A new way to count white sharks
Anna is collecting genetic information from white shark fin clips to assess this species’ population size in South Africa. Using close-kin mark-recapture analysis instead of traditional methods, she hopes to provide an accurate account of South Africa’s white shark population size. She also aims to develop a monitoring protocol that can use genetic samples collected during shark net and drumline patrols by the KwaZulu-Natal Sharks Board. This information is needed in South Africa, where the conservation of a protected species is balanced against concerns about bather safety, and where sharks are caught in bather protection gear.
Anna Simeon
My interest in the ocean began when I was growing up in Alaska, where I was lucky enough to spend lots of time on the water and among the region’s coastal communities. I always knew that I wanted to study marine ecosystems, but my scientific journey has been anything but straightforward.
My first real foray into research was during an internship in the Caribbean, studying the ecology of coral reefs. Intrigued by the tropics, I moved to Guam to complete my Master’s degree and launched diving expeditions to Micronesia, Japan and the Mariana Islands to collect samples of marine plants...
Estimating South African Great White Shark (Carcharodon carcharias) Population Size using Close-Kin Mark Recapture
To estimate the size of South Africa’s great white shark population by means of the innovative method of close-kin mark-recapture analysis, which uses genetic relatedness of the individuals sampled.
South Africa’s great white sharks Carcharodon carcharias present a unique case study of the complicated politics surrounding human–wildlife interactions. Measures to protect swimmers, such as the shark nets maintained by the KwaZulu-Natal Sharks Board, are a fine trade-off between the public desire for accident prevention and the conservation of viable shark populations. Accurate population estimates are required to keep this balance, and this project uses genetic information from routinely collected fin clips to provide that information.
South Africa’s great white sharks have been protected from targeted fishing since 1991 and the need to monitor their population has been highlighted by the South African government, but recent robust population estimates are unavailable and new ones are difficult to obtain. To date, research on the population of great white sharks in South Africa has relied on traditional mark-recapture methods, which depend on tagged animals being ‘recaptured’ to produce population estimates. The first study of this kind used dart tags to mark adult sharks, whereas others have used individual dorsal fin markings as ‘tags’ to estimate population size. Each of these methods requires extensive field work, which makes them expensive to continue.
In contrast to traditional mark-recapture studies, a close-kin mark-recapture analysis relies on genetic markers to identify parent–offspring pairs that constitute a ‘recapture’ of the parental individual. Because the ‘tag’ is contained within the individual’s genetic data, each animal needs to be encountered only once and therefore does not have to be recaptured or seen again. The KwaZulu-Natal Sharks Board’s programme of maintaining devices to protect swimmers requires frequent monitoring of its nets and drumlines, which means that there are regular opportunities for additional samples to be collected. These future samples can be sequenced, added to the existing genetic data already collected for this study and used for population monitoring.
- To determine the population of great white sharks in South African waters by using close-kin mark-recapture analysis.
- To establish a protocol for future population monitoring using genetic samples routinely collected during shark net and drumline patrols. These data, which are critical for the KwaZulu-Natal Sharks Board’s goal to balance conservation and public safety, will provide crucial information for future policy and conservation decisions.
Summary of main research results/outcomes
Management of Great White Sharks in South Africa currently lacks accurate population size estimates, which is essential for effective conservation strategies. Here, we applied close-kin mark-recapture to 78 shark samples collected from the KZN Sharks Board bather protection program to estimate population size. From the genetic data of these samples, we discovered that nearly half of these samples were related in some way. While surprising, these relatively large groups of related individuals suggested violation of key assumptions of the approach, and so any population estimates have to be taken with caution. Nevertheless, large family groups increase the potential for inbreeding and inbreeding depression, which may be detrimental to population viability. Further research into the movement patterns and connectivity of sharks between sites along the South African coast as well as the cause of large family groups are needed to establish firm management recommendations.