• SharkieSophie

Clash of the Marine Titans

Updated: Apr 26

Several species of sharks are described as apex predators in their communities, including, but not limited to the great white (Carcharodon carcharias), the broadnose seven gill shark (Notorynchus cepedianus) and the tiger shark (Galeocerdo cuvier). You may be familiar with the term “apex predator”, which scientists use to describe a species which is at the top of its respective food chain, above all other predators. Apex predators are critical for ecosystems health and maintenance of biodiversity, as they exert “top-down” control of the system; balancing mesopredator, herbivore and producer populations below them in the chain. These guys are the biggest, baddest, beasts in their habitats; all the smaller guys run from them and no-one is able to eat them. Right?


White shark carcass (Image credit: Marine Dymanics, South Africa, Source: www.allthatsinteresting.com)

Actually, that is not necessarily the case because it is possible to have multiple apex predators co-existing within one ecosystem. This can occur because each top-dog occupies a slightly different “niche” (the position of a species and its ecological role within an ecosystem), so apex predator species are able to persist side-by-side.


For example, in False Bay, South Africa both great whites and sevengills can be found. Both predate upon pinnipeds (such as cape fur seals (Arctocephalus pusillus pusillus)) and cetaceans (such as common dolphins (Delphinus delphis)) and other, smaller sharks (such as bronze whaler sharks (Carcharhinus brachyurus)). Yet, it could be argued that the great white comes out as the real top-dog because they will target sevengills as their prey. Or they used to be the top of the top-dogs…


The great white (left) and broadnose sevengill shark (right)

Since 2009 there has been a marked increase in the abundance of orca (Orcinus orca) in False Bay. As the orca preyed upon dolphins and humpback whales (Megaptera novaeangliae), they also occupied a different niche to that of the sharks. However, since 2015 (believe it or not), pods of orca have been witnessed hunting both sevengill sharks and the mighty great white!


Study site in South Africa

Since November 2015, there has been multiple incidents when sevengill presence at their aggregation site in the kelp forests of Miller’s Point dropped markedly overnight; leaving the area almost completely abandoned. These events were followed by the discovery of sevengill carcasses in the area (Engelbrecht et al, 2019). Also since 2015, great white relative abundance has been decreasing in False Bay, to an 18-year-low in 2018. There have been several incidents of white shark carcasses being discovered, which were followed by periods of several days, or even weeks, when white sharks were entirely absent, which is normally unheard of (Hammerschlag et al, 2019).


What is especially fascinating is that the carcasses of these sharks show the killer has a very distinctive signature; in every case, the majority of the body of both great whites and sevengills were intact, but the the pectoral girdle was broken open and the liver removed through the tear in the abdominal wall.


The carcass of a broadnose sevengill shark found in False Bay

It appears that the previous balance of white shark and sevengill in False Bay, is being upended by the predation from killer whales, but what is confounding is that orca have, in fact, been transient residents of False Bay in the past without such effects being reported in the shark community. So why have orca suddenly started preying upon the sharks?


In January of 2015 whale watching charters in False Bay reported sightings of 2 new transient orca, which they named Port and Starboard (due to their opposing collapsed dorsal fins). These orca have been spotted in the vicinity of Miller’s Point around the time of sevengill deaths, and have been sighted up the coast at Struisbaai and Gansbaai around the time of white shark deaths.


Cue the CSI theme music… it seems we have our suspects!


Pods of orca are certainly capable of hunting large sharks, but not all groups of orca bother. Orca pods have highly organised social structures based upon a matriarchy and each pod tailors its own unique diet and behaviour. It has even been suggested that vocalisations are specialised by group, in something akin to a dialect. We refer to these differences as “ecotypes”, meaning, despite being classified as a single species, orca are capable of varying enormously between pods. These ecotypes are often driven by differences in diet, leading to changes in behaviour and morphology. For instance, some orca pods are specialised to target marine mammal prey predominantly, whereas other groups take a more generalist approach and have a broader diet. It seems that, in False Bay, there are 2 new kids on the block and they are partial to shark liver snacks.


Orca in South Africa

We cannot predict whether these shark killing sprees will become a regular occurrence in the future, or whether other groups of orca will follow Port and Starboard’s example, but what we do know is that apex predator sharks are critical to the health of the marine ecosystem in False Bay. If shark populations decline due to increasing predation or because they flee the area, this could have a catastrophic effect on the rest of the ecosystem known as a “trophic cascade”. Future research will be critical to understand why these orca have shifted their ecotype to target sharks in False Bay, because if anthropogenic activity (such as pollution or overfishing) has caused the shift in their behaviour, it is possible that management strategies need to be implemented in order to indirectly protect South African sharks.



For more on this topic, you can check out Where Have All the White Sharks Gone? and When the Great Whites are Away the Sevengills will Play.



References

Engelbrecht TM, Kock AA & O’Riain MJ (2019). Running scared: When predators become prey. Ecosphere, 10:1, e02531. 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, 9:1908.


By Sophie A. Maycock for SharkSpeak.

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