Watching the kids: nursery use in juvenile sharks
Understanding how different species of shark use St Joseph Atoll helps to define what they need from their environment and how we can best protect it. At least once a week the SOSF-DRC team visits the atoll to tag juvenile sharks at different locations in the lagoon. Juvenile Blacktip reef sharks (Carcharhinus melanopterus) and Sicklefin lemon sharks (Negaprion acutidens) are caught with a small gillnet. Small passive integrated transponder (PIT) tags are inserted under each animal’s skin and then act as a unique identifier for that animal. Tissue samples for genetic analysis and standard measurements of each animal are taken and their sex and weight are also recorded. These data help us to understand population dynamics, pupping success, growth and survival rates and tell us about the areas of the atoll used by the different species and how these change as the young sharks grow. This long-term project builds on the existing database of the longest mark-and-recapture study in the Indian Ocean region, which was set up by Dr Ornella Weideli for her PhD research.
Since its inception in 2012, the SOSF-DRC has been managed and run by different people in various capacities. Lab managers, research officers and research assistants have been integral to the daily operations of the centre over these years and our long-term projects have been handed on from past to present management.
Currently the SOSF-DRC is run by Dr Robert Bullock and Henriette Grimmel in a joint-management capacity.
The following is a history of past management staff:
- Dr Rainer von Brandis (scientific director) 2006/2012–2016, previously did his PhD on D’Arros and managed the research lab for the previous owners of the island
- Chris Boyes...
Monitoring juvenile shark populations
The key objective of this project is to catch, measure and tag juvenile sharks at St Joseph Atoll in order to better understand population status and abundance, distribution, ecology, growth and survival rates of juvenile Blacktip reef sharks and Sicklefin lemon sharks.
Fishing pressures continue to increase in Seychelles and although sharks are no longer a target species, unreported, unregulated and illegal fishing remains a significant issue. Globally, shark populations are continuing to decline and the risk of extinction remains high for many species. Due to these threats, it is becoming ever more important to learn more about how key habitats are used by sharks in the early stages of their lives. Areas such as nursery sites can offer vital refuges from fishing threats and support the recruitment of juveniles into adult populations. St Joseph Atoll has recently been gazetted a marine protected area, which means that long-term information on how threatened species use the site can help to define how the marine protected area will be designed and managed.
Sharks are key predators in aquatic food chains and play a vital role in the health of oceans. D’Arros Island and the neighbouring St Joseph Atoll are home to a diversity of elasmobranch species that occupy the surrounding reefs and lagoon habitat. Two shark species, the Blacktip reef shark and the Sicklefin lemon shark, use the atoll’s lagoon as a nursery site and populations of juveniles are found in it year-round. The Blacktip reef shark is listed as Vulnerable on the IUCN Red List and the Sicklefin lemon shark has recently been uplisted to Endangered, which makes it one of the most highly threatened species using the St Joseph site.
Accurate and reliable monitoring is critical for the effective conservation of threatened species. Information about population sizes and dynamics is lacking for both Blacktip reef and Sicklefin lemon sharks and the collection of population data is listed as a research priority in their respective Red List accounts. Mark-recapture methods are the most common technique used to study animal populations and are a powerful tool to estimate population sizes and survival rates and to monitor long-term population trends. This in turn helps to inform conservation managers and improve the effectiveness of protected area management.
The key objective of this project is met by several specific study objectives:
– To estimate the population sizes and dynamics of these shark species over the long term.
– To investigate the distribution of individuals within the lagoon habitat and how it changes as sharks grow.
– To monitor the growth and survival rates of these sharks across long-term scales.
– To measure environmental parameters across different sample sites and determine their effects on the distribution of sharks at different tidal phases and times of day.