For over two decades, scientists have been monitoring the disappearance of great white sharks in False Bay, South Africa. These apex predators have historically been abundant in the region, which makes the dramatic decline in their numbers particularly concerning to scientists. A new study conducted by researchers from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science has determined the most likely reasons behind the sharks’ disappearance. The study also examines the far-reaching consequences of their absence on the ecosystem.
The Great White Sharks of False Bay
Great white sharks were once abundant in False Bay, but their populations began to decline significantly after 2015.
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False Bay is located southeast of the Cape Peninsula in South Africa, about 13 miles from Cape Town. The bay is a popular swimming and surfing destination for both locals and tourists. False Bay has long been considered a hot spot for great white sharks (Carcharodon carcharias), where they could be found prowling the coasts year-round. Scientists note that populations of great white sharks remained relatively stable during the first 15 years of monitoring. Yet after 2015, their numbers began to decline significantly. Researchers found there was an 82% decrease in great white shark sightings between 2016 and 2020. No great white sharks have been seen in False Bay since mid-2018. This raises the question: Where did the sharks go?
What Happened to the Sharks?
The exact reasons behind the sharks’ disappearance are still uncertain. However, the authors suggest that the number of great white sharks removed by the KwaZulu-Natal Sharks Board is high enough to substantially impact regional shark populations. The KwaZulu-Natal Sharks Board’s mission is to protect swimmers from shark attacks along the KwaZulu-Natal coast. Unfortunately, even the documented level of shark removals is likely driving the population decline. Researchers also fear that the actual number of removals may be underestimated.
Some studies have suggested that shark populations remain stable and that there has merely been a redistribution of shark populations from False Bay to other areas in response to orca (Orcinus orca) predation. However, the authors of the current study argue that the data do not support this theory, as the population decline was observed before orcas appeared in the region. Great white shark mortality due to orcas has not been documented in False Bay, although it has been observed in Gansbaai, in the Western Cape Province. While orca predation may contribute to the decline of great whites in False Bay, it is not considered the primary cause.
The Impact of the Sharks’ Disappearance on the Ecosystem
The absence of great white sharks in False Bay has resulted in an increase in Cape fur seal populations.
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Researchers have found evidence of a trophic cascade after the loss of great white sharks from False Bay. A trophic cascade refers to the impact that organisms at one level of a food web have on those at other levels within an ecosystem. Scientists explain that this research makes it clear that the removal of an apex predator has significant consequences for marine ecosystems.
Researchers used underwater video surveys, conducted more than ten years apart, to compare the food web before and after the sharks’ disappearance. The absence of great white sharks has led to an increase in Cape fur seal and sevengill shark populations. As a result, the populations of their prey—small fish for the seals and small, bottom-dwelling sharks for the sevengill sharks—have declined. A scarcity of prey can lead to reduced reproductive success and starvation among predators. Without apex predators to regulate prey populations, the ecosystem’s biodiversity and stability may suffer. Researchers warn these changes could ultimately lead to enduring impacts on ocean health.
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