Sharks In The Water Stanford Case Study

Slim chance of shark attacks, according to California-focused study

  • Scuba divers are 6,897 times more likely to be hospitalized for diving-related decompression sickness than for white shark bites.
  • Ocean-goers are 1,817 times more likely to drown than die from a shark attack.
  • Scuba divers have a 1-in-136 million chance of being bitten.
  • Surfers have a 1-in-17 million chance of being bitten

Source: Stanford News Service

According to a study out of Stanford's Hopkins Marine Station, shark attack risk on the California coast has dropped by more than 91 percent since 1950.

The study, published in Frontiers in Ecology and the Environment, weighted the number of reported attacks in the last 60 years in the state against the number of people using the ocean for swimming, diving, surfing and other sports. While the number of statewide attacks has remained relatively steady (about one per year on average in the '50s to one-to-two per year now) the number of people in the ocean has ballooned.

"So right now your chances to have a shark interaction is much lower than it was in the '50s," says Francesco Ferretti, a shark researcher who studies the human impact on ocean health.

The best hypothesis for explaining this pattern, Ferretti says, is the amazing recovery of pinnipeds -- seals, sea lions, elephant seals -- in California. These animals were basically hunted to extinction at the beginning of the last century, exploited for fur and fat. Their recovery began in the mid-20th century. "Now they are skyrocketing in many places around California," Ferretti says.

Ferretti and co-researchers suspect that since seals tend to live in large groups in predictable areas -- such as the Farallon Islands or Año Nuevo, north of Monterey Bay, white sharks know where to go to get a meal. It's possible they're not spending as much time roaming up and down the coastline as they once did.

As predators, sharks play key roles in ocean ecosystems.

"The sharks are kind of the police of the ocean," says Ferretti. "They keep in balance things that [would] get out of control."

The presence of top predators helps ensure stability and balance in fish stocks.

For example, it's suspected that on the East Coast, killing sharks contributed to the collapse of a scallop fishery in North Carolina, because sharks kept the rays -- which eat scallops -- in check.

Ferretti's data is practical too. He says learning about the ecology and movement of sharks can do much more to decrease the risk of shark attack than culling sharks.

Risk of shark attacks is highest, he says, in the months of October and November in Northern California. For surfers, for example, risk can be reduced 25-fold by surfing in March instead, and by more than 1,600-fold by surfing in Southern California, between Los Angeles and San Diego.

Yesterday, the Jeffreys Bay surf tournament in South Africa came to a grinding halt after two sharks, suspected to be great whites, shook surfer Mick Fanning clear off his board. Broadcasters panicked, swore, and watched the chaos unfold, trying to comprehend and explain what was happening on live television. After a swift punch to one of the sharks—yes, he punched a shark—Fanning escaped on a rescue Jet Ski, bite-free. The attack is understandably receiving a ton of attention. But the commotion may also be racking up fear that is more fiction than fact, according to a new study, which finds that—at least off the coast of California—your chances of being bitten by a shark are the lowest they’ve been in decades.

“It’s important for the public to really understand shark attacks in terms of risk,” says Sal Jorgensen, a great white shark researcher at the Monterey Bay Aquarium in California. “What’s important is looking at the probability of being attacked, and that’s decreased dramatically.”

Jorgensen and his colleagues pulled together more than 6 decades of data, from 1950 to 2013, on injury-causing shark attacks from a database that has carefully followed and recorded attacks along the California coast. During this period, there were 86 attacks, and an increase from 0.9 to 1.5 attacks per year. However, the number of beachgoers in that same time frame exploded from 53 million in the 1950s to 165 million in 2013—a 211% increase. Doing the math, your chances of being bitten by a shark in California waters has actually decreased 91% over the past 63 years, the team will report later this month in Frontiers in Ecology and the Environment.

“If you put more people in the water, you have more people that could possibly interact with a shark,” says Francesco Ferretti, lead author of the study and a marine ecologist at Stanford University’s Hopkins Marine Station in Pacific Grove, California. “Take the lottery—the more people that buy tickets for the lottery, the more that there is a chance that someone will win. Or in this case, lose.”

But why are shark attacks themselves up? One possibility is that their numbers are on the rebound after years of shark hunting, thanks to the work of conservationists, but at this point, there aren’t enough data to confirm whether populations are indeed rising. Another possibility is that the recovery of seal and sea lion populations off the coast of California, thanks again to successful marine protection policies, are giving the sharks more reason to stay in these waters.

Still, researchers both on and off the study stress the importance of sharing shark information, like the whereabouts and abundance of sharks during specific months, with the public. They encourage beachgoers to base their decisions on the data to decrease dangerous shark-human encounters. For example, the study’s results showed that surfers are about 1500 times safer surfing between San Diego and Los Angeles in March, compared with surfing in Mendocino County in October and November.

“I think the thing that’s most important here is the need for education,” says Chris Lowe, an unaffiliated shark expert at California State University, Long Beach. “Studies like this show the big numbers and show what probabilities are true—we’ve been trying to communicate that for a while, and we just have to keep at it.”

doi:10.1126/science.aac8881

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