A shark marked for the study. (Credit: Jewell et al. (2019) Biology Letters)
The high, undulating algae, known as seaweed, grow in dense underwater forests off the coast of southern Africa. It was believed that seaweed forests provided a safe haven for Cape seals of great white sharks. Then, researchers place high resolution cameras similar to GoPro in predators.
Researchers find that, instead of being deterred by underwater flora, sharks dive into the dense forests of seaweed in search of prey. It is a new discovery for shark researchers, who had previously thought that forests were banned for hungry sharks.
"It's a completely new perspective on what they do," said Oliver Jewell, a marine biologist at Murdoch University in Australia who led the new research.
A shark being marked for the study. (Credit: Jewell et al. (2019) Biology Letters)
Great white sharks are ambush hunters. Usually, seals are lowered at dawn or dusk, while pinnipeds swim to and from rocky outcrops. But in previous research, Jewell and his colleagues discovered that near Dyer Island, a reserve in the southern tip of Africa, sharks were kept close to the seal colonies throughout the day.
"We wanted to know why, but it was hard to be sure without seeing what the sharks were doing below the surface," Jewell explained.
The researchers connected high-definition video cameras to eight sharks while nine to 12-foot predators swam freely in the water, making sure they got the correct angle to see what the sharks could do below the surface. Each camera recorded eight hours of images during daylight hours for the next one to three days before leaving the animals.
Video and tracking data from the cameras revealed that the forests did not deter predators. Almost all sharks moved repeatedly into the dense jungle of algae, researchers report in the magazine Letters of biology of the real society. A shark even spent most of its time inside the thick canopy.
The findings challenge the previous work. "A previous study found that Cape fur seals were taking refuge from white sharks in the kelp forest," Jewell said. "What we discovered is that white sharks go to the algae forest behind them and are more than capable of navigating and feeding in and through dense algae."
But even entering the seal's hideout is not a guarantee of a meal. The seals evaded the sharks with expert deviation tactics. They made bubbles, swam deeper in the seaweed or settled close to the seabed. The evasive strategies were effective. None of the sharks managed to snag a seal during the study, although that may be due to the fact that successful catches are quite rare in general.
However, "to really put it in the camera, a camera connected to a shark too, was a great discovery and an incredible feeling," said Jewell.