The great whites vanished. Then these sharks took over
What has happened to False Bay's fearsome great white sharks is a mystery
Great white sharks have disappeared from False Bay and no one knows why.
Chris Fallows, who has monitored the sharks at Seal Island for 18 years, says the apex predators’ departure began in 2015. By 2018, their seal meals at the island had sunk to zero from a peak of seven an hour in 2004.
In their place, sevengill sharks have moved in from the kelp forests where they have traditionally hidden away to escape their cousins’ fearsome jaws.
More than 60 consecutive sampling days between August 2017 and May 2018, no white sharks were sighted off the “rookery” of 60,000 Cape fur seals.
“The reasons for the white shark population declines, combined with prolonged periods of disappearances during 2017 and 2018, remain unknown,” Fallows and fellow academics say in a study published on Wednesday in the journal Scientific Reports.
“It is possible these patterns reflect population-level declines for the region, for example, from overfishing or habitat loss.
“It is also possible that False Bay’s white sharks have shifted their distributional range elsewhere due to shifts in environmental conditions.”
Study leader Neil Hammerschlag, from the University of Miami, said research into the sharks’ disappearance was a priority.
Fallows, of Apex Shark Expeditions, is also a photographer, and his pictures of great whites hunting seals by leaping out of the water have made him famous.
Since 2000, when he began recording observations at Seal Island, he recorded 6,333 different sharks and 8,076 attacks on seals.
“In 18-plus years of working at Seal Island, we had never seen sevengill sharks in our surveys,” said Fallows.
“Following the disappearance of white sharks in 2017, sevengill began to show up for the first time and have been increasing in number ever since.”
During periods of great white absence in 2017 and 2018, the researchers made 120 sevengill sightings and witnessed an individual attacking a live seal.
“On March 28 2018, at 9.45am, an estimated 2m sevengill shark rose to the surface and attacked a young-of-the-year Cape fur seal,” said Hammerschlag.
Historically, the only well-known aggregation site for sevengills in False Bay was within inshore kelp beds at Millers Point, 18km from Seal Island.
The study suggested the appearance of sevengills at Seal Island was due to the disappearance of great whites, allowing them to exploit the area without risk of predation from great whites or competition with them for prey.
But the mystery of what is really going on beneath the surface of False Bay remains.
“We are not aware of any major environmental or biological perturbations across the study period that we can attribute to the appearance of sevengill sharks at Seal Island in 2017 and 2018, with the exception of the disappearance of white sharks from our surveys,” said Hammerschlag.