ARE GREAT WHITES FLEEING ROGUE KILLER WHALES OR ARE HUMANS A GREATER THREAT?

 

Reports of the mysterious disappearance of great white sharks from Cape Town have gone viral across South Africa and around the world. In fact, the decline in great whites has been noticeable and reported on since 2015 with a number of articles speculating as to the causes and expressing concern about the impact on marine ecosystems due to the loss of an apex predator. Great whites, through the eco-tourism and documentary filmmaking sectors, also contribute significantly to Cape Town’s local economy.

In August, the City of Cape Town and Shark Spotters released a statement on the phenomenon, saying that "great white sharks have been noticeably absent from False Bay on the Cape coast during 2019, prompting questions as to when the apex predators will return." The statement also notes that between 2010 and 2016, an average of 205 white shark sightings a year were recorded but that since then, the numbers have dropped significantly, with only 50 seen since the start of 2018 and none spotted in 2019. So where are the sharks and will they be back? There are a variety of theories but the most popular appears to be the arrival of killer whales.

Between May and June 2017, it was reported that a total of four dead great whites had washed ashore at Gansbaai. Necropsies showed that they were all victims of predation, with their livers removed and wounds and bite indentations consistent with Orca attacks. Since 2009, there has been a steady increase in the frequency of killer whale sightings. A local whale watching charter documented the arrival of two new killer whales in the bay in January 2015. In 2017, it was suspected that these same two killer whales were responsible for the death of the four great whites.

Initially the pods in the bay were observed feeding only on marine mammals such as common dolphins and Cape fur seal. So, why did the orcas start killing sharks? Evidence points to the arrival of a different kind of killer whale. Dr Alison Kock (a marine biologist from the Cape Research Centre at UCT) and other scientists, are speculating that a new sub-group of killer whale has arrived in the bay, which habitually feeds on sharks. Due to the unique predatory niche occupied by sharks, the increased presence of these particular killer whales in False Bay could have profound impacts throughout the ecosystem.

Meanwhile, the Department of Agriculture, Forestry and Fisheries (DAFF) has said that great whites have not completely disappeared from the False Bay coast. Responding to the City of Cape Town's statement, the department said white shark cage diving operators have spotted sharks around Seal Island, and that if white sharks are showing indicators of displacement within False Bay, there may be a need to evaluate the general coastal ecosystem health within the bay.

Although many people (including scientists) are favouring the killer whale theory to explain the disappearance of the sharks, others believe that a combination of factors are responsible. Shark researcher, Professor Neil Hammerschlag from the University of Miami, and naturalist Chris Fallows, who runs Apex Shark Expeditions have – since 2000 – been conducting a long-term collaborative study. Together with his colleagues in South Africa’s shark tourism business, Fallows believes the main culprit is unregulated fishing, with long-line fisheries targeting smaller species of sharks, including soupfin and smooth-hound sharks, which are a favoured prey species of great whites. Fallows says these boats have intensified their efforts in recent years and have driven stocks of smaller shark species to collapse, which may have led to starvation of juvenile great whites and driven other great whites elsewhere. The fishery may also be illegally hooking and killing young great whites and protected hammerhead sharks.

Dr. Sara Andreotti, speaking on CAPE TALK also refuted speculation that the presence of orcas along the coast had led to the vanishing of the great whites. She believes this could be the result of changes in the climate, overfishing, pollution, and shark nets in KZN ... and that while great whites are protected in South Africa, their food source is not. In other words, what could be a localised extinction, would be mostly attributable to the impact of humans rather than killer whales: "I was probably the least surprised about the disappearance of the white shark in False Bay considering that back in 2012, we estimated the population to be in between 300-500 sharks around the coast. We also did a genetic study to confirm this result and it confirmed that the white shark population in South Africa was in very big trouble. ... this is not a crash that suddenly happened, or an orca event that immediately made the sharks disappear. It is a decline that has been happening for years.”

Marine biologist Alison Towner, who does extensive research on great white sharks, also says there could be a number of factors contributing to the change in shark sightings: “It is very likely a combination of factors impacting white shark distribution, including, environmental, prey species removal, industrial overfishing, pollution as well as other threats.”