Loss of great white sharks triggers domino effect down food chain, study shows


CAPE TOWN — South Africa’s False Bay was once known as a global hotspot for great white sharks. But within the span of a few years, between 2015 and 2019, this apex predator vanished from the area, leading to profound ecological changes, according to a new study.

The study suggests that the disappearance of the great white shark (Carcharodon carcharias) from the bay on South Africa’s southwestern Atlantic tip triggered a trophic cascade in the local marine ecosystem — a domino effect that rippled down the food chain.

Scientists speak of the phenomenon of a trophic cascade when the change in prevalence of a top predator leads to changes across all levels of the food chain below them.

With the loss of the sharks, populations of their prey species like Cape fur seals (Arctocephalus pusillus pusillus) and sevengill sharks (Notorynchus cepedianus) in the study area increased, while the number of small fish and smaller sharks that sevengills prey upon declined.

The changes also coincided with shifts in the behavior of animals that live in the orbit of great whites.

Study lead author Neil Hammerschlag, a marine ecologist and CEO of the Canada-based Shark Research Foundation, analyzed data from a long-term monitoring program that tracked white shark presence around False Bay’s Seal Island between 2000 and 2020. He and other scientists documented a stable great white population until 2015, followed by a sharp decline that led to their complete disappearance from the bay by mid-2018. Data show that within three and a half years, great whites had disappeared from the area.

Analyzing data from standardized boat-based surveys, baited remote underwater video stations (BRUVS) and citizen science seal sightings, the authors found that the significant drop and eventual disappearance of great white sharks led to major ecological shifts throughout the food chain.

As great whites were disappearing, some animals directly below them in the food chain began to be seen in unusual locations.

“We started seeing sevengill sharks at the boat instead of great whites, which was mind-blowing,” Hammerschlag said. These sharks had typically stayed away from where their predators were hunting, preferring the kelp forests of False Bay, more than 20 kilometers (12.5 miles) from Seal Island.

At the top of the food chain, great white sharks in False Bay preyed on Cape fur seals and other sharks like sevengill sharks. Seals, in turn, primarily prey on small pelagic fish like horse mackerel (Trachurus capensis) and anchovies (Engraulis encrasicolus), while sevengill sharks feed on fish and smaller sharks, such as pyjama catsharks (Poroderma africanum) and smooth-hound sharks (Mustelus mustelus).

The study authors monitored a whole range of species in the great white sharks’ surroundings, but homed in on sevengill sharks and seals. “We focused on those [species for which] we had a clear understanding of predator-prey relationships,” Hammerschlag said.

“As Neil showed, in less than 10 years, the entire ecosystem of an entire bay completely changed,” said Enrico Gennari, a marine biologist and director of the Oceans Research Institute in South Africa, who has been monitoring white sharks in the bay for many years.

Gennari has collaborated with Hammerschlag in the past but was not involved in the new study.

The decline of great white sharks and the rise of sevengill sharks was accompanied by a sixfold (522%) increase in Cape fur seal sightings over the study period. The researchers noted that, just like sevengill sharks, seals began appearing in areas they previously avoided due to the threat of predation. “They used to stay close to the shore around Seal Island. Now they are rafting in deep waters and in locations where they historically would have been eaten,” Hammerschlag said.

The disappearance of the white sharks might also have contributed to less stress among Cape fur seals indicated in an earlier study by Hammerschlag that analyzed seal feces over time. Hammerschlag said he observed a drop in the seals’ cortisol levels in correlation with decreasing white shark numbers.

“Without great whites, fewer seals and sevengill sharks are being eaten, and also, they’re not afraid anymore — their physiology changes,” he said. “It looked like they no longer feared the sharks.”

After observing changes in the occurrence and behavior of species that great white sharks prey on, the scientists looked further down the trophic chain. They analyzed data on anchovies and Cape horse mackerel, which seals, and to a lesser extent sevengills, prey on. Replicating a baseline study from 2015, the authors describe a decline in these species, especially the mackerel, which they linked to the increase in the number of seals.

