Photo credit: www.yahoo.com
No marine creature evokes fear quite like the great white shark. Known for its streamlined design, formidable teeth, and a reputation—often exaggerated—for preying on humans, the great white (Carcharodon carcharias) stands as a pinnacle predator in oceanic ecosystems.
However, even this majestic shark has its own terrors. Since 2017, researchers have noted a significant decline in great white populations along the South African coast, where these sharks typically gather in large numbers. Initially, this startling exodus was attributed to human influences like overfishing.
Yet, in 2022, findings painted a different picture. The primary culprit turned out to be a duo of orcas (Orcinus orca), affectionately nicknamed Port and Starboard due to the distinctive kinks in their dorsal fins. These intelligent mammals have been found to target great whites, specifically their nutrient-rich livers, leaving the once-abundant sharks scarce.
Gansbaai, a fishing town renowned for its thriving shark population, has seen remarkable shifts over the last few years. The area, famously dubbed the great white shark capital of the world, is now experiencing a noticeable decline in sightings.
The situation has escalated to the point where since 2017, numerous great white sharks have been found washed ashore in Gansbaai, many tragically missing their livers and even hearts—clear indications of an orca attack. Furthermore, Port and Starboard have been linked to a dramatic uptick in killings of other shark species, including the broadnose sevengill shark, with reports of at least 17 sharks killed in a single assault.
The scars on these unfortunate sharks have been attributed to the same pair of orcas. Scientists suspect that many other great white sharks have also fallen victim to these predatory attacks without being found.
Research indicates that orcas can effectively drive great whites from their habitats. A 2020 study highlighted how great whites vacate preferred hunting areas off San Francisco whenever orcas come nearby.
In a subsequent study published in 2022, marine biologist Alison Towner and her team analyzed long-term data from tagged sharks. They discovered that the presence of orcas caused great whites to increasingly abandon formerly reliable hunting zones.
“After an encounter with orcas in Gansbaai, individual great whites were missing for extended periods,” Towner explained. “We are witnessing a broad avoidance strategy similar to how wild dogs behave in response to lions in the Serengeti. The more frequently orcas visit an area, the longer the great whites tend to stay away.”
Over five years, researchers tracked 14 GPS-tagged great whites that fled when orcas were around. The decline in sightings of these iconic sharks has been profound across various localities.
This trend is alarming. Historically, great white sharks were seldom absent from Gansbaai for extended periods—only noted absent for a single week in 2007 and for three weeks in 2017. Current absences, however, are unprecedented and seem to be a continuing issue.
Research published earlier this year documents two separate instances of Port and Starboard attacking sharks to consume their livers.
This predation is stirring ecological concerns. With fewer great whites in the water, copper sharks (Carcharhinus brachyurus) are beginning to occupy the ecological void left behind. Normally preyed upon by great whites, the copper sharks are now facing threats from orcas as well.
Researchers have noted these orcas’ proficiency in hunting sharks, showcasing a high level of skill. “Without the regulatory presence of great white sharks, Cape fur seals can overexploit resources, potentially leading to a decline in critically endangered species such as African penguins,” Towner added. “The intricate web of marine ecosystems is influenced by both top-down and bottom-up pressures, including the significant removal of abalone, which impacts kelp forests that serve as critical habitats.”
“While this remains a hypothesis, ecosystems can only endure so much, and the repercussions of orcas targeting sharks could be extensive,” she cautioned.
The motivation behind the orcas’ predation on sharks is fascinating. Shark livers are abundant in nutrients, packed with fats and oils that support the animals’ long migrations across vast ocean expanses.
However, it remains unclear how orcas have learned to target shark livers as a preferred food source. Some researchers propose that orcas may be adjusting their hunting practices in response to dwindling populations of their usual prey. For instance, an unrelated pod in the Gulf of California has devised new methods for pursuing whale sharks.
With the global decline of great white populations, the emergence of orcas as proficient shark hunters raises further conservation concerns.
“The orcas are specifically targeting younger great whites, further stressing a population already compromised by their slow growth and late maturity,” Towner noted.
“Increasing awareness through citizen science initiatives, such as fisher reports and tourism observations, alongside ongoing tracking studies will be crucial for understanding the long-term effects these predator interactions may have on the delicate balance of these coastal ecosystems.”
This research has been featured in the African Journal of Marine Science, and African Journal of Marine Science.
A previous version of this research was released in July 2022.
Related News
Source
www.yahoo.com