In South Africa’s waters, a notorious pair of orcas, nicknamed Port and Starboard, are shaking up marine ecosystems with their predatory focus on sharks. Since 2017, at least nine great white sharks have washed ashore at Gansbaai, missing vital organs such as livers and, in some cases, hearts a hallmark of orca attacks. The duo has also been linked to the mass killing of at least 17 broadnose sevengill sharks in a single day.
The distinctive wounds on the shark carcasses trace back to these orcas, and scientists suspect the pair may be responsible for many more unreported great white deaths. Research suggests this predation is driving great white sharks away from their preferred hunting grounds, altering the ecological balance of these waters.
Marine biologist Alison Towner of the Dyer Island Conservation Trust led a study in 2022 using long-term tracking data to show that orcas are forcing great white sharks to avoid areas they once frequented. After an orca attack, individual great whites disappeared from Gansbaai for weeks or even months.
“What we seem to be witnessing is a large-scale avoidance strategy,” Towner explained, likening it to the way wild dogs in Tanzania avoid lions in the Serengeti.
Over five years, researchers tracked 14 GPS-tagged sharks fleeing areas with orca activity. These extended absences are unprecedented; since record-keeping began, great white disappearances lasted only a week in 2007 and three weeks in 2017. Now, sightings of great whites in some bays have plummeted significantly.
The absence of great white sharks has allowed other species, such as copper sharks, to move into the ecological niche. However, the orcas are now hunting copper sharks with the same precision as their great white predations.
This ripple effect could destabilize the marine ecosystem. Without great whites to restrict Cape fur seals, the seals could prey on critically endangered African penguins or compete for small pelagic fish, further straining the food web.
Marine ecosystems are already under pressure from “bottom-up” issues like overfishing of abalone, a key kelp-grazing species. Combined with the “top-down” impact of orca predation, the balance is fragile. “There is only so much pressure an ecosystem can take,” Towner warned.
Orcas may be targeting sharks for their livers, which are rich in fats and oils that fuel the sharks’ long migrations. How orcas learned to extract these nutrient-dense organs remains unclear, but their specialized hunting behavior may be a response to declining numbers of other prey.
In the Gulf of California, an unrelated orca pod has developed techniques to hunt whale sharks, highlighting the adaptability of these apex predators.
The added predation pressure on great white sharks is alarming, given their already declining populations. Orcas are targeting subadult sharks, which further threatens the population due to the species’ slow growth and late maturity.
Towner calls for increased vigilance through citizen science, such as reports from fishers and tourism vessels, alongside continued tracking studies to monitor the long-term impact of these predations.
As the oceans’ top predators clash, the delicate balance of marine ecosystems is at stake.
