Killer whales rage in South African waters.
Since 2017 a couple male orcas †Orcinus orca) have sexed at least eight great white sharks †Carcharodon carcharias) near the coast of Gansbaai, shred the fearsome fish and eat their liver in all cases except one. Some sharks — which scientists regularly find dead and mauled on nearby beaches — were even heartbroken.
Such eerie displays are not all that uncommon for killer whales, which have been observed to kill everything from small fish to sturdy sharks, sea lions and even blue whales† Rarely have the violent escapades of two individual whales had such a clear and immediate impact on their habitats, say the authors of a new study published Wednesday (June 29) in the African Marine Science Magazine†
According to the researchers, sightings of great white sharks in the area have dropped dramatically since the two serial killer cetaceans began their spree five years ago. Using a combination of long-term observations and tagging data, the researchers found that dozens of sharks actively avoid regions of the Gansbaai coast when the orcas are nearby. Labeled sharks sometimes disappeared for weeks or months at a time, leaving the area historically dominated by great whites.
“What we appear to be seeing, however, is a large-scale avoidance strategy, mirroring what we see being used by wild dogs in the Serengeti in Tanzania, in response to the increased presence of lions,” lead study author Alison Towner, a senior biologist who studies white sharks at the Dyer Island Conservation Trust in South Africa, said in a statement† “The more often the killer whales visit these sites, the longer the great white sharks stay away.”
Orcas series
Gansbaai is located about 120 km east of Cape Town. According to the study authors, the year-round presence of great white sharks in the area has made Gansbaai a popular tourist destination for shark enthusiasts.
At up to 16 feet (4.9 meters) in length and with multiple rows of up to 300 serrated, blade-like teeth, great whites are among the best predators in the water. But as of 2015, Gansbaai’s sharks met their match when a pair of adult male killer whales descended on the shore.
Like cartoon criminals, the killer whales were easily identified by their battle scars; the dorsal fin of one whale was folded to the right, while the other was bent to the left. These features inspired local whale watchers to name the port and starboard orcas. Almost as soon as whale watchers spotted the duo on Gansbaai, a great white shark washed up dead on the beach.
Between 2016 and 2017, five more great white sharks were found dead — four with their bodies ripped open and their livers ripped out. (Unlike bony fish, sharks have a very large, fatty liver, which can provide attackers with a lot of energy and nourishment, Live Science previously reported). The deaths of each shark matched observations from port and starboard in the area. Although their official death toll is eight, there are likely more undiscovered shark carcasses lost to the sea.
In the six months after the first shark’s death, daily shark sightings in Gansbaai dropped from more than six sightings per day to just one. Detections of sharks tagged with electronic transmitters also declined. Prior to the 2017 attacks, between three and eight tagged sharks were detected in the area daily. A few months later, that number dropped to zero. Transmitter data showed that individual sharks sometimes swam hundreds of miles away from Gansbaai within a few days of an orca attack. In some cases, sharks stayed away for six months or more before returning.
This massive migration of sharks has had downstream effects on the local ecosystem, the researchers said. Most notable is a new mid-level predator, the bronze whale shark (Carcharhinus brachyurus) has moved to the area to take over the niche that previously filled great whites. These sharks are often preyed upon by great whites, but now that the big boys are gone, sightings of bronze whalers have increased significantly, the researchers found. This shift could disrupt the entire food chain and put pressure on the entire ecosystem, the team added.
What is driving this killer whale eruption in the first place? As ferocious as they are, killer whales typically don’t prey on great white sharks, the researchers said. However, scientists have observed a rare flat-toothed subspecies of the killer whale in South Africa that appears to specialize in shark hunting. Port and Starboard may belong to this group, the team wrote. The sudden appearance of killer whales off Gansbaai may be “related to a decline in prey populations, including fish and sharks, changing their distribution pattern,” Towner said.
Further observations of Gansbaai’s whales and sharks are needed to fully solve this ocean murder mystery.
Originally published on Live Science.