Great white mysteries unravelled

BREACH: Great whites often grab the headlines, but we know very little about them. Now Cape Town scientists have discovered an intriguing new fact about the movement patterns between the sexes. Photo: Morne Hardenberg

Melanie Gosling

Environment Writer

IF you spot a great white shark close inshore in False Bay this summer, your first thought will probably not be to wonder what sex it is – particularly if you’re in the water.

Nevertheless, scientists can now tell you that you would almost certainly be looking at a female great white.

That they can say this is the result of groundbreaking research by Cape Town shark scientists who tracked the movements of 56 tagged white sharks of both sexes in False Bay for nearly three years.

Shark scientists are excited about the finding and say it is yet another piece of the puzzle they have found in their quest to build up a picture of this creature, the subject of so much publicity, yet one about which so little is known.

They say that the more they know, the better they’re able to manage them.

The research, led by Cape Town shark scientist Alison Kock and published online yesterday in the journal PLOS ONE, forms part of Kock’s doctoral research at UCT.

“It’s very exciting. We never expected to find that. We’ve known for some time that white sharks come inshore in False Bay in spring and summer, but now we’ve found that it’s the females that do that, and the males don’t.

“What we don’t know is where the males go. That’s the big question,” Kock said yesterday.

The 56 tagged sharks ranged from 1.7m to 5m.

Great whites congregate around Seal Island in autumn and winter, where they feed on seals, particularly the young. They move away from the island in spring and summer to take advantage of the migratory fish that move inshore.

“In Australia they have found that male and female sharks at Neptune Island had different timings at the seal colony, but no one had ever shown that the sexes actually segregate at this level for both adults and juveniles.”

While people may regard the presence of sharks inshore as an increased risk to themselves, being closer inshore poses an increased risk for the sharks too.

“There’s a lot more disturbance at the coast – fishing, pollution, habitat destruction from coastal development, so the risks to the sharks are elevated for the females compared to the males.”

Does her research suggest attacks on humans by great whites are most likely to have been by females, given that attacks are mostly inshore?

“We do wonder about that, and can’t say. We’ve picked up one male this summer, but that was at the island. It could be that occasionally a male comes inshore.”

One of the ways this new research can feed into management is in the case of shark nets. Kock said had the city council decided to erect shark nets at beaches, they would unknowingly have been killing only females.

Traditional shark nets, like those used in KwaZulu-Natal, are designed to entangle and kill sharks, to reduce their numbers. The nets also kill many other species, such as dolphins.

The nets to be erected at a portion of Fish Hoek beach are small-meshed, designed to exclude, not kill, sharks.

“Luckily people in Cape Town are still of the mindset not to have shark nets or go out and kill sharks. But that view can change. Every time there is an attack, the pressure is on.”

l Kock is co-ordinating a UCT summer school course on white sharks today and tomorrow. See www.summerschool. uct.ac.za