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.
The Shark Spotters programme in Cape Town, South Africa, improves beach safety through both shark warnings and emergency assistance in the event of a shark incident. The programme contributes to research on shark ecology and behaviour, raises public awareness about shark-related issues, and provides employment opportunities and skills development for spotters.