Inside Stories

Rethinking False Bay’s top predators

The Shark Spotters

Words by Leigh de Necker

Cover image

Photo by WildestAnimal | Shutterstock

So you think you know which shark sits at the top of the food chain? No doubt when you hear the phrase ‘the ocean’s apex predator’ one species rises to the top of your mind above the rest: the white shark Carcharodon carcharias – and deservedly so. There is, however, another species of shark, humbly lurking in temperate kelp forests worldwide, that is emerging as a rival to the charismatic white shark for the crown of top predatory shark in the waters of False Bay.

The broadnose sevengill shark Notorynchus cepedianus, also known simply as the sevengill or the cowshark, aggregates seasonally in the shallow water off Miller’s Point, on the eastern side of False Bay. Its secretive and mysterious nature, coupled with its low conservation priority, means that there is little information about its ecology and it is therefore classified as Data Deficient on the IUCN Red List. For the past few years, I have been studying sevengills in False Bay as part of my Masters degree. By investigating their diet and feeding dynamics, I hope to contribute valuable dietary data to the conservation and management of these enigmatic sharks.

Traditionally, stomach content analysis has been the most popular method used in research into the diet of sharks. It does, however, have a number of limitations, foremost of which is the difficulty of sampling large, living predators and getting out of their stomachs whatever may be inside them. Consequently, most stomach content studies have relied on lethal sampling methods – dead sharks – although they provide only a snapshot of an individual’s diet. In addition, many sharks are captured with empty stomachs or unidentifiable prey items, which means that large numbers of sharks have to be caught to provide a representative diet for a given species. For threatened and protected species in particular, lethal sampling is not ideal.

An alternative or complementary, method that addresses many of the limitations of stomach content investigation is stable isotope analysis, which is based on the premise that you are what you eat. A predator’s stable isotope values reflect those of the prey it has consumed, so by comparing small tissue samples (no larger than the size of a little fingernail) from sevengills with those of their potential prey species, I was able to determine the sharks’ prey preferences and the relative quantities of each prey species in their diet. This method also enabled me to gather integrated information about the long-term diet of sevengills (what they had been eating for the two to 24 months before the sample was taken), as well as reveal the position the sevengill holds in the food web relative to other predators, the white shark in particular, and relative to their various prey species.

My study analysed a total of 39 muscle samples (33 female, six male) and 28 blood plasma samples (25 female, three male) collected from sevengill sharks between 2013 and 2015, all of which were safely sampled, tagged and released alive. These were compared with muscle samples from 161 prey samples from 32 different marine species. Additionally, seven white shark samples were analysed for further comparisons with those of sevengills in order to understand the feeding dynamics between the two top predatory sharks in False Bay.

Cover image

Photo by WildestAnimal | Shutterstock

My research found that sevengills in False Bay appear to feed predominantly on a variety of coastal prey species. Inshore chondrichthyans (sharks, skates and rays) were found to be their favourite food items, with Cape fur seals and inshore teleost (bony fish) species also being important. The sevengill is therefore considered to be a generalist species, as individuals consume a variety of prey items from a variety of habitats. Diet is often regarded as a species’ trait, but as apex predators that occupy areas with abundant prey resources, sevengills may be ‘picky’ when deciding what they eat. We identified variability in the feeding patterns of these sharks, suggesting that individual specialisation may take place to some extent, whereby certain individual sevengills will choose preferred prey items from the False Bay menu that are different from the selection of others.

Interestingly – and unexpectedly – there was significant variation in diet between mature and immature female sevengills, with the smaller sharks having a higher proportion of Cape fur seals in their diet relative to the mature females. This was in contrast to previous studies conducted on the stomach contents of sevengills (Ebert 1991), which found that larger and more mature sevengills ate more mammal prey than their smaller, immature counterparts. It is possible that the abundant opportunities to scavenge on seal pups around Seal Island in False Bay provide immature sharks with access to this resource, whereas mature sharks may be restricted to coastal waters away from seal colonies for reasons that are related to reproduction.

Overall, however, the proportion of seal in the diet of sevengills is too high to be attributed solely to scavenging. Little is known about the species’ ability to actively catch live seals – sevengills are thought to feed at night, which makes it challenging to observe their hunting behaviour. It is especially difficult to compare its ability to that of a species like the white shark, which is known to hunt live mammal prey successfully. However, there is no doubt that the sevengill is a top predator and therefore its importance in the False Bay ecosystem cannot be disputed. In fact, my research puts the sevengill in a higher position than the white shark in the food web. This result mirrors that of a previous study (Cortés, 1999), which placed the sevengill in the highest position in the food web relative to the 149 shark species compared in the study, including the white shark. This may be due to the fact that sevengills consistently feed on seals, possibly all year round, whereas white sharks move between habitats and only feed on seals seasonally.

This is the first study to use stable isotopes from the muscle tissue and blood plasma of sevengills to gain insight into the species’ diet and relative position in the food web within False Bay. The results confirm that the sevengill shark is an apex predator and a generalist that feeds on a variety of prey items across diverse habitat types. Stable isotope research such as this provides a better understanding of a species’ role within the ecosystem, which is vital for determining conservation and management strategies for sharks.

When this work is mirrored by efforts on the sympatric white shark, the opportunity to explore the trophic and ecological role of top predatory sharks in a coastal bay will become an important milestone in shark ecology.