![]() ![]() “White sharks eventually returned, and the numbers stabilized, only to disappear again for longer periods after more sightings of the implicated killer whale pair, and subsequently of other killer whales with straight fins.” “Boat surveys and telemetry data showed that white sharks left the greater Dyer Island region in May 2017, which is considered the peak season for white shark abundance…and remained absent from the region for six weeks,” Towner and her colleagues said in the study. But since Port and Starboard arrived, 14 tagged sharks have been observed noping out of the area and sightings by boat have plummeted. Great whites are normally abundant at Gansbaai, which attracts tourists from around the world to cage-dive with these massive hunters. The team combined information from the “necropsies'' of the shark carcasses, sightings from boats, and telemetry data of shark movements obtained from tagged individuals to document the mass emigration of this iconic species from what was once a major hub for them. Towner and her colleagues considered alternate explanations for the recent drop of great whites at Gansbaai, including human fishing pressures and sea surface temperatures, but the comprehensive evidence they gathered strongly implicates Port and Starboard in the flight of sharks from the area and hints that this pair “might be members of a rare shark-eating morphotype, known to hunt at least three shark species as a prime source of nutrition in South Africa,” according to the study. “Predator–prey interactions between white sharks, other coastal sharks, and killer whales are now increasing in South African waters and are expected to have pronounced impacts on the coastal ecosystem.” “We provide evidence that killer whale predation on white sharks directly induced emigration of sharks from Gansbaai, and individual white sharks did not return for weeks or months,” the team added. “The loss of top predators can disrupt ecosystems…but the introduction of novel apex predators into ecosystems is not well understood.” “Risk-induced fear effects exerted by top predators are pervasive in terrestrial and marine systems, with lasting impacts on ecosystem structure and function,” said researchers led by Alison Towner, a senior white shark biologist at the Dyer Island Conservation Trust and a PhD candidate at Rhodes University, in the study. The recent arrival of these killer whales in the area, followed by the sudden decline in sharks, suggests “that white sharks respond rapidly to risk from a novel predator,” a reaction that could “have long-lasting ecosystem effects” in this area, and many others where these hunters meet, according to a study published on Wednesday in the African Journal of Marine Science. ![]()
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