When the Great Whites are Away the Sevengills will Play
Updated: Apr 26, 2021
South Africa is world famous for its great white sharks (Carcharodon carcharias). White sharks can be found year-round in from KwaZulu-Natal to the Western Cape, especially around seals rookeries, such as that found at Seal Island in False Bay. However, great whites are not the only large sharks which can be found in South Africa. In fact, in recent years, another large shark seems to expanding into more and more space in South Africa, as the great whites seem to disappearing. But why is this happening? What shark could possibly usurp the mighty great white?
Great white sharks exist within a complex food web in South Africa. They live alongside other large sharks, such as tiger sharks (Galeocerdo cuvier), oceanic whitetips (Carcharhinus longimanus), hammerheads (Sphyrna zygaena and S. lewini), whale sharks (Rhinocodon typus) and basking sharks (Cetorhinus maximus) (Hammerschlag et al, 2019).
White sharks are generalist predators, eating a broad diet of fish, squid, crustaceans, octopi and “elasmobranchs” (other sharks and rays). Amongst the species of shark they like to eat, bronze whalers (Carcharhinus brachyurus) and broadnose sevengill sharks (Notorynchus cepedianus) are popular items. When they reach a size of 3 metres in total length great whites undergo an “ontogenetic” diet shift to also include marine mammals (such as seals, dolphins and whales) into their diet (Hammerschlag et al, 2019).
Whilst white sharks can be seen year-round in South Africa, their abundance seasonally fluctuates; in the winter (December - February) white sharks flock to seal colonies, like Seal Island in False Bay. This is because, after the pupping season in November, naive cape fur seal (Arctocephalus pusillus pusillus) youngsters tentatively taking to the water, make a nutritious easy meal for the white sharks. Conversely, in the summer the white sharks move inshore to feed on fish and elasmobranchs (Hammerschlag et al, 2019).
However, white shark abundances in South Africa have been declining recently and for the first time since observations began, white sharks have been entirely absent for extended periods of time. In 2017 and 2018 a research team reported that white sharks were repeatedly absent for 10 or more consecutive days, even in the colder months when abundances should have been at their highest (Engelbrecht et al, 2019 & Hammerschlag et al, 2019).
What is especially interesting about this shift in habitat use, is that the movements of the white sharks seem to affect the subsequent movements of another species of shark: the broadnose sevengill. Sevengills refuge within inshore kelp beds, like at Miller’s Point in Flase Bay, to hide from the predatory white shark. Yet, when the white sharks are away broadnose sevengill sharks are sighted away from Miller’s Point, in the vicinity of Seal Island, where they have not been seen before. Whatsmore, as white shark abundances slowly decline, sevengill populations have been slowly rising (Hammerschlag et al, 2019).
Despite being predated upon by white sharks, seven gills are themselves “apex predators”; feeding at the top of their food chain on large fish, seals and elasmobranchs. Therefore, it appears that, when the white sharks vacate the area, seven gills exploit the release from predation pressure and/or competition to hunt seals around Seal Island (Hammerschlag et al, 2019).
But why are the white sharks leaving? There seem to several, potentially inter-related hypotheses. Firstly, it seems that white shark absences over periods of days are likely in response to the presence of predators… Yes, you read that correctly… a predator which can scare off a white shark!… Orca. Pods of orca (Orcinus orca) are capable of hunting a great white and in recent years a new transient pod of orca have been sighted in South Africa around the time that large shark carcasses have been washing up on beaches. Sharks are known to flee an area and not return after smelling that another shark has been killed nearby. So, it seems great whites are fleeing from these orca (Engelbrecht et al, 2019 & Fisher, 2021).
Maybe the great white is not the absolute top of its food chain, after all!
Secondly, the overall trend of declining white sharks numbers is caused by an even worse killer… A creature more dangerous than sharks, more deadly than orca… Human beings.
White sharks have been pushed to the brink of population collapse by human activities, including culling in response to attacks on humans, death in shark nets and drumlines installed to protect bather beaches, overfishing for fins, oil and meat, and unnecessary
mortality as "bycatch" or from entanglement in "ghost nets". They are now classified as 'vulnerable' by the IUCN and trade in their products is limited by their listing on Appendix II of CITES.
If South African white sharks continue to decline, it is very possible that the entire community in False Bay could permanently change. Whilst that may be good news for the sevengills, it could be very bad for the community as a whole; leading to “trophic cascades” which may alter the ecosystem indelibly. If we want to keep our white sharks in South Africa, it will be critical to ensure a healthy population size in the area, which can only be achieved through protective measures which are ratified and adhered to globally!
To learn more about the disappearance of great whites in South Africa, you can read Where Have All the White Sharks gone? and Clash of the Marine Titans.
References
Engelbrecht TM, Kock AA & O’Riain MJ (2019). Running scared: When predators become prey. Ecosphere, 10:1, e02531. Access online.
Fisher R (2021). Possible causes of a substantial decline in sightings in South Africa of an ecologically important apex predator, the white shark. South African Journal of Science, 117:8101. Access online.
Hammerschlag N, Williams L, Fallows M & Fallows C (2019). Disappearance of white sharks leads to the novel emergence of an allopatric apex predator, the sevengill shark. Scientific Reports, 1:9, doi: 10.1038/s41598-018-37576-6. Access online.
By Sophie A. Maycock for SharkSpeak.