. Some whales use those same frequencies to communicate. (Fish — including whale sharks — don’t use sonar to communicate, so this is likely not a big problem for them.)

A container ship in East China’s Jiangsu Province.CFOTO/Future Publishing via Getty Images

As a result, in noisier areas, some whales seem to be getting louder, for the same reason you might yell when you’re talking to friends at a loud bar, said Daniel Costa, a marine ecologist at the University of California Santa Cruz who was not involved in the whale shark study. “Whales have already started speaking louder to make up for the increased noise,” he said.

Scientists have also discovered that sound can interfere with communication and disrupt behaviors such as hunting prey, sleeping, and mating. More than 150 studies have found that noise has significant effects on marine life, according to a recent review. (I recommend listening to some of this six-minute audio track that accompanies the paper. You can hear what the ocean sounds like with and without shipping.)

Big cargo ships also pollute the air with carbon emissions. They use some of the dirtiest fuel on the planet and produce a similar amount of carbon emissions as the aviation industry (roughly 3 percent of global greenhouse gas emissions), as Vox’s Umair Irfan reports.

Those emissions accelerate climate change, which harms all kinds of marine life. Ironically, warming can even make some animals more prone to ship strikes. For example, North Atlantic right whales are traveling north into Canadian waters in the spring and summer as the ocean warms, where until recently they weren’t protected against ship strikes. “Climate change keeps reshuffling the deck,” making it hard for regulations to keep up, Costa said.

Toward a safer sea

Making oceans safer for marine giants is conceptually simple, and one option is to route ships away from animal hot spots. A 2015 study, for example, found that shifting a shipping lane near Sri Lanka just 15 nautical miles south could reduce the risk of ships hitting blue whales by 95 percent. (Advocates are now pushing for this change.)

Even just slowing ships down can make a huge difference. The chance that a cargo ship will kill a whale falls to below 50 percent when it’s moving at around half speed (10 knots, or 11.5 miles per hour), compared to nearly 100 percent when it’s moving more quickly, according to one 2006 study.

This so-called “slow steaming” is also less noisy and requires less fuel. Just a 10 percent speed reduction may lead to a 19 percent cut in greenhouse gas emissions, as Irfan reports. And the fuel bill is cheaper, too.

But there’s a big drawback to ships slowing down or going on a different route: It takes longer to deliver goods. That’s one reason studies like this don’t always translate into shipping restrictions. That drawback also makes alternative approaches, such as designing quieter ships or adding wildlife deterrents or propeller guards, appealing (although the benefits of these technologies aren’t well established).

But other than shopping locally to reduce shipping, it’s something we can do right now, Womersley said, and the payoff would be massive. Many marine giants are at the top of the food chain, where they stabilize ocean ecosystems. They also help fertilize the ocean and capture huge amounts of carbon that could otherwise fuel climate change, as old-growth trees do. These animals are also awesome. They don’t just include the largest fish on Earth, but the largest animal to have ever lived (the blue whale). All of which makes getting our goods a little bit later seem like a pretty reasonable trade-off.