Thick as a torpedo and heavily scarred, the gigantic shark rises from the deep, circling the bait near the surface. At possibly 15 feet long, she is the biggest shortfin mako Riley Elliott has ever seen: "We were just struck with awe."
But it's what happened next that really floored the marine scientist.
A blue shark ten times smaller darted onto the scene and aggressively pushed the mako—the ocean's fastest shark—away. The bigger fish obliged, allowing the blue to eat in peace.
Shark scientist Riley Elliott observes unusual social behavior in blue sharks and mako sharks as they compete for food off the coast of New Zealand.
"What we saw broke a lot of theory in shark behavioural biology that size is the trump card," says Elliott, of the University of Auckland in New Zealand. Shark experts have always believed there's a clear hierarchy in how sharks interact with one another—namely, that size matters. But Elliott's observation seems to upend that.
While filming a new episode for Nat Geo WILD's SharkFest, Elliott traveled off the coast of New Zealand into the remote southwestern Pacific with a specific plan: To simulate a rare feeding event in the open ocean and observe how blue sharks and shortfin makos—both open-water sharks that tend to stick to their own kind—interact when forced into close proximity.
After baiting the water, up to 10 blues and six makos of various size and gender swarmed around the food—the first time this behaviour has been observed this far south in the open ocean.
Elliott and his team observed the animals slamming into each other, nipping tails, and popping their gills—social behaviours associated with feeding that are not often seen between highly migratory species like the blue and the particularly anti-social mako.
Why do sharks swarm?
"We basically observed these animals throwing caution to the wind. They put down their swords because they see that food is available," says Elliott. As the most exploited shark in the world—mostly for its fins—the blue is also one of the least studied, something Elliott is trying to change.
OUT OF THE BLUE
The open ocean is often called a blue desert. It's relatively low in life-forms with the exception of food-rich oases such as continental shelves and volcanic seamounts.
It's near a seamount that Elliott—a National Geographic Society grantee—and his team set up their experiment. They cut tuna into metre-cubed "ice pops" and chummed a half-mile trail over five days to lure sharks into the vicinity.
James Anderson, a biologist at the University of Hawaii at Manoa, noted that documenting such interspecies interactions is difficult—most studies that have examined shark behaviour in baited situations deal with only one species.
"So gaining a better understanding of the dynamics of social relationships between species is interesting and novel," Anderson says by email.
There's a caveat: Behaviours seen in a baited situation are not necessarily natural, he adds.
"They might be the best proxy we have for behaviours that might be seen in response to a drifting food source, such as a whale carcass, but by the same token, a large drifting whale carcass does not force the animals to have to compete for the same resource, in the way that some fish carcasses on the end of a rope would do," says Anderson, who is also a National Geographic Society grantee.
"Sharks could feed from different parts of the whale carcass without having to come together."
'JANE GOODALL THEORY'
To Elliott, the most "ground-breaking" part of his experiment was observing what he calls Jane Goodall theory—the idea that animals' behaviour is based on their own life experiences and personalities.
He says that could explain why the tiny blue shark dominated a mature mako—it had a lot of gumption, while the mako may have been more reserved. (Makos that size "don't get big by being dumb," Elliott quips.)
Anderson adds there may be other factors at play during that interaction: For instance, the major injury on the mako's nose and face may have impaired her vision, which could have prevented her from engaging in risky behaviours.
Above all, Elliott wants his work to educate the public and in turn stop the 90 percent decline in shark populations globally over the past 20 years—particularly in blue sharks.
For instance, though the slim, nocturnal hunters "aren’t the pinup boys of the shark world," they're abundant throughout the world's oceans and are major nutrient recyclers, he says.
Blue populations are still robust in the South Pacific—making it crucial to study the animals' behaviour and lifestyle in one of its last strongholds. (See "14 Not-Fake Shark Pictures From a Real Nat Geo Photographer.")
"My passion is to give people insight into this unexplored area of shark research," says Elliott. "There’s no better way to capture an audience than with imagery they’ve never seen before."