There has been a dramatic decline in great white shark sightings in False Bay, Cape Town, and it looks like shark-eating killer whales could be to blame.
These killer whales have moved to Cape waters seeking fresh hunting grounds following declines in prey species in other parts of the world, writes Dr Alison Kock, a marine biologist from the Cape Research Centre at the University of Cape Town, in The Conversation.
Four years ago, several dead sevengill sharks were found by scuba divers at a popular dive site inside the Table Mountain National Park marine protected area.
Initially humans were blamed. But after more dead sharks began to surface, it was clear that killer whales were responsible for the killing.
According to Kock, this was unheard-of behaviour. These super predators, which were first spotted just a decade ago in False Bay, previously stuck to marine mammals (like seals).
Two killer whales, nicknamed “Port” and “Starboard”, were sighted near the sevengill aggregation site at the time of both incidents in 2015 and 2016. In 2017, it is suspected that these same two killer whales were also responsible for the death of five great white sharks further up the coast in Gansbaai.
Further sightings of killer whales have been recorded, with the latest being in November at Miller's Point in Simons Town and Buffels Bay.
Kock and other scientists now speculate that a new sub-group of killer whales, who specialise in shark hunting, have arrived in SA to feast.
“Evidence from our literature review points to the arrival of a different killer whale, one which targets sharks. Typically, it used to occur offshore. But that seems to have changed.”
When food is scarce, research shows that killer whales can adapt. In the 1990s killer whales began targeting sea otters and caused massive population declines in Alaska.
This in turn had a knock-on effect as the number of sea urchins (which sea otters eat) exploded. And as sea urchins’ primary food is kelp, this increase resulted in the deforestation of kelp forests in the region.
According to Shark Spotters, the 2017/18 summer season recorded an all-time low number of great white sharks spotted across eight beaches in Cape Town.
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