"I would dig through scientific literature to see what we knew about it scientifically," Kajiura says, "and was surprised that no one has ever studied this."
So in 2011, he started doing aerial surveys of sharks to determine how many were hanging around, when they came and went, and what environmental factors might correlate to their movement. He's a pilot, so he began flying over the ocean in his Cessna 172, usually bringing along a student, covering stretches between Miami Beach and Jupiter Inlet once a week. He would get back to his office at Florida Atlantic University and look at the high-definition footage caught on video from equipment rigged up to his plane. He could then count the number of sharks in the water between the beach and about 200 meters offshore.
"I go every week form December to April," Kajiura says. "Shark season."
Last week, the professor explains, "I was flying along and thought, 'There are a lot of sharks today; let me post that video.'" The footage has since gone viral. He estimates there were 10,000 blacktip sharks at the time, with 99 percent of them hovering between Palm Beach and Jupiter. "And those are just the sharks in the video survey. We know there's more because we could see them on other side of the plane [out of the video frame]. That was a gross underestimate."
And they will all stick around until mid- to late March, when they start heading north.
But don't fear — blacktips, he says, are generally skittish and move away from people in the water, though they are responsible for more bites on humans than any other shark — but mostly in the area of Melbourne or New Smyrna Beach, where the water is cloudier and they mistake a surfer's hand for the small fish they eat.
Kajiura says he wanted to learn where sharks go when they leave here, so a few years ago, his team started fishing for sharks also. The critters would be caught around Riviera Beach, implanted with a transmitter, and released. This allows researchers to see where the sharks migrate along the coast.
One hundred percent of the 50 or so sharks he's caught around Palm Beach County are male, he says. "Maybe they're going to South Beach or something," he joked, before saying that he has no knowledge of homosexual behavior in sharks.
More likely, the females are just slow and tired and not migrating as far as the males because they are very pregnant at this time of year after mating around July and getting ready to give birth 11 months later, in June. Colleagues in Cape Canaveral have found female sharks there.
Ten thousand blacktip sharks might seem like a lot, but Kajiura explains that those are all the ones along the Eastern Seaboard. "It looks like a lot when they're all together," but in the summer, they'll move up near Virginia and spread out, he says.
Sharks are actually "in danger of overexploitation," the professor says, and he believes a widely used estimate of 100 million sharks per year killed by humans is accurate. They are killed for meat but, more problematically, as bycatch. Sharks "only have a handful of pups every two years, and it takes them a long time to reach sexual maturity," Kajiura says.
The prof grew up in Ontario, Canada, watching Jacques Costeau. He studied at Florida Tech and the University of Hawaii. His work, he says, is funded by Shawn Colgan and the Colgan Foundation.