Cape Town – Baby seals are often eaten by great white sharks that congregate around Seal Island in False Bay, but new research has shown that these youngsters get wise to the big predators and start changing their behaviour to avoid becoming shark breakfast.
Alta de Vos from Rhodes University, the lead author in the study published in Marine Mammal Science this month, said yesterday there were many more young seals attacked by great whites around the island than adult seals.
She and the other researchers had originally thought this was because sharks preferred young seals to adults. When they looked at the situation more closely, they found this was not the case.
It was because the adult seals were wiser than the babies, and avoided the shark “danger zone” at certain times of the day.
Great whites congregate around Seal Island during winter. Although scientists have not established why, they say it may have to do with the availability of plenty of seal pups born in December/January and which start to go to sea to feed in April/May. Or it may be because other shark food is not available during winter – or something entirely different.
De Vos said that earlier researchers had established that great whites mostly attacked seals between 7am and 10am – the “high-risk period”. This was probably because the light at this time of day made it it easier for sharks, cruising about 20m below the surface, to see the seals above them, but more difficult for seals to spot sharks below them.
There is also a high-risk area. Most shark attacks occur within 400m of the island, but the danger zone for attacks extends up to 1.5km from the island.
Once the researchers put radio tags on some of the adults and juveniles, they were able to see when individual seals left and returned to Seal Island over several months. What they found was that at the start of shark season, in the autumn, juvenile seals would go to sea at any time of the day.
“Juveniles were putting themselves more at risk at the beginning of the season by crossing the danger zone willy-nilly, at any time of the day,” De Vos said.
But by spring, none of the tagged seal youngsters ventured into the danger zone during the high-risk period, just after sunrise.
Alison Kock, another of the researchers, said the most interesting finding was how the youngsters adapted their behaviour after exposure to great white sharks.
“At the beginning of the season (autumn) when white sharks first arrive at the island, the young seals behave naively by swimming at the surface a lot, either alone or in pairs, during high risk times. But a few months later they had wisened up and started mimicking the more savvy adult behaviour, avoiding the high-risk sunrise period when conditions are ideal for ambushing by white sharks,” Kock said.
As a comparison, re-searchers tagged sharks at Egg Island near Paternoster, where shark predation on seals was low. They found that the young seals at Egg Island made no changes in their behaviour from autumn to spring.
Without great whites to contend with, the Egg Island youngsters left and returned to the island haphazardly throughout the study period.