Great White Shark Filmed Breaching at Night -- A First
Although seals are a common meal for great white sharks, under the cover of darkness, one might expect the marine mammals to catch a break. Dramatic new footage, however, proves great whites sometimes fancy a midnight snack.
The video shows a 13-foot (4-meter) shark launching itself from the water to snare a seal at the surface—the first time such "breach feeding" has been documented at night.
The footage appears in Sharkville, which aired July 25 on the National Geographic Channel. (National Geographic News and the National Geographic Channel are both a part of the National Geographic Society.)
Welcome to "Sharkville"
South African marine biologist Ryan Johnson and his team got the shot in a section of South Africa's Mosselbaai (Mossel Bay) dubbed Sharkville, where a large population of great whites lives nearly year-round not far from a popular beach.
The sharks don't bother the bay's human bathers. But a colony of some 5,000 seals on a small island just a half mile (800 meters) from shore isn't so lucky.
Johnson, founder of the South African Marine Predator Lab, first discovered that Mosselbaai's great whites breach the water's surface to feed at night through a lot of hard work and a bit of luck.
In 2005 he'd been following about 165 feet (50 meters) behind a shark fitted with an acoustic tracking tag for 103 straight hours.
"It was nighttime, so we didn't expect a lot of action," he remembered.
"And 30 meters [100 feet] from us, it erupts from the water with a seal in its mouth and sits there chewing on it," he said. "There was enough moonlight to see this going on."
Johnson said the event forced his team to reconsider how the sharks feed.
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.
Midnight Cravings?
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."
Posts
No comments:
Post a Comment