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What’s on the Menu for Sevengill Sharks?

MSc student: Leigh de Necker (UCT)

Supervisors:
 Prof Justin O’Riain (UCT), Dr Alison Kock (Shark Spotters, UCT, SAIAB), Dr Adam Barnett (James Cook University)

Funded by:
 UCT, Two Oceans Aquarium


Large sharks are often major predators of other megafauna, and therefore occupy trophic positions at, or near, the top of marine food webs. The removal of top predators directly affects populations of their prey, triggering a cascading effect to the bottom of the food web. The sevengill shark is an apex predator and therefore, by quantifying their feeding ecology, we can gain insights into their feeding relationships and the potential regulatory effects they may have on the structure and function of the False Bay ecosystem.

Stable isotope analyses are based on the premise that “you are what you eat.” This analysis was performed on a total of 39 muscle biopsies (33 female, six male), and 28 blood plasma samples (25 female, three male) collected from sevengill sharks between 2013 and 2015. A total of 161 prey samples were analysed from 32 different species. By comparing isotope results from tissue samples from sevengills with those of their prey, we are able to gain detailed insights into their diet. Additionally, seven white shark samples were analysed for comparisons with those of sevengills to understand the feeding dynamics between these two top predatory sharks in False Bay.

Chondrichthyans (species of sharks and rays) were found to be the most important prey to sevengills. No significant seasonal differences were found in the diet of sevengills, and no differences in diet were observed between male and female sharks. Interestingly and unexpectedly, there is a significant variation in diet between mature and immature female sevengill sharks, with smaller sharks having a higher proportion of higher trophic level prey, Cape Fur seals in particular, in their diet, relative to mature females. The proportion of seal in the diet of sevengills is too high to be solely attributed to scavenging, however, there is a high level of uncertainty regarding their ability to actively capture live seals, especially when compared to species known to successfully hunt live mammal prey, such as the white shark. Surprisingly, sevengills were found to occupy a higher trophic level than white sharks. It is likely that sevengills are feeding on seals all year round, whereas white sharks move between habitats, exploiting seals seasonally. However, it important to gain detailed knowledge on movement patterns in order to better understand the complexity in the feeding dynamics of sevengill sharks and the potential resource partitioning that may exist with white sharks.

Sevengills are considered an opportunistic/generalist species as they consume a variety of prey. Diet is often regarded as a species’ trait, but as top predators, occupying areas with abundant prey resources, sevengills may become “picky” in what they eat. The variability noted in the feeding patterns from sharks in False Bay, suggest that individual specialisation may take place to some extent, whereby certain individuals feed on select prey types.

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