Rachel is using animal-attached tagging technologies to investigate the behaviour and energetics of reef manta rays at D’Arros and St Joseph. Her project aims to develop a deeper understanding of habitat use at this important aggregation site. What are the mantas doing whilst they are here and what does this tell us about their ecological role in coral reef systems?
Growing up on the coast of Western Australia in Perth, I always had a love for and fascination with the natural environment, particularly the ocean. My parents were both environmentalists and because of this I was fortunate to grow up exploring remote places around the world. Some of my fondest memories revolve around these experiences. At the age of 12 I begged my parents to let me learn how to scuba dive, and it was being immersed in the marine environment that helped me solidify the sense of wonder and curiosity that has driven me to learn more about this...
To quantify the behaviour and activity regimes of free-swimming reef manta rays within a marine protected area at D’Arros Island, Seychelles, using novel biologging devices. We aim to understand the spatio-temporal patterns in time–energy budgets of the manta rays; gain insights into their functional ecology at coral reefs; and quantify the overlap of key behaviours within the marine protected area.
The reef manta ray is a threatened species, yet we have no detailed understanding of how much time and energy it spends on key life-history processes. The factors that determine behaviours and their costs are crucial to predict and mitigate how human disturbance, climate change and habitat alteration impact these rays. Isolated as it is, D’Arros Island offers a unique opportunity to gather important information about the energetics of reef manta rays within a relatively undisturbed area, thus providing an important global baseline. We also know little about what drives high rates of residency and site fidelity in manta rays. By identifying the physical and biological processes that influence the species’ movements and behaviour, as well as estimating the energetic costs underlying specific behaviour, this study will improve our ability to recognise critical habitats for reef manta rays and provide a broader understanding of what determines their movements and ecological role in coral reef systems in Seychelles and on a global scale.
The reef manta ray is a large mobulid ray with a tropical and subtropical distribution throughout much of the Indian and Pacific oceans. It is typically resident in productive near-shore environments, such as coral and rocky reefs of island groups, atolls and continental coastlines. Although individuals are capable of long-distance movements of up to 1,150 kilometres (715 miles), they seldom travel so far. There is low connectivity between widely separated sites and a high degree of residency, which makes the reef manta ray vulnerable to local depletion, which may lead to regional extinction.
Given the potential for reef manta rays to influence nutrient flow, their disappearance from aggregation sites may negatively affect local reefs. Whereas many reef-associated species forage over short distances within coral reefs, reef manta rays travel longer distances to feed on zooplankton communities offshore. This foraging behaviour, coupled with the rays’ residency at aggregation sites, may facilitate the transport and recycling of nutrients between isolated reefs and offshore environments, benefiting coral growth and productivity within these reef systems. Our current understanding of the feeding ecology of reef manta rays, however, has relied on indirect and/or passive measures of foraging behaviour. Fine-scale multi-sensor tag technology will enhance our knowledge.
The aggregation of reef manta rays at D’Arros Island provides an opportunity to document the behavioural and energetic baselines for the species, which are necessary to subsequently quantify anthropogenic threats. D’Arros is a significant aggregation site for reef manta rays and since 2013 has been the focus of a programme monitoring their movement patterns and foraging behaviour within the archipelago. Until now, it has not been possible to quantify the rays’ behavioural regimes at this site, but the development of biologging tags that can be mounted on wild animals without the need for capture has now made this possible. For the first time, we have the opportunity to better understand the drivers of behaviour and habitat use patterns of reef manta rays in the Seychelles, and the functional role that these rays may play in coral reef environments.
Dillys will be collecting plankton samples and environmental data around D’Arros island and St. Joseph atoll, where highly resident reef manta rays feed, to quantify the prey density threshold. She will process these samples in the lab and use the biomass data to estimate the prey density threshold; and using the environmental data, she will also identify potential drivers of prey density patterns. This will provide insights on reef manta ray feeding ecology globally.
This project will investigate the scale at which structural complexity correlates with coral reef productivity and how this varies spatially in an isolated and largely unperturbed reef system. The project will further explore the application of structure-from-motion photogrammetry to quantifying spatial gradients in reef complexity and bleaching extent.
I will be using drones on St Joseph Atoll, to conduct aerial population densities of both green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) sea turtles, whilst outlining which habitat types are occupied by each species. I will use this information to produce habitat management recommendations that will be applicable in other areas of the Seychelles, as well as on a global scale.