There is no sea monster whose name, correctly or wrongly, induces fear as much as the great white shark.
The great white (Carcharodon carcharias) is commonly considered as one of the ocean's top predators, with its sleek form adapted for hunting, keen teeth, and (perhaps unfair) reputation for eating human flesh. That is correct - yet there is something that even the great white dread.
Since 2017, scientists have discovered that sharks have become exceedingly uncommon off the coast of South Africa, where they normally cluster. Initially, the unexpected absence was blamed on human activity, such as overfishing.
New study has revealed the actual culprit: a pair of orcas (Orcinus orca), which hunt the sharks and gulp their tasty, nutritious, vitamin-rich livers.
Once upon a time, the fishing hamlet of Gansbaai on the South African coast was a shark-spotting hotspot, so densely packed with the predators that adjacent Dyer Island is known as the world's great white shark capital. However, sharks' presence has been decreasing in recent years.
Furthermore, eight great white sharks have come up in Gansbaai since 2017, seven of them without livers (and several without hearts), indicating an orca assault. These sharks' wounds are unique, and they have been linked to the same pair of orcas. Scientists suspect the couple is responsible for many more great white deaths that haven't shown up on shore.
Other research has shown that the presence of orcas can effectively drive great white sharks away. One study published in 2020 discovered that great white sharks would always flee from favorite hunting grounds off the coast of San Francisco if an orca is in the area.
Using long-term sighting and tracking data from tagged sharks, a team of scientists led by Dyer Island Conservation Trust marine biologist Alison Towner discovered that orcas are the reason sharks are starting to avoid what used to be some of their favorite sites.
"Initially, following an orca attack in Gansbaai, individual great white sharks did not appear for weeks or months," Towner noted.
"What we seem to be witnessing though is a large-scale avoidance (rather than a fine-scale) strategy, mirroring what we see used by wild dogs in the Serengeti in Tanzania, in response to increased lion presence. The more the orcas frequent these sites, the longer the great white sharks stay away."
The scientists monitored 14 GPS-tagged sharks as they departed the region when orcas were there over the course of five years. Sightings of great white sharks are also much lower in numerous bays.
This is significant. Only twice previously have great white sharks been reported missing for a week or longer at Gansbaai since records began: one week in 2007, and three weeks in 2017. According to the experts, the new absences are unusual. Furthermore, they are affecting the ecology.
In the absence of great white sharks, copper sharks (Carcharhinus brachyurus) are filling the ecological void. Great white sharks prey on these sharks; when there are no great whites nearby, orcas pursue the coppers instead. They're doing it with the competence of predators who have hunted enormous sharks before, according to the researchers.
"However, balance is crucial in marine ecosystems, for example, with no great white sharks restricting Cape fur seal behavior, the seals can predate on critically endangered African penguins, or compete for the small pelagic fish they eat. That's a top-down impact, we also have 'bottom up' trophic pressures from extensive removal of abalone, which graze the kelp forests these species are all connected through," Towner stated.
"To put it simply, although this is a hypothesis for now, there is only so much pressure an ecosystem can take, and the impacts of orcas removing sharks, are likely far wider-reaching."
It's also interesting thinking about why orcas would be targeting sharks. Their livers are nutrient-dense, large, plump, and full of lipids and oil, which sharks need to power their epic migratory voyages across the ocean. But it's unknown how the orcas discovered this, or why they may choose shark livers as a source of nourishment.
Scientists suggest in an unpublished research that certain orcas are adjusting to preferentially target sharks, possibly in reaction to dwindling populations of their preferred food. However, considering the global decline of great white populations, the addition of an efficient predator is reason for concern.
"The orcas are targeting subadult great white sharks, which can further impact an already vulnerable shark population owing to their slow growth and late-maturing life-history strategy," Towner explained.
"Increased vigilance using citizen science (e.g. fishers' reports, tourism vessels), as well as continued tracking studies, will aid in collecting more information on how these predations may impact the long-term ecological balance in these complex coastal seascapes."
The team's research has been published in the African Journal of Marine Science.
