Salvador Jorgensen has spent more than 15 years studying great white sharks near California's coast. The senior research scientist from the Monterey Bay Aquarium and his team have attached tracking tags to 165 of the toothy predators, which routinely visit islands west of San Francisco and prey on elephant seals. But something odd happened one autumn: “In 2009, 17 of those tagged white sharks were simultaneously swimming around the Farallon Islands, when they abruptly departed. Not just one or two but all 17, in a matter of hours,” Jorgensen recalls. “Normally the sharks hang around for weeks or months at a time.” So why did they flee? Great white sharks are perhaps the most widely feared predators in the ocean, but it turns out they may have something to fear, too: orcas, also known as killer whales.
Jorgensen and his colleagues drew this conclusion in a recent study that combined their shark-tagging data with a nearly three-decade survey of wildlife abundance around Southeast Farallon Island. Great whites have been seen abandoning this prime feeding area when killer whales come too close for comfort—even if the mammals are simply passing through for a few hours. And the sharks do not just disappear for a day or two—they stay away for the entire season. Researchers recorded a fourfold to sevenfold reduction in the number of elephant seals killed by great white sharks during years in which they were scared away. The findings were described in a study published in April in Scientific Reports.
Sharks have existed for at least 450 million years, whereas cetaceans (whales and dolphins) evolved just 50 million years ago. “For sharks to have survived and thrived in our ocean for so long, they have their bag of tricks,” Jorgensen says. “One of those tricks is knowing when to fold.” What was particularly surprising to him is that it can take almost a full year before the sharks feel comfortable returning. Some orcas specialize in eating salmon and other fish; others prefer pinnipeds (a group that includes seals and walruses), and a third type feasts on sharks. At least one orca has been observed killing and eating an adult white shark at the Farallon Islands, back in 1997. It is not clear whether the sharks avoid orcas out of fear of getting eaten or because they compete over the same seal prey. Either way, this extreme caution may simply be a prudent survival strategy for the sharks.
The eastern Pacific great whites do have other hunting grounds. “There's a lot more feeding habitat for white sharks because the [pinniped] rookeries are expanding,” thanks to intensive conservation efforts, says ecologist Chris Lowe of California State University, Long Beach, who was not involved with the new study. The sharks' willingness to give up good feeding opportunities at the Farallon Islands suggests that going elsewhere is preferable to sticking around and facing the risk—however slight—of becoming an orca's next meal.
Scientists are not sure how the sharks detect the orcas. The waters around the Farallon Islands are murky, and great whites have been seen escaping the area even when their adversaries were far beyond sight or hearing range. Jorgensen says the most likely explanation is that the sharks “were able to smell something in the water that alerted them.” They could be sniffing out the orcas themselves or some chemical cue given off by another stressed-out shark after a run-in, he says. This idea has some support: Jorgensen and his colleagues monitored the movements of a group of great white sharks that were hundreds of kilometers away from the Farallon Islands when orcas arrived. Sometime after the orcas came and left, these sharks showed up. “But they just poked [around] and left almost immediately,” he says. Perhaps they somehow sensed that the orcas were—or had recently been—in the area.
Ecologists often use the phrase “landscape of fear” to describe the way predators influence the movements and behaviors of their prey, resulting in a cascade of impacts on the ecosystem. For example, in one recent experiment, island-dwelling raccoons that heard dogs barking spent less time foraging on beaches and around tide pools. That led to increases in fish, worm and crab communities. And that in turn led to decreases in snails—easy prey for a growing crab population.
How the sharks' avoidance of orcas might likewise be affecting marine ecosystems remains a mystery. “We know very little about how these apex predators might interact with each other in the wild ocean,” Jorgensen says. That is in part because white sharks, orcas and elephant seals are all still recovering from a century of mistreatment by humans. “The assumption is [these interactions] have been there historically—it's just that all these animals were basically eliminated from the ecosystem for more than 100 years,” Lowe says. “There's no reason to believe that orcas weren't [hunting] both seals and sharks 300 or 400 years ago, before people really started exploiting those animals.
This article was originally published with the title "Scaredy-Sharks" in Scientific American: Science News, Expert Analysis, Health Research , , 12-14 ()
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