Something scares the great white sharks and it’s not man

They leave their territories

The great white shark (Carcharodon carcharias) is an amazing creature that inspires awe and terror in humans. He’s considered the ocean’s top hunter, but there’s something even he fears. Since 2017 there has been a decline in shark populations off the coast of South Africa. In fact, the city of Gansbaai is considered the Mecca of white sharks and thousands of tourists and researchers flock every year to admire or research them.

Initially, their population decline was thought to be due to overfishing, but the carcasses of eight great whites that washed up on the shores of Gansbaai since 2017 have made scientists reconsider.

The wounds on their bodies are distinctive and are undoubtedly attributed to an orca attack. In fact, scientists believe that a particular pair of killer whales is responsible for all the deaths of great whites we have seen in the past five years, leading the rest of the sharks to leave the area.

Research has shown that when an orca is present, sharks tend to leave the area they are hunting and do not return to it for months.

Initially, after one orca attack in Gansbaai, certain great white sharks were not seen again for weeks or months. What we are seeing appears to be large-scale avoidance, like what we see with the Serengeti wild dogs in Tanzania when the presence of lions increases. The more orcas that frequent these areas, the more great white sharks stay away.

The researchers saw 14 great whites that had been fitted with GPS leave the area when the pair of orcas appeared. The absence of great whites, however, disturbs the balance of the ecosystem in the area. Brachytail sharks (Carcharhinus brachyurus) are flocking to fill the ecological gap. They are usually the prey of great whites, but without great whites orcas hunt sharks.

Balance is critical to marine ecosystems, for example, without great whites restricting seal behavior, seals feed on the endangered African penguin species or compete with small pelagic fish for food. More simply, there is a certain amount of pressure an ecosystem can take and the effects of removing sharks from orcas will have an effect.

Orcas show a preference for hunting great whites, due to their nutrient-rich liver, and a yet-to-be-published study claims that orcas have adapted to prefer sharks due to the decline in the population of their prey. However, as great white populations decline globally, the extra pressure from orcas worries scientists.

The research was published in the African Journal of Marine Science .