A team of marine biologists departed from Honolulu recently on a large research vessel bound for a remote region of the subtropical Pacific Ocean. There, they hope to solve one of marine science's most confounding and persistent mysteries—the reason why great white sharks swim every spring to an area of the ocean dubbed by researchers as the "White Shark Cafe."
For almost 20 years, scientists who tag and track great whites have observed their long migration between the coastal waters of North America and this large stretch of sea about 1,200 nautical miles east of Hawaii. More recently, tracking devices attached to sharks have revealed that once the predators reach the Cafe zone, they make deep dives to depths exceeding 3,300 feet. The sharks—especially the males—do this repeatedly, dozens of times per day, for weeks on end, and scientists have virtually no clue what they're up to. Finding out could shed new light on a little explored region of the Pacific.
In a quest for answers, the researchers—led by Stanford University marine biologist Barbara Block—embarked from Honolulu on April 20th on the 290-foot-long Falkor with an arsenal of advanced tools, like tracking devices, sampling nets, and cameras. Experts in several fields, including deep-water marine ecosystems and advanced genetic sampling, are onboard to help operate camera-fitted submersibles, run acoustic sampling systems, deploy small trawl nets, and analyze DNA drawn directly from the water column. The scientists also hope they'll have the opportunity to fit a camera on the back of a great white shark and produce footage that sheds light on what the animals are doing in the Cafe zone.
Block said she hopes that a clearer understanding of when, where, how, and why the sharks visit the Cafe will lead to strict protection of these waters from industrial fishing. Great whites are already protected in most of the world, but Block said that to most effectively guard them against fishing—especially long-line gear, a major threat to many shark species—will require marine reserves "that connect the great white highway from Monterey to the Cafe and on to Hawaii." In fact, the United Nations is now considering granting protected status to the White Shark Cafe by designating the area a UNESCO World Heritage Site.
Centered at about 25 degrees latitude north, the White Shark Cafe encompasses a band of ocean with weak winds, relatively little upwelling, and low biological productivity, at least near the surface.
"The White Shark Cafe has long been considered sort of the desert of the Pacific, but the sharks are telling us there's something very, very important there, and maybe rather than an ocean desert it's a secret ocean oasis," said Salvador Jorgensen, a senior research scientist with the Monterey Bay Aquarium who has compared the animals' migration to Burning Man, the annual weeklong gathering of neo-hippies, artists, and the tech elite in the Nevada desert.
The scientists will be testing two existing hypotheses for why sharks that live the rest of the year along the West Coast of North America frequent the Cafe region: They are likely either going there to mate or to take advantage of a seasonal food source.
"Like any cafe, it's kind of a meet-and-greet spot," Block said. "There's clear evidence that we have a lot of males hanging around and females coming through, so it's possible that it's a place of some sort of sexual interaction."
If the sharks are hunting for food, it begs the question of what they're after that's so compelling. The scientists have no idea. Researchers have long believed that great whites shift from eating fishes as juveniles to eating marine mammals once they've surpassed about seven feet in length.
"So why would an adult white shark, which we know is a predator of pinnipeds, want to come out here and spend half of its annual cycle in this open-sea area?" Block asked. "What might be in the ocean water column that would attract something like the white shark to spend lots of time here?"
Researcher Aaron Carlisle, a marine biologist with the University of Delaware who is also aboard the Falkor, said it's clear that great whites are eating while in the Cafe, though he isn't certain what. He led research published in 2012 in the journal PLoS One in which he and his co-authors—including Jorgensen and Block—analyzed tissue from great whites collected in California. Through stable isotype analysis, the scientists were able to determine that the animals do feed to a limited degree while migrating far offshore. However, the sharks, the authors observed, tend to re-appear in California coastal waters looking "lean," and they concluded that, "although white sharks feed offshore, it appears foraging may not be the primary motivation for offshore migration."
More than six years have passed since that study, and Carlisle, Block, Jorgensen, and other top shark researchers still don't have the data to decide whether food or sex is drawing the sharks to the Cafe. Block said that mako sharks and salmon sharks—each a close cousin to the great white—also spend part of each year in offshore open-ocean waters.
Over the next four weeks, the scientists will be sampling deep waters with trawl nets with which they hope to catch creatures—possibly squid, Block said—that might be attracting the sharks. They also will be receiving data from a pair of autonomous seafaring robots called Saildrones outfitted with dozens of sensors. The Saildrones are already at sea, gleaning data from the ocean below and transmitting it to the Falkor, which is owned by the Schmidt Ocean Institute. The researchers also have the support of a deep-diving remotely operated vehicle and a free-swimming robot called a Slocum Glider.
But one of the first things they need to do is find the sharks. To do this, the scientists have spent the past year tagging coastal great whites with pop-up satellite tags. The tags were pre-programmed to begin popping off the sharks at appointed times this month and in early May. When each tag floats to the surface, it begins transmitting a signal. The plan is to make a beeline at the cruising speed of 10 knots straight toward these tags, not only to retrieve them and download tracking data but also, hopefully, to rendezvous with one of the animals. That will take some luck. It will take even more luck to lure the shark close enough for the scientists to pin a camera to its back.
To attach satellite tags to the sharks, the scientists use bait—often hunks of tuna—to draw the animals near the boat and then, using a long lance, stab the shark just below the dorsal fin, leaving the device trailing from its back. Cameras are fitted in much the same way, except that they are placed on the sharks' dorsal fins with a trigger-activated clamp at the end of a pole.
Onshore, other researchers are keenly waiting for results. Scot Anderson, a shark biologist who works with the Tagging of Pelagic Predators program and the Monterey Bay Aquarium, has collaborated with the expedition scientists on past research, including the past year's shark tagging project. He noted that it's anyone's guess just where the satellite tags will begin surfacing. "But hopefully the tags will pop up close together," Anderson said.
Block, too, said the size of the area creates fundamental challenges, and she compared the mission to "looking for a tagged moose in Maine with a VW bus."
Chris Lowe, director of the Shark Lab at the California State University at Long Beach, said many scientists have become skilled at finding sharks and clamping cameras to their dorsal fins in nearshore feeding aggregations.
"Whether they can find sharks and attract them to a boat out in the middle of the Cafe will certainly be a challenge, but it's worth a shot," he said.
Jorgensen said there has never been such a focused effort to study this part of the ocean, and he expects his team will turn up some interesting findings, especially with their camera-fitted underwater robot and, ideally, a shark cam. If they don't solve the White Shark Cafe mystery entirely, at least on this voyage, he said he won't be disappointed.
"The 'quest' is really half the fun," he said by email. "If it takes longer, it will be that much more satisfying in the end."