Sharks and seals

There are thousands of marine species, including 27 sharks, that play an important part in the balance of life in the bay, but in the minds of people there is one that overshadows them all: the great white shark. Since the 1920s, the white shark population in False Bay has been recognised as exceptional and the area’s shark abundance is almost unrivalled elsewhere.

But what conditions support this profusion of white sharks? Seals, for starters. A consistent and abundant supply of them gives these marine giants – even though they are not yet fully adult – an ideal refuge to grow up in before they disperse into the wide ocean.

'They're not going to ignore a food source like this!', says Dr Allison Kock.

Kock is a Cape Town-based marine biologist. False Bay’s Seal Island, a small rocky outcrop, is home to the second largest breeding colony of Cape fur seals in South Africa. This means the area is one of the most vital for white sharks anywhere in the world.

Because of the close relationship between predator and prey, the behaviour of the sharks in the bay reflects the breeding cycle of the seals. ‘From November to January [summer], the males arrive and set up their hareems. The seals are mating in deeper water and the females are giving birth on the island,’ says Kock, a project leader funded by the Save Our Seas Foundation who has been working with False Bay’s sharks for more than 15 years.

‘At this stage, the pups are only drinking [milk] from their moms, they are not yet going into the water and supplementing their diet with fish. So they are not available to the sharks.’ At the onset of winter in about April, however, the seals’ seemingly languid lifestyle is disrupted. This is when white sharks start spending most of their time near the island, and it’s no coincidence that this is also when the naive pups begin venturing into the shallows. At about four months old, the young seals are inexperienced and the sharks capitalise on their vulnerability. Winter in False Bay is the season of breaching, when four-metre-long, 1.5-tonne sharks launch themselves completely out of the water in pursuit of seals.

By the time spring rolls around again, though, the young-of-the-year seals are not so young any more. They have wised up, and they know how to avoid being eaten. The sharks move off and the cycle begins anew.

This dance of predator and prey, life and death, plays out within a few kilometres of the False Bay shore. But where do the sharks go when they aren’t at the island? This question was answered only a couple of years ago when research showed that white sharks in the bay, while present year-round, spend their time differently depending on the season. In winter, both male and female sharks hang out around Seal Island; in summer, though, males disperse along the South African coast while females move closer to shore. Summer is a time when other sharks and fish are also inshore, providing another primary food source for the white sharks.

Critically, this finding means that during the busiest time of the year for Cape Town’s beaches, white sharks and people are using the same space, and inevitably – although rarely – their paths cross.