California: Clash of the titans: white sharks vs. orcas
When orcas and white sharks cross paths, only one can prevail as the true apex predator. New research from the Monterey Bay Aquarium published in Nature Scientific Reports details these rare, sometimes brutal encounters — and their ecological implications.
It’s a study decades in the making because observations of the two creatures interacting is a rarity.
Scot Anderson, a white shark expert and seasonal researcher for the Aquarium, still remembers one such run-in more than 20 years ago near the Farallon Islands, a short boat ride west of downtown San Francisco.
“The first time it happened was kind of shocking to everybody,” Scot says. “Before we had seen anything like that, people would ask, who’s the baddest predator?”
The first scorecard came on October 4, 1997, when orcas killed and partially ate a white shark within view of a whale-watching boat. Scot was heading out from nearby Bolinas when he heard what was happening over the radio.
“We just went straight there,” he says. “We got there right as it was finishing. I saw the two orcas sticking their heads out of the water and squealing like they do when they have a kill. And the shark just sank away.”
That was apparently all it took to drive off white sharks for the rest of the season. Scot scoured the area for nearly two weeks afterward and saw zero.
White sharks skip town
Orcas don’t hunt at the Farallones every year, but white sharks do. The federally protected islands represent a small but concentrated colony of elephant seals — important white shark prey sometimes also targeted by transient pods of orcas.
Since 2006, Aquarium researchers, along with Stanford University partners, have deployed more than 165 acoustic tags to white sharks along California’s central coast. Data collected from those tags, says Aquarium Senior Research Scientist Dr. Salvador Jorgensen, made it possible “to capture one of these rare events with the full force of our tracking technology.”
One November afternoon in 2009, 17 tagged sharks were detected around the Farallones when two pods of orcas showed up. Observers saw the orcas feasting on seals, and while all the tagged sharks survived, others without tags may have been picked off.
The orcas departed just a few hours later, but the tagged white sharks were spooked. By nightfall, all 17 had left the Farallones. Many showed up soon after around Tomales Point to the north or Año Nuevo Island to the south, two other elephant seal rookeries frequented by white sharks.
“We realized at that point that, although these events are rarely ever even witnessed, the implications are huge,” Sal says. “It calls into question the use of ‘apex predator’ to describe white sharks.”
Who’s the baddest?
“You can really only have one apex,” says Scot, who coauthored the paper alongside Sal and several others. “There’s only one top-top.” White sharks, Scot argues, are outclassed by bigger, smarter animals in orcas, which can communicate and hunt strategically in groups.
Since the 2009 encounter, the team combined tagging data with long-term seal and shark surveys conducted by collaborators at Point Blue Conservation Science and revealed similar run-ins at the Farallones in 2011 and 2013. The same pattern has also been reported in Australia and South Africa.
But one key unknown is whether orcas are targeting white sharks as prey, or simply bullying their competitors — sometimes fatally. If it’s the latter, the opportunity to eat the vanquished shark’s liver, as the orcas did in 1997, would be what Sal calls an “ancillary benefit.”
The liver, made up largely of oil, is where white sharks store energy. It sometimes weighs more than a ton. “It’s probably one of the densest sources of calories you could find in the ocean,” Sal says. “Orcas do know what side of the bread the butter is on.”
What still lurks below
After a pod of orcas scares off the white sharks for the season, the survey data show that elephant seals in the area benefit, Sal says.
“In years where orcas appeared, the rate of seals being eaten by sharks decreased fourfold,” explains Sal. “So when orcas come to town, there might be a small tax and a few seals get eaten, but following that couple hours, the seals enjoy a predator-free island for the rest of the season.”
And that benefit could last for many months.
Meanwhile, the white sharks that flee must compete for food elsewhere, which may undermine their fitness, since they need to build substantial energy reserves 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,” Sal says. “It turns out these ‘risk effects’ are very strong even for large predators like white sharks — strong enough to confine their hunting activity to less preferred, but safer areas.”
While it remains unclear whether orcas deliberately prey upon white sharks, Sal says, “I think this shows that food chains are not always linear.” In the hierarchy of ocean predators, orcas may not occupy a place directly above the sharks so much as off to one side.
So-called “lateral interactions” between top predators are fairly well understood on land, Sal says — but are much harder to document in the ocean. Because these interactions are so rare, fully understanding this apex predator relationship will take time. Doing so will help us better protect the vital intricacies of our global ocean.
Featured photo: Pod of orcas swims in Monterey Bay. Photo © Jim Capwell/www.divecentral.com
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