(ORDO NEWS) — No other sea creature is as terrifying – rightly so or not – as the great white shark.
With its sleek hunting body, sharp teeth, and (somewhat undeserved) reputation for feasting on human flesh, the great white shark (Carcharodon carcharias) is widely recognized as one of the ocean’s top predators. And this is true, but there is something even a big white is afraid of.
Since 2017, scientists have recorded that sharks have become extremely rare off the coast of South Africa, where they usually accumulate. Initially, human activities, such as overfishing, were blamed for the strange disappearance.
Now, a new study has detailed the true culprit: a pair of killer whales (Orcinus orca) prey on sharks and devour their tasty, nutritious, vitamin-rich livers.
Once upon a time, the fishing town of Gansbaai on the South African coast was a shark-watching mecca of sorts – it was so densely populated by predators that nearby Dyer Island is considered the great white shark capital of the world. However, over the past few years, the presence of sharks has begun to decrease.
In addition, since 2017, eight great white sharks have washed ashore in Gansbae, seven of them without a liver (and some without a heart) – a hallmark of killer whale attacks.
The wounds on these sharks are distinctive and were inflicted by the same pair of killer whales. It is likely, scientists say, that this pair is responsible for many other deaths of great white sharks that did not wash ashore.
We know from other research that the presence of killer whales can be quite adept at warding off great white sharks. One 2020 study found that great white sharks will invariably flee their preferred hunting waters off the coast of San Francisco if a killer whale appears in the area.
In a new study using data from long-term observation and tracking of tagged sharks, a team led by marine biologist Alison Towner at the Dyer Island Conservation Foundation found that killer whales are causing sharks to avoid their favorite places.
“Initially, after a killer whale attack in Gansbae, individual great white sharks did not appear for several weeks or months,” Towner explained.
“However, we seem to be seeing a large-scale avoidance strategy (rather than a small-scale one) that mirrors what we see in wild dogs in the Serengeti in Tanzania in response to increased lion presence. The more killer whales visit these areas, the longer white sharks stay away. “.
Over the course of five years, the team tracked 14 GPS-tagged sharks as they left an area where killer whales were present. Great white shark sightings have also declined, quite significantly, in several bays.
It is very important. Since the start of records in Gansbae, great white sharks have been recorded only twice for a week or more, in 2007 for one week and in 2017 for three weeks. The new absences, the researchers say, are unprecedented. Moreover, they change the ecosystem.
In the absence of great white sharks, copper sharks (Carcharhinus brachyurus) come to the vacated ecological niche.
These sharks are preyed upon by great white sharks; when there are no white sharks, killer whales prey on copper sharks. And, remarkably, they do so with the skill of predators experienced in hunting large sharks, the researchers say.
“However, balance is critical in marine ecosystems, for example, if great white sharks do not limit the behavior of Cape seals, then seals may prey on endangered African penguins or compete for the small pelagic fish they feed on.
This is a top-down effect, but we also have bottom-up trophic pressure caused by the extensive removal of abalones, which are grazing kelp, through which all these species are connected,” Towner said.
“To put it simply, while it’s still a hypothesis, the ecosystem can only withstand that kind of pressure, and the impact of killer whales removing sharks is likely much wider.”
It is also worth considering the reasons killer whales may prey on sharks. Their livers are a rich food source, huge, plump, full of fats and oils, which sharks use to fuel their epic migratory journeys across the ocean.
But it’s unclear how killer whales figured this out, or why they might be looking to shark liver as their preferred food source.
Perhaps, scientists suggest in a yet-to-be-published paper, some killer whales are adapting to preying on sharks, perhaps in response to a decline in the numbers of their favorite prey. However, with great white shark populations declining around the world, additional pressure from an efficient predator is worrying.
“Orcas attack sexually mature great white sharks, which could have an additional impact on an already vulnerable shark population due to their slow growth and late maturation,” Towner said.
“Increasing vigilance through ‘citizen science’ (eg fishermen’s reports, cruise ships) as well as continued tracking research will help gather more information about how these predators may affect the long-term ecological balance in these challenging coastal seascapes.”
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