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July 2019    Download the Entire Issue (PDF) Available to the Public Vol. 45, No. 7   RSS Feed for Undercurrent Issues
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Are Great Whites the Oceanís Most Fearsome Predators? New Findings Say No

from the July, 2019 issue of Undercurrent   Subscribe Now

Great white sharks, long considered the sea's most bloodthirsty killers, are actually cream puffs compared with killer whales. A study in Nature Scientific Reports took the first comprehensive look at the rarely seen, sometimes violent, encounters between two of the world's largest ocean carnivores and found that great whites run like scaredy-cats whenever killer whales show up at their feeding grounds at the Farallon Islands, near San Francisco.

"What we saw was that when orcas came close to the island during shark season, all of the sharks would take off," said Salvador Jorgensen, a Monterey Bay Aquarium researcher and the study's lead author. "As a predator that has been successful for millions of years, that may be the card white sharks know how to play that has kept them alive so long -- knowing when to fold."

The report, which used acoustic tracking data from 165 white sharks between 2006 and 2013 and observations on Southeast Farallon Island dating to 1987, found that the sharks would all vanish around the islands, even if orcas were just passing through. The fear is so deep rooted that the sharks stayed away from the islands for the rest of the feeding season even after the killer whales left, sometimes as long as a year.

The data confirms what many scientists suspected after a famous attack in October 1997 near the Farallones by orcas on a great white in full view of a whale-watching boat (the orcas ripped out and devoured the shark's liver) -- that killer whales are the undisputed champions of the food chain. Researchers believe pods of killer whales were also responsible for the deaths of at least five great whites that washed ashore in South Africa with their livers missing over the past few years. It's not known whether killer whales attack because they consider great whites a food source or competition, but shark livers are a major source of calories, weighing up to 25 percent of their body weight.

Offshore orcas eat schooling fish and Pacific sleeper sharks, and roaming pods that scientists call "transients" prey mostly on marine mammals. Some of the 140 or so transients identified as existing off California's shores were present at the Farallon Islands every time tagged sharks were observed bolting en masse between 1992 and 2013. The sharks skedaddled even though the whales attacked only pinnipeds and smaller prey during the two hours they were in the vicinity. One shark returned a week later and a few tagged sharks that weren't present during the original orca sighting later visited, but they all left within minutes or, at most, hours. Each time, the fleeing sharks reconnoitered near AŮo Nuevo Island farther south or Point Reyes National Seashore up north, but they avoided the Farallones.

It is still a mystery how the sharks --solitary hunters and usually not close enough to see the orcas -- knew the predators were around. Orcas communicate with one another at a higher frequency than white sharks can hear, but they are known to remain silent when hunting prey that have the ability to detect their higher tones. "My gut feeling, and this is the topic of future work, is that sharks are able to detect orcas using their sense of smell," Jorgensen said.

To him, the most interesting thing about the clashes between these seagoing titans is how they impact the ocean ecosystem. Elephant seal predation decreased fourfold in the years orcas visited the Farallones. Sharks, meanwhile, were forced to compete for food elsewhere, which may have reduced the energy reserves needed to fuel their long ocean migrations.

"We don't typically think about how fear and risk aversion might play a role in shaping where large predators hunt and how that influences ocean ecosystems," Jorgensen said. "It turns out these risk effects are very strong, even for large predators like white sharks."

-- Condensed from an article by Peter Fimrite published in the San Francisco Chronicle

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