[imagesource: Dave Hurwitz]

Researchers have spent years documenting the gradual disappearance of great white sharks from the waters off Cape Town.

It’s been widely reported that Port and Starboard, two infamous orcas (or killer whales) that frequent these waters, have taken a liking to great white shark liver.

A November 2020 study, carried out by a government-appointed team of nine experts, also found that the sharks’ disappearance was likely “a result of recent orca occurrence and predation, rather than being related to the fishing activity of the demersal shark long-line fishery”.

At the same time, respected marine scientists still maintain that overfishing of smaller species has also played a role.

This week saw a new study published in the African Journal of Marine Science, led by Alison Towner, a senior white shark biologist at the Dyer Island Conservation Trust.

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“Killer whales are highly specialized hunters,” said Alison Towner, a marine biologist at the Dyer Island Conservation Trust and lead author of the new research, in an email to Gizmodo.

“This specific type has learned how to target sharks for their lipid-rich, nutrient-dense liver, which can be up to one-third of the shark’s body weight.”

Image: Dyer Island Conservation Trust

Using tagging and sighting data, Towner’s team determined that the great white sharks have stopped visiting certain regions along the South African coast where the orcas have been feeding on the subadult sharks. “The more the orcas frequent these sites, the longer the great white sharks stay away,” Towner said.

Port and Starboard, easily recognisable due to their ‘floppy’ dorsal fins, have actually thrown the entire ecosystem into turmoil due to their fondness for great white shark livers.

The new study pins at least eight great white deaths on the duo. Since 2017, eight carcasses have washed up on the shores of the Gansbaai coast, with seven missing livers and the heart and testes of one carcass also torn out. It’s likely that Port and Starboard were responsible for further great white deaths where the carcasses didn’t wash up on shore.

The dearth of great whites in certain areas has led to the bronze whaler shark emerging as the apex predator:

The reduced presence of great whites has also increased local numbers of their prey, Cape fur seals, which are in turn eating the critically endangered African penguins.

“To put it simply, although this is a hypothesis for now, there is only so much pressure an ecosystem can take, and the impacts of orcas removing sharks are likely far wider-reaching,” Towner said.

In recent weeks, a number of great whites have been spotted around Gansbaai, much to the relief of shark cage diving companies.

We now know that Port and Starboard are back in Cape Town. Yesterday, Simon’s Town Boat Company owner Dave Hurwitz posted pictures from a sighting in False Bay:

The text from the post includes:

Port & Starboard, our famous (or infamous) killer whales showed up in False Bay today after terrorising the shark population from Cape Point all the way up to Algoa Bay for the past month.

They are a formidable pair and can travel great distances in very short periods of time, for example: Cape Point on the 15th June – De Kelders on the 16th – Mossel Bay on the 20th.

Image: Simon’s Town Boat Company

This is what makes killer whales such successful predators, they’re so unpredictable that their prey don’t know where or when they’ll next pounce.

Today they were foraging in a few of their usual spots in False Bay but what was different is that instead of following their regular route, they kept doubling back & also moving much further offshore.

Hurwitz added that the highlight of the encounter was “managing to record their underwater vocalisation with a hydrophone”.

The duo move rapidly and cover huge distances so you have to strike it lucky to catch a sighting.

[sources:gizmodo&facebook]