Great whites flee an orca invasion
The world’s most photographed population of great white sharks has disappeared from the South African coast where ‘‘rogue’’ killer whales have moved in.
The great whites of False Bay are celebrated for their spectacular breaching around Seal Island, a learnt behaviour used to capture prey.
The phenomenon has featured in scores of wildlife documentaries, including the BBC’s Planet Earth, and each year draws 80,000 tourists to False Bay and Gansbaai, a 21⁄2-hour drive east along the coast, to watch from boats or dive in cages.
However, during the sharks’ winter hunting season, which runs from June to October, not a single great white has been seen. ‘‘To our knowledge the absence of great white sharks from False Bay has not been recorded or reported before,’’ Marian Nieuwoudt, a Cape Town environment official, said.
A number of factors have been cited by scientists to explain their absence. The arrival in 2015 of two killer whales in the area coincided ‘‘with the timing of the significant change in behaviour of white sharks’’ according to Alison Kock, a shark specialist.
The male killer whales, known to researchers as Port and Starboard, have adapted to become their own subgroup of ‘‘super-predators’’ preying on Cape Town’s shark population, including the great whites. Scientists have been struck by their hunting technique – a clean tear between the pectoral fins of their prey to remove only their livers, leaving all other organs intact.
Killer whales, or orcas, are nature’s most cunning and organised predators. Yet their attacking of great whites, which have no other predators apart from humans, is highly unusual and has unsettled the hierarchy of their hunting grounds, researchers believe.
Between 2010 and 2016, there were more than 200 shark sightings recorded each year off Cape Town’s False Bay beaches. By last year that number had fallen to 50 and none so far this year. Tracking devices implanted into about 50 great whites have not been detected along South Africa’s coast or elsewhere. In their place, sevengill sharks, normally scavengers, have moved into the waters of Seal Island, four miles from the False Bay mainland and home to more than 60,000 Cape fur seals.
Scientists are keen to discover whether the disappearance of great whites is part of the species’ complex movement patterns or a reflection of a sudden drop in numbers. The species’ worldwide population is estimated to be between a few hundred and 3000.
In Gansbaai, where anxious tour operators try to market cage diving with bronze whaler and sevengill sharks, few visitors are taking the bait.
‘‘When most people hear there are no great whites, they don’t want to go out,’’ Chris Fallows, a shark expert who helped to produce the footage for Blue Planet, said. ‘‘It is like Africa losing its lions. The great white shark is an iconic and enigmatic animal and our coastline is so much poorer for not having them here in the same numbers.’’