Not much is known about the broadnose sevengill shark as they are not a large commercial species. However, new research shows sevengills are important apex predators in the False Bay ecosystem, an important milestone in shark ecology.
The white shark (Carcharodon carcharias) has a reputation as one of the ocean’s top predators. But in False Bay’s temperate water kelp forests. another charismatic aquatic individual is challenging the white shark for the title of top predator: the broadnose sevengill shark (Notorynchus cepedianus). also known as the cow shark. Unlike the white shark with its torpedo-shaped body and water-slicing dorsal fin, the sevengill seems to lack most typical attributes. However, hidden behind a deceptively gummy smile are the sharp serrated teeth of a very successful hunter. In fact, new research has just placed the cow shark at the top end of the False Bay ecosystem.
Sevengills are considered an opportunistic/generalist species as they consume a variety of coastal prey species from different habitats. Their preferred prey is other sharks, skates and rays, but Cape fur seals and bony fish also form part of their diet. Being top predators, it is thought sevengills have become picky eaters due to the wide menu selection, allowing individuals to specialise. The most significant finding is sevengills constantly feed on seals. whereas white sharks move between habitats, feeding on seals seasonally.
I was surprised by the findings that sevengill sharks fed more frequently higher up on the food chain than white sharks. It highlights the importance of studies like this to better understand the relative importance top predators have on their prey, as this, in turn, impacts our understanding of the structure and function of the ecosystem.” says SANParks marine biologist Dr Alison Kock. Prey in False Bay is abundant; sevengills and white sharks are likely to use their shared menu differently as hunting activity may differ in technique, season and time of day. Essentially, based on reports from boats near Seal Island, the species seem to stay out of each other’s way- sevengills were only visible when the white sharks weren’t present.
Broadnose sevengill shark facts
- 7: the number of gills slits in the sevengill shark. Sharks typically have five gills slits.
- The sevengill shark appears to have no dorsal fin because it is situated further back on its body
- Also known as the cow shark, they can reach a maximum length of 290cm and are thought to live up to 30 years.
- Sevengill males are considered mature at 4-5 years old, while females mature at 11-12 years when they reach a total length of approximately 220cm
- Due to this shark’s mysterious behaviour it is classified as Data Deficient on the International Union for Conservation of Nature (IUCN) list.
Diet varies significantly between mature and immature female sevengill sharks. Smaller females have a higher proportion of Cape fur seals in their diet. Juvenile females are believed to scavenge on, and potentially hunt. seal pups around Seal Island, while mature sharks are restricted to coastal waters away from seal colonies for reproductive reasons. Scientists believe the sevengill shark hunts at night. making it challenging to observe hunting behaviour.
Sevengills are a low-value fishery species across most of their global distribution, but their preference
for coastal temperate waters makes them a vulnerable catch, even if not directly targeted. Globally, there are limited management policies or conservation considerations for this species. and commercial exploitation in South Africa is currently unrestricted. To date, sevengill sharks have not been studied widely due to a lack of funding and interest. Other charismatic and commercially important species. such as great whites. often receive all the attention and as a result. data on sevengills is limited. “Hopefully studies like these not only fill knowledge gaps, but also raise the profile of this important predator.” explains Kock.
Written by Leigh de Necker, marine biologist and aquarist at the Two Oceans Aquarium
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