A pair of orcas are brutally killing great white sharks and ripping out their livers off the coast of South Africa, prompting great whites to abandon this part of their normal range en masse, reports a new study.
Since 2017, eight great white shark carcasses have washed up on the shores of the Gansbaai coast, which is located in South Africa’s Western Cape. Seven of them were missing livers and the heart and testes of one carcass was torn out. Scientists have pinned the grisly killings on two male orcas, named Port and Starboard after the direction of their collapsed dorsal fins, and conclude that the carcasses represent the small fraction of victims that happened to end up on land.
The recent arrival of these killer whales in the area, followed by the sudden decline in sharks, suggests “that white sharks respond rapidly to risk from a novel predator,” a reaction that could “have long-lasting ecosystem effects” in this area, and many others where these hunters meet, according to a study published on Wednesday in the African Journal of Marine Science.
“Risk-induced fear effects exerted by top predators are pervasive in terrestrial and marine systems, with lasting impacts on ecosystem structure and function,” said researchers led by Alison Towner, a senior white shark biologist at the Dyer Island Conservation Trust and a PhD candidate at Rhodes University, in the study. “The loss of top predators can disrupt ecosystems…but the introduction of novel apex predators into ecosystems is not well understood.”
“We provide evidence that killer whale predation on white sharks directly induced emigration of sharks from Gansbaai, and individual white sharks did not return for weeks or months,” the team added. “Predator–prey interactions between white sharks, other coastal sharks, and killer whales are now increasing in South African waters and are expected to have pronounced impacts on the coastal ecosystem.”
Towner and her colleagues considered alternate explanations for the recent drop of great whites at Gansbaai, including human fishing pressures and sea surface temperatures, but the comprehensive evidence they gathered strongly implicates Port and Starboard in the flight of sharks from the area and hints that this pair “might be members of a rare shark-eating morphotype, known to hunt at least three shark species as a prime source of nutrition in South Africa,” according to the study.
The team combined information from the “necropsies'' of the shark carcasses, sightings from boats, and telemetry data of shark movements obtained from tagged individuals to document the mass emigration of this iconic species from what was once a major hub for them. Great whites are normally abundant at Gansbaai, which attracts tourists from around the world to cage-dive with these massive hunters. But since Port and Starboard arrived, 14 tagged sharks have been observed noping out of the area and sightings by boat have plummeted.
“Boat surveys and telemetry data showed that white sharks left the greater Dyer Island region in May 2017, which is considered the peak season for white shark abundance…and remained absent from the region for six weeks,” Towner and her colleagues said in the study. “White sharks eventually returned, and the numbers stabilized, only to disappear again for longer periods after more sightings of the implicated killer whale pair, and subsequently of other killer whales with straight fins.”
“Prior to these predations there were only two instances since data collection began where white sharks were absent for a week or more: one week in 2007, and one 3-week period at the beginning of 2016,” the team added.” At Gansbaai, the likelihood is that, in response to the killer whale presence there, white sharks fled in different directions and at different rates, some offshore out of detection range by coastal receivers and some longshore.”
This precipitous drop in great whites has already disrupted both the region’s tourism industry and the wider ecosystem. For instance, the population of bronzer sharks, a prey species for great whites, has exploded in the area now that one of their main predators has fled the scene. The researchers warn of more far-reaching impacts that are not yet known and point to other examples of orcas around the world who have similarly shifted their hunting patterns, in part due to human-caused pressures.
“Though killer whales have previously been incidentally sighted in South Africa, there has been an increase in sightings in the last decade,” the team noted. “This change could be related to a decline in prey populations, including pelagic fishes and sharks causing changes in distribution patterns of killer whales.”
“The movements of the two male killer whales ‘Port’ and ‘Starboard’, and other killer whales known to interact with longline fisheries are the subject of a current review study,” the researchers added.
In this way, the orcas that are terrorizing sharks around the Gansbaai coast may also be fleeing from another top predator: humans. To better understand these dynamics, and the effects they may have within orca ranges worldwide, Towner and her colleagues recommend continued monitoring of marine ecosystems with a variety of measures.
“Increased vigilance using citizen science (e.g. fishers’ reports, tourist 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 concluded.