"The seals aren't easy to catch, and to be able to do it at night is a big step up for them," he said.
The Great (Photographic) Shark Hunt
Catching a breach-feeding event on film at night was an even more difficult task.
Johnson's team fitted a hungry shark—one they'd named Big Mama—with an acoustic "pinger" tag so they could follow her movements from a boat.
They then baited Big Mama with a robotic seal decoy that was tough enough to survive her teeth but forgiving enough not to injure her jaws.
The system created a realistic chance of a breach-feeding event within sight of the boat. But a big problem remained—how to film the shark's actions in the dark.
Because great whites are warm blooded, Big Mama's body was some 9 degrees Fahrenheit (5 degrees Celsius) warmer than the water, which was 63 degrees Fahrenheit (17 degrees Celsius).
When Johnson's team was filming, the great white shark showed up clearly on a thermal camera, and image intensifying lenses were used to capture reflected light and create a familiar "night vision" view.
When great white sharks hunt from below in daytime hours, they spot their prey by silhouette against the lit background above them.
Mosselbaai's seals use the cover of darkness to swim to their hunting waters and back—perhaps to avoid being easy targets for sharks.
Their silhouettes, however, might still be visible from below against the moon or the ever-brightening lights from the growing town of Mosselbaai.
"The lights from the city are cast down over the island, and maybe that gives the sharks enough visibility to be able to perform these procedures at night," Johnson speculated.
Shark scientists Pete Klimley and Scott Davis remotely tracked great white shark movements around Año Nuevo Island, off central California, and were not involved with Johnson's research.
"You could speculate forever" on how exactly the sharks might manage their nighttime hunting, said Klimley, a marine animal behavior specialist at the University of California, Davis.
"It probably has to do with some moonlight illuminating the prey, or [perhaps] some bioluminescence. Or it may also be that the seal is making a sound and sharks are very sensitive to sound," he said.
Klimley and Davis said their research suggested nighttime feeding activity, but they did not directly observe the phenomenon.
"I always suspected that the sharks were indeed continuing to forage," Davis said. "Just because we can't see them during the night doesn't mean they aren't feeding."
According to Johnson, knowing how the sharks are feeding could help people "make more informed decisions about where and when they hop into the water."