The impact on small fish populations was less pronounced than that seen for the seals and sevengill sharks, which is a pattern seen in other studies looking at trophic cascades.

The study authors acknowledged that there are data gaps: sampling periods were limited, and they lack the complementary data sets from other survey methods that are better suited to measuring change in certain species. They point out, however, that while broader supporting data are necessary to confirm their findings, there has certainly been an impact on the ecosystem following the loss of white sharks in False Bay. Just how far down the food web that impact goes is something that needs further investigation, according to the study authors.

But just why great white sharks have disappeared from False Bay remains hotly debated. Some scientists suggest the presence of orcas (Orcinus orca), known to prey on great whites, may have driven the sharks to relocate farther east. Others, including Hammerschlag and Gennari, attribute the decline to ongoing mortality caused by shark nets and drum lines installed off the South African coast by the KwaZulu-Natal Sharks Board, which is a provincial public entity mandated to protect bathers against shark-related incidents. Those measures put in place to prevent sharks from swimming too close to the shore also cause shark mortality by trapping the animals in mesh barriers (shark nets) or culling them with baited hooks (drum lines).

The KwaZulu-Natal province faces the Indian Ocean on South Africa’s eastern coast, hundreds of kilometers from False Bay in the southwest of the country.”

“We know the distributions of white sharks in South Africa used to span from False Bay to Mozambique. So, all white sharks moved between False Bay and KwaZulu-Natal,” Gennari said. “If they are killed in KwaZulu-Natal, the population number goes down and the distribution shrinks from both the edges of the distributions.”

What is uncontested is that the disappearance of any apex predator from any food chain impacts their prey populations. Apex predators play a crucial role in maintaining ecosystem balance by regulating prey populations. Hammerschlag warned that without white sharks, False Bay’s ecosystem may become increasingly unstable, leading to long-term ecological changes that are difficult to predict.

“We need white sharks in our waters to maintain the balance of the ecosystem,” Gennari said. He added the decline in great white numbers isn’t only happening in False Bay. While the study was focused there because of the availability of long-term data, similar declines have been recorded in other former great white hotspots in South African waters, including Mossel Bay, Gansbaai, Plettenberg Bay, Algoa Bay, and off the KwaZulu-Natal province on South Africa’s Indian Ocean coast.

Citations:

Hammerschlag, N., Herskowitz, Y., Fallows, C., & Couto, T. B. A. (2025). Evidence of cascading ecosystem effects following the loss of white sharks from False Bay, South Africa. Frontiers in Marine Science, 12. doi:10.3389/fmars.2025.1530362

Hammerschlag, N., Fallows, C., Meÿer, M., Seakamela, S. M., Orndorff, S., Kirkman, S., … Creel, S. (2022). Loss of an apex predator in the wild induces physiological and behavioural changes in prey. Biology Letters, 18(1). doi:10.1098/rsbl.2021.0476

De Vos, L., Watson, R. G. A., Götz, A., & Attwood, C. G. (2015). Baited remote underwater video system (BRUVs) survey of chondrichthyan diversity in False Bay, South Africa. African Journal of Marine Science, 37(2), 209-218. doi:10.2989/1814232x.2015.1036119

Ripple, W. J., & Beschta, R. L. (2004). Wolves and the ecology of fear: Can predation risk structure ecosystems? BioScience, 54(8), 755-766. doi:10.1641/0006-3568(2004)054[0755:wateof]2.0.co;2

Bowlby, H. D., Dicken, M. L., Towner, A. V., Waries, S., Rogers, T., & Kock, A. (2023). Decline or shifting distribution? A first regional trend assessment for white sharks (Carcharodon carcharias) in South Africa. Ecological Indicators, 154, 110720. doi:10.1016/j.ecolind.2023.110720

This article by Victoria Schneider was first published by Mongabay.com on 23 April 2025. Lead Image: A great white shark seen in False Bay, Cape Town, South Africa before these sharks disappeared from the area. Image courtesy of Chris Fallows.

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