Published in The Herald (South Africa) on 13 September 2024
By Guy Rogers
.PNG)
We urgently need to know more about the movement of the white shark — not least because of its effect on the survival of the endangered African penguin.
That is the view of white shark specialist Dr Alison Towner, who was speaking at an SA International Maritime Institute seminar at Nelson Mandela University yesterday.
Towner said white sharks had abruptly disappeared from Western Cape waters between 2015 and 2017, altering the natural balance on the coastline.
“In a 2022 case study we did in Gansbaai, we found that the Cape fur seals that the white sharks used to prey on were becoming more bold.
“The seals were increasing their pressure on sardines and anchovies which the penguins also relied on for food.
“There were also incidents where seals attacked penguins directly.”
Speaking earlier, economist Prof Stephen Hosking said it was vital to establish a sound valuation of the ocean economy because only in that way could different applicant uses be fairly assessed and accommodated.
This was necessary because of dramatic human population growth, dwindling land resources, rising incomes in countries such as China driving increased consumerism, and rapid changes in climate and technology.
“At the same time there is an international optimism that the ocean holds the promise for renewed wealth creation and sustainable development.
“This optimism is shared in Africa and in initiatives like Operation Phakisa in SA.”
He said a comprehensively valuated “virtuous” ocean economy would highlight the need for environmental and economic sustainability, as well as inclusivity.
Towner said extensive monitoring and research still needed to be done to understand exactly what happened to the Western Cape’s white sharks.
The species disappeared from their previous strongholds in False Bay and Gansbaai, where the sharks had underpinned an $8m (about R142m) a year cage-diving industry.
She and colleagues based in Gansbaai established that their disappearance from the region was the result of a sudden onslaught by a particular pair of orcas.
Their findings, the first of their kind, showed how the orcas were flipping the white sharks onto their backs and tearing out their highly nutritious livers.
Towner said subsequent studies showed that the culprit pair, nicknamed Port and Starboard for their distinctively slanted dorsal fins, had passed on their savage skill to other killer whales, in a “cultural transfer” typical of highly sophisticated predators.
“There are many questions, including what made the orcas suddenly arrive, and have the white sharks declined or just moved?
“And if they have moved, where to?
“The hypothesis best supported by the shark incident file is that white sharks have partially shifted their distribution away from the Western Cape and that they have moved east to places like Algoa Bay, the Wild Coast and KwaZulu-Natal.
“But we need more monitoring and research to establish this for sure.”
She said though a 2012 census put the SA white shark population at about 1,000, there were many unknowns.
“In fact, the animals we see off our coast constitute a Southern African population, and they range sometimes much further than that.”
She said it was a concern that there had been no evidence of population increase after the sharks were declared a protected species in 1991, and that the average size of female white sharks caught in KwaZulu-Natal nets had declined.