Whale sharks, the world’s largest fish, spend much of their time swimming slowly, swallowing mass quantities of tiny creatures such as krill, as befits such a colossal filter-feeder. But this portrait is incomplete—the giants have more complex hunting habits than previously thought.
New observations in Western Australia’s Ningaloo Reef show these animals can hunt in tandem with other predators including tuna, other sharks, and even diving seabirds. In a recent clip, captured in March 2020 by photographer Tom Cannon, at least three whale sharks were seen swooping in on a ball of baitfish, a behavior rarely caught on camera.
“I’ve watched the footage hundreds of times, and it still blows my mind,” says Emily Lester, a postdoctoral fellow at the Australian Institute of Marine Science and lead author of a research article, publish in February that details the encounter.
While krill and other plankton make up the bulk of whale sharks’ diets, scientists have long known that small fish like anchovies and sardines—and even the occasional squid—do appear on the menu. Pinning down the specifics of when, where, and why these animals opt for more sizable sustenance, however, is a difficult task.
“Whale sharks can be really challenging to study despite their huge size because they’re so mobile,” says Lester. Not only can the behemoths swim across entire oceans, they’re also known to dive thousands of feet beneath the surface.
Whale shark meetups like the one at Ningaloo—which occurs each year between March and August—offer a unique opportunity for scientists and ocean enthusiasts to observe these elusive animals in shallow, nearshore waters. Even then, notes Lester, happening upon a feeding event like this is akin to finding a needle in an ocean-sized haystack.
It’s likely that whale shark bait ball feeding occurs more frequently than it’s caught on camera. Anecdotal evidence of the behavior has been documented at aggregations throughout the world’s tropical oceans. Eyewitness reports of such events in Western Australia, for example, go back more than 20 years. But underwater photographic evidence like this is rare. Each new piece of footage holds unique clues about what drives whale sharks to the bait ball table, and which strategies they use to make the most of it once they get there.
When you’re longer and heavier than a school bus, eating is all about conserving energy. The whale sharks in the Ningaloo footage were seen ram feeding–charging at speed through the center of the bait ball—as well as positioning themselves vertically beneath it, primed to hoover fish into their mouths. Both of these tactics require more effort than swimming slowly along, mouth agape, which makes it imperative to land a calorie-dense gulp when foraging this way. (Learn more: Whale sharks may live up to a century, Cold War bomb dating reveals.)
Ningaloo Reef is renowned for its biodiverse waters. Situated 750 miles north of Perth, the protected UNESCO world heritage site is home to relatively intact predator populations. Feasting beside the dappled giants in Cannon’s footage were other large fish such as tuna, trevally, and whaler sharks, as well as a divebombing cohort of wedge-tailed shearwaters, seabirds in the petrel family. Lester and her colleagues wonder if the Ningaloo whale sharks may be leaning on their agile neighbors to do some of the heavy lifting.
“The interactions made my mind race with a whole bunch of questions,” Lester says. “We know that when bait balls are around, other predators do these spatially-coordinated attacks to maximize their foraging efficiency. What if the same thing is happening here?”
Slow and ungainly?
Whale sharks certainly can manage the feat alone: a group of seven whale sharks were filmed corralling a school of anchovies in the Gulf of Tadjoura, off the coast of Djibouti, in 2017. This encounter, and reports of others like it in the region, occurred during the off season–a time when whale sharks typically migrate elsewhere. It’s possible that for certain individuals, the benefit of high-calorie fish feasts outweighs that of lengthy swims in search of smaller prey. Targeting the same bait ball as other predators might further enhance the sharks’ energy gains.
“By themselves, whale sharks will definitely chase fish around but, bless their spotty hides, they're relatively slow and ungainly,” says marine biologist Simon Pierce, who has been conducting research on whale sharks for more than 15 years, and was not involved with the recent study.
“They don't seem to be very successful on their own, but when other predators are there attacking the baitfish, forcing them to contract into a defensive ball, the largest mouth gets the most fish.”
Whale shark mouths can reach a whopping five feet across. While that’s bad news for baitfish, the birds in Cannon’s footage were not in any danger. Despite having around 300 rows of tiny teeth, whale sharks do not chew. What’s more, whale shark throats are only about as wide as a grapefruit, which makes it unlikely that anything as big as a shearwater could go down the gullet.
Having a giant shark scoop up the lion’s share of your meal seems like a major disadvantage, but there could be some benefit to the birds and other fish as well. “I wonder if this is a two-way interaction,” Lester muses. “After the whale sharks charge through the bait ball, can those other predators capitalize on the isolated fishes that are separated?” More work is needed to tease apart the dynamics at play, but these observations provide an intriguing start.
“This is only one interaction, and we got incredibly lucky to get footage of it,” Lester says. “But I’m hoping that we get even luckier in the future.” Ningaloo’s thriving ecosystem together with the countless ocean enthusiasts who document it, put that goal within reach.