The world's largest fish are vanishing without a trace

Ship collisions with the world’s rarest whales have garnered global attention. But until recently, scientists didn’t realize these same vessels are likely killing whale sharks, an endangered species and the world’s largest fish.

In a new study, over 75 researchers came together to quantify the threat shipping may pose to whale sharks, which live in warm tropical and sub-tropical waters around the world, including the Indian, Pacific, and Atlantic Oceans.

The scientists mapped whale shark aggregation hot spots—known as constellations because of the sharks’ star-like patterns across their backs—in 26 countries and overlaid them with information on the positions of large ships, provided by Global Fishing Watch, a nonprofit that uses technology to increase transparency around how our oceans are used and managed. (Learn the secrets of whale shark migration.)

They found the sharks were most in danger in heavily trafficked regions of Ecuador, Mexico, Malaysia, the Philippines, Oman, Seychelles, and Taiwan. While it’s unknown how many sharks are dying, their proximity to so much shipping activity suggests the burden is high, says lead author Freya Womersley, a researcher at the Marine Research and Conservation Foundation and the University of Southampton in the U.K.

“For the first time, we were able to map most whale shark constellations globally thanks to contributions from the whale shark research community,” Womersley says.

Whales, such as right whales, stay near the surface to breathe air, making them particularly vulnerable to hitting ships. Although whale sharks, which often reach 32 feet long, don’t need to come up to breathe, they spend around half their time cruising at the surface feeding on plankton.

“The data the study pulled together from tagging whales sharks all over the world just shows how much time they're spending in that high-risk zone,” says Michael Heithaus, a shark ecologist at Florida International University who was not involved in the study, published in May in the journal Science of the Total Environment.

As shipping increases globally, these experts say it’s vital to act now to protect sharks: There are more than 100,000 ships moving goods worldwide, and the number could grow by as much as 1,200 percent by 2050.

Vanishing without a trace

Why have shark deaths gone unnoticed? A main reason is the lack of evidence.

“Marine mammal bodies can float or wash up on shore, so the issue is a lot more visible,” says Womersley. Sharks, however, are negatively buoyant, so they sink to the bottom when they die. (Read why scientists are spotting fewer whale sharks.)

This also means the damage of large ships to whale sharks is likely underestimated. Many whale sharks that have collided with small vessels carry around large scars, for instance.

“Whale sharks are big and pretty hardy so if they have a non-lethal injury, they can often bounce back,” says Heithaus. “But when they get hit by one of these big ships, they're not going to survive.”

It’s unknown if the number of animals being killed is large enough to affect population sizes. Over 400 individuals aggregate in Mexico’s Yucatán Peninsula each year, while nearly 500 sharks have been identified off Madagascar.

“We're trying to put together a puzzle piece by piece to help protect the species before it's too late,” Womersley says.

‘A real win-win’

Two tactics already work well to prevent ship strikes among marine mammal species: reducing vessel speeds and re-routing ships. Womersley says governments, industry, scientists, and nonprofits can use this knowledge to work together to avoid killing whale sharks. (Read why sharks are still being killed at high rates.)

For instance, by simulating vessel movements, the new study showed that reducing vessel speed by 75 percent increased transit time by just five percent, while making it much easier for ship captains to see and avoid sharks. Another simulation showed re-routing ships around core habitats was even less disruptive for shipping, with transit time increasing by just 0.5 percent—an average of around 2.4 hours per vessel.

Heithaus was surprised how relatively small changes could have an impressive impact for the sharks without really harming the industry.

“You have a real win-win in terms of conservation without a massive trade-off for people,” he says.

He adds people are generally willing to make small changes if they will make a big impact, particularly for a charismatic species such as the whale shark, which is worth millions of dollars worldwide to local communities through ecotourism.

Designating whale shark habitats that are off limits to large ships—temporarily, seasonally, or when a certain number of sharks are present—could also make the waters safer for whale sharks and other species, says Womersley. 

For example, on the U.S. East Coast, speed restrictions are enforced at certain times of year to protect endangered North Atlantic right whales.

“We have a duty to protect one of the world's most beautiful and charismatic species, which has been on this Earth millions of years before humans.”