Johannesburg, South Africa — Not a single great white shark has been spotted this year in a bay on South Africa's coast which, until recently, was home to a significant colony of the predators. False Bay, on Cape Town's eastern coast, has long been one of the world's premier locations to spot and interact with white sharks - and a key attraction for the region's $2.5 billion per-year tourism industry.

Thousands of tourists and scientists have flocked to False Bay to observe the massive sharks in their natural habitat from boats or, for the more adventurous, from inside cages lowered into the sea.

A multi-million-dollar research and documentary-making industry has even been established around the sharks, which are famed for leaping out of the water, or "breaching," as they hunt seals in the bay.

But not a single white shark has been recorded there since January, in spite of organized efforts; shark spotters are employed to look out for the animals, mainly for the safety of surfers and swimmers. I remember as a child frequently being told to get out of the sea when sharks were spotted.

Between 2010 and 2016, an average of 205 white shark sightings were recorded per year. The number of sightings fell to just 50 in 2018. And this year, nothing.

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Why did they go?

It appears this "shiver" of sharks, as the groups are known, has moved elsewhere. Scientists have no conclusive answer as to why they relocated, but one theory is that a pod of orcas moved in and pushed them out.

Until the killer whales arrived, the great whites were at the top of the local food chain. Shortly after the orcas moved in, the carcasses of large great whites began to wash up along the False Bay coast.

Forensic evidence has shown orcas killed them. Specifically, bite marks showed the great whites were targeted for their livers; the orcas ripped the organs from the sharks' bodies and then discarded the remains intact.

Supporting this theory, a few weeks ago great whites were spotted in Mossel Bay, about 200 miles from Cape Town, but when orcas arrived in the area the sharks vanished. A similar phenomenon has occurred off the coast of San Francisco — orcas appeared, sharks vanished, and when the orcas finally left the area, the sharks came back.

But other scientists dispute this theory, and say whatever the direct cause of the sharks relocation, humans are partly to blame.

The most serious threat to great whites is from fishing; sharks get caught on baited hooks and suffer devastating injuries. Some experts say fishing is also contributing to the great white sharks' disappearance because, while the predators are protected in South African waters, their food — including smaller shark species — are not.

There is no limit on the number of smaller shark species that licensed long-line fishing boats can remove from South African waters.

Marine biologists suspect that over-fishing has collapsed the stocks of the smaller shark species. That, they say, may have led to the starvation of juvenile great whites and driven adults elsewhere in search of more stable food supplies.

Will they come back?

Again, scientists don't know for sure.

If the orcas are part of the problem, then there is hope. As noted above, the great whites slowly returned to San Francisco's coastal waters after the orcas left the area.

Sharks are migratory and they can travel massive distances. One great white, named Nicole, after the actress Nicole Kidman, was tagged in South Africa in 2005 and swam all the way to Australia and back. But there is no guarantee the False Bay sharks will return to their old hunting grounds.

The impact

The impact on tourism in the Cape Town region has already been significant. Shark tours and cage diving bookings are down 50%.

But the impact on the ecosystem is of even more concern.

A great white shark jumps out of the water as it bites a fake decoy seal near False Bay, off Cape Town, South Africa, in this July 4, 2010 file photo. CARL DE SOUZA/Getty

White sharks influence hundreds of species in the bay, either through direct predation or by the fear of predation. Seals have been swimming more openly away from land, and smaller sharks have emerged from the kelp forests. Bronze whaler sharks have been seen swimming near beaches.

The continued absence of the great whites will likely bring more changes to the delicately balanced ecosystem, many of which marine biologists can't predict.