Whale sharks, the largest fish on Earth, have been mysteriously vanishing from the ocean without a trace over the last 75 years—and marine scientists couldn’t figure out why. When they die, the bodies of these massive, endangered sharks, which are typically 18 to 32 feet long and weigh about 15 tons, sink to the seafloor, so researchers can’t easily do a post-mortem.
Now, new research suggests that cargo ships and other large vessels are likely responsible for killing these gentle giants, contributing to a worldwide population decline of around 50 percent in the last 75 years. A new study in the journal Proceedings of the National Academy of Sciences finds that colliding with large ships “may be a greater cause of death for the world’s largest fish … than anyone previously realized,” Freya Womersley and David Sims, marine ecologists at the University of Southampton and study co-authors, write for the Conversation.
To probe the sharks’ unexplained disappearances, the researchers compared the satellite-tracked movements of 348 whale sharks with maps of global shipping routes in the Atlantic, Indian and Pacific oceans from 2005 to 2019. They found a lot of similarities: 92 percent of sharks’ horizontal space and nearly 50 percent of their vertical space overlapped with large vessel traffic, putting them on a potential collision course with ships that are larger than 300 gross tons.
Sharks were most at risk while spending time near the water’s surface (where they regularly feast on zooplankton), the researchers found. The most dangerous places for whale sharks to swim, according to the data, are the Gulf of Mexico, the Arabian Gulf and the Red Sea, where some of the world’s busiest ports are located.
The overlapping territories are problematic because sharks swim horizontally at speeds that are up to 10 times slower than vessels, the researchers note in the paper. And, when the sharks do notice an oncoming ship, their reaction time is slow and minimal until the vessel gets very close. Sharks that get hit by fast-moving ships stand little chance of surviving.
Though scientists don’t know how many whale sharks have died as a result of run-ins with cargo ships, they did see some harrowing anecdotes in the data: Trackers showed some whale sharks swimming along normally, then slowly sinking to the seafloor. Writing for the Conversation, Womersley and Sims described this as “the smoking gun for a lethal ship strike.”
After accounting for technical failures, they also found that 24 percent of the satellite tracker tags attached to the sharks stopped working in busy shipping areas, which the researchers believe indicates that the whales died after being struck by vessels.
And cargo ships aren’t just killing whale sharks, they’re also harming as many as 75 other marine species, from dolphins to manatees to penguins. So while it’s handy to be able to order a product that’s made on the other side of the world and have it arrive in a matter of days, the growing ocean shipping industry—which expanded from 1,771 vessels in 1995 to more than 94,000 in 2020, according to the researchers—is leading to huge, unintended consequences for marine wildlife.
“Shipping is a serious problem for giants of the sea,” Robert Harcourt, a marine ecologist at Australia’s Macquarie University who was not affiliated with the study, told Vox’s Benji Jones. “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.”