“People would anecdotally say the sharks won’t swim into the kelp, not the great whites,” said Oliver Jewell, a doctoral student at Murdoch University in Australia.

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Great white sharks typically hunt at dusk or dawn, near the ocean’s surface, and will jump out of the water to snag cape fur seals — just like on T.V. But in the middle of the day, cage divers and researchers near Dyer Island Marine Reserve, off South Africa’s west coast, had noticed sharks lurking in areas close to the island’s cape fur seal colonies, which are surrounded by kelp forests.

They assumed the sharks, too big to maneuver through the dense vegetation, avoided the kelp forests where seals hide. But this footage reveals there’s more to a sharks’ predatory strategy than previously known.

“We kind of thought they would wait for the seals to come out of the kelp but no, they’re not that patient,” said Mr. Jewell, who led the research that captured this footage, which was published Wednesday in Biology Letters. “They want to go in and find them.”

In other well-studied white shark populations in South African waters, sharks aren’t found near kelp, because there isn’t much kelp. But here, mixing oceans stir up nutrients so kelp — and the animals it feeds — thrive.