The sudden disappearance of great white sharks from South Africa’s False Bay has resulted in major changes across the local marine ecosystem, according to a new study.
Famed for their spectacular breaches as they hunt seals, great white sharks (Carcharodon carcharias) were once a common sight in the waters off False Bay. But in recent years, sightings have plummeted – and researchers say their absence is causing a knock-on effect throughout the food web.
The research, published in the journal Frontiers in Marine Science, was led by Neil Hammerschlag from the University of Miami Rosenstiel School and draws on more than 20 years of data.
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What’s happening in False Bay?
Historically, great white sharks could be found year-round in False Bay, where they would spend the summer months patrolling the inshore areas and winter in the waters around Seal Island, a rocky outcrop roughly 10km off the Bay’s northern shoreline.
But the great white sharks have now vanished, say the researchers, who spent two decades monitoring the species using boat-based surveys.
"The overall trend in the relative abundance of white sharks was relatively stable in the first fifteen years of monitoring; however, their relative abundance began to precipitously decline after 2015, leading to a complete disappearance of white sharks from the standardised surveys since mid-2018."
The scientists believe the decline could be due to a mix of human impacts – including shark nets, which are installed along the coast to protect swimmers – and predation by orcas, which have recently been seen hunting the sharks.
Ripple effect
Alongside the boat-based surveys, the study involved the use of underwater video surveys taken more than a decade apart to provide a snapshot of the food web both before and after the disappearance of white sharks from False Bay.
“The number of individuals of a given species recorded on the videos not only informs us about their numerical abundance, but also their behaviour, as species under increased predation risk often become more elusive and are thus less likely to be detected on our cameras,” explains Yakira Herskowitz, a co-author of the study.
Following the loss of great white sharks from the area, the study shows that cape fur seal (Arctocephalus pusillus) numbers have gone up. Sevengill sharks (Notorynchus cepedianus) have also become more common. As a result, fish that seals rely on for food are now under pressure, and smaller shark species that sevengills hunt are also in decline.
“These changes align with long established ecological theories that predict the removal of a top predator leads to cascading effects on the marine food web," says Hammerschlag.
Why it matters
The findings reinforce the vital role apex predators such as sharks play in keeping ecosystems balanced. “Without these apex predators to regulate populations, we are seeing measurable changes that could have long-term effects on ocean health,” says Hammerschlag.
The study emphasises the importance of shark conservation efforts around the world.
Find out more about the study: Evidence of cascading ecosystem effects following the loss of white sharks from False Bay, South Africa
Main image: great white shark. Credit: Getty
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