Port and Starboard are two killer whales that are causing panic in the waters of South Africa. The pair of cetaceans has killed at least eight white sharks since 2017 and scientists suspect that there are more victims who have been lost in the waters. They tear sharks apart and eat their livers, sometimes their hearts as well . White sharks are avoiding the area, historically under their domain, for fear of attack by killer whales.
These hair-raising acts are not uncommon for killer whales as these animals have been seen killing everything from small fish to large sharks, sea lions and even blue whales. What is strange is that the violent escapes of two killer whales have such a clear and immediate impact on the habitat, point out the authors of a study recently published in the African Journal of Marine Science .
The researchers affirm that sightings of white sharks have been drastically reduced in the area since Port and Starboard began their raids in 2017. Using a combination of long-term sightings and tagging data, the researchers found that dozens of sharks have been avoiding regions of Gansbaai’s coastline when killer whales were around. Tagged sharks would sometimes disappear for weeks or months, abandoning territory that has historically been theirs.
“What we seem to be witnessing is a large-scale evasion strategy , mirroring what we see wild dogs using in the Serengeti, Tanzania, in response to the increased presence of lions,” the author said in a statement. The study’s lead, Alison Towner, a senior biologist studying white sharks at the Dyer Island Conservation Trust in South Africa. “The more orcas frequent these places, the longer the white sharks stay away.”
The fishing town of Gansbaai is located about 120 kilometers east of Cape Town. According to the study’s authors, the year-round presence of great white sharks in the area has made it a popular destination for shark lovers .
White sharks, with their almost five meters in length and several rows of up to 300 teeth, are among the top aquatic predators. What the sharks of Gansbaai did not know is that in 2015 a pair of male killer whales would arrive on the coast that would change everything.
Orcas were easily identifiable by their war wounds. The dorsal fin of one of them had bent to the right, while that of the other was bent to the left. The local whale watchers therefore named them Port and Starboard. Almost as soon as they were seen in Gansbaai, a great white shark washed up dead on the beach.
In the absence of white sharks, coppery sharks ( Carcharhinus brachyurus ) are moving in to fill the vacant ecological niche. Coppery sharks are preyed upon by whites. In the absence of the latter, the orcas are hunting copper. And, above all, they are doing so with the skill of predators that have had experience hunting large sharks, the researchers said.
“However, balance is crucial in marine ecosystems; for example, if there are no great white sharks to restrict the behavior of Cape fur seals, the seals may prey on African penguins, which are critically endangered , or compete by the small pelagic fish they eat. That’s a downward impact, we also have ‘upward’ trophic pressures from the extensive removal of abalone, which graze the kelp forests that these species are connected by,” Towner said. “In short, although this is a hypothesis for now, there is a limit to the pressure an ecosystem can withstand, and the repercussions of killing sharks by killer whales are likely to be much more far-reaching.”
And why are killer whales feeding on sharks? And with their livers in particular? It turns out that the liver of sharks is very nutritious , it is huge and full of fats and oils that the sharks use as fuel to endure their great migratory journeys through the ocean. What is not known is how the killer whales became aware of all this or why they preferentially seek livers for nourishment.
To answer the above questions, scientists speculate that some killer whales may be adapting to preferentially hunt sharks, perhaps in response to declining numbers of their preferred prey. However, with great white shark populations declining around the world, the added pressure of an effective predator is cause for concern .
“Orcas are targeting subadult white sharks , which may further affect an already vulnerable shark population due to their slow growth and late-maturing life strategy,” Towner said. “Increased surveillance through citizen science (eg, reports from fishermen, tourist boats), as well as continued monitoring studies, will help gather more information about how these depredations may affect the long-term ecological balance in these complex coastal seascapes.
Referencia: Towner AV, et. al. 2022. Fear at the top: killer whale predation drives white shark absence at South Africa’s largest aggregation site. African Journal of Marine Science. DOI: https://doi.org/10.2989/1814232X.2022.2066723
