Across D’Arros Island, St Joseph Atoll and the broader region we have an array of stationary acoustic receivers that are constantly recording information about the presence of individual animals that we have tagged with acoustic transmitters. When a tagged animal moves within its range, the receiver records the ID tag (and thus the animal), date and time. Together as a network, these receivers tell us about the movements and distributions of different animals across different spatial scales. We have a total of 89 receiver stations, and we have tagged many individuals of different species of shark, ray, fish and turtle, so a lot of data is constantly being recorded on these receivers. Our staff download the data from and maintain these receivers every six months.
The key objective of this project is to monitor the patterns of habitat use and behaviour across a variety of species that use the D’Arros and St Joseph site.
Understanding the movements of different species in space and time gives us insight into the role of these species within ecosystems, as well as the requirements they have from those ecosystems. Movement data gathered for threatened species are particularly important for informing the protection of those species.
Acoustic telemetry is the process of using sound and distance to determine the movements of animals. Two pieces of equipment are involved: a transmitter and a receiver. Each transmitter emits a series of sound pulses that is detected by a stationary receiver. The receiver records the unique pulse series of a transmitter and logs the transmitter date and time of the detection.
By working with a network of receivers, it is possible to track movements between them. This tells us about the amount of time an animal spends in any one area, the different kinds of habitats it prefers and how large an area it may occupy.
When used over the long term, acoustic telemetry accumulates a wealth of extremely valuable data that have real value for conservation, providing as they do important information about how movements of threatened species overlap with fishing areas for example, or helping to design effective marine protected areas that take the movements of key species into consideration. The long-term collection of data also tells us about changes in the use of space over the lifespan of different animals and how these affect their management and conservation.
The key objective of this project is met by several specific study objectives:
To supplement much of the research conducted on site (e.g. coral monitoring and reef fish surveys) and to help better understand the lives of the animals using D’Arros Island and St Joseph Atoll, it is important to measure and monitor environmental conditions. One of the most important of these is sea temperature. A series of small loggers deployed at different areas and depths around the islands constantly record the water temperature. Additionally, via our on-site weather station, we collect meteorological data every day, including on wind, rainfall and air temperature.
D’Arros Island and St Joseph Atoll have been identified as one of the most important areas in Seychelles and the broader Western Indian Ocean for hawksbill Eretmochelys imbricata and green Chelonia mydas turtles, with both species using the beaches for nesting. We continually monitor their presence at the islands, collecting data both from individual animals present on the beaches and from the tracks they leave behind. From these we can estimate the population sizes and growth of these two turtle species.
A large population of reef manta rays Mobula alfredi occupies the near-shore waters of D’Arros Island. Each of these rays has unique markings on its ventral surface (belly) that enable us to identify different individuals. From this we gather information about how many mantas are using the site, where they spend their time and for how long. We use various photograph and video techniques to identify manta rays at D’Arros, including the deployment of cameras at a regularly visited cleaning station. Working with the Manta Trust, we incorporate our data into a broader regional understanding of the population and ecology of these threatened animals in Seychelles.