Sharks in seasonal seas

To determine the seasonal and long-term habitat use of three reef-associated shark species around D’Arros Island and St Joseph Atoll and to establish whether it is influenced by monsoon-driven wind and surface-current regimes. This will be achieved by analysing a decade of acoustic telemetry data alongside corresponding wind and surface-current datasets.

Although considerable research has documented the habitat use and movement patterns of reef-associated and threatened shark species, significant gaps remain in our understanding of how large-scale oceanographic processes influence shark behaviour at fine spatial scales, especially on coral reef systems. In the Indian Ocean, monsoon cycles are a dominant driver of seasonal variability in wind patterns, surface currents, water temperature, productivity and reef hydrodynamics. Emerging evidence indicates that climate change is altering monsoon systems globally, including in the timing, duration and intensity of monsoonal winds and currents in the Indian Ocean. Such changes may have cascading ecological consequences for coral reef ecosystems and the species that depend on them. D’Arros Island and St Joseph Atoll represent a remote, relatively undisturbed coral reef system and serve as a critical habitat for several threatened shark species. The site’s strong seasonal exposure to monsoon-driven winds and currents makes it an ideal natural laboratory to investigate how environmental forces shape predator–habitat relationships.

Reef sharks play essential ecological roles as predators, contributing to reef resilience and trophic balance. Their movement patterns are influenced by environmental conditions and a combination of biological needs such as foraging, prey availability and reproduction. Acoustic telemetry has revealed that many reef shark species exhibit strong site fidelity, but they can also display seasonal shifts in habitat use. The Indian Ocean monsoon system is one of the most influential climate features in the region, alternating between the north-west monsoon and the south-east monsoon, each generating distinct wind and surface-current patterns. These seasonal changes can alter wave exposure, water flow, turbidity, prey availability and energetic costs of movement, all factors that are likely to affect how reef-associated predators use different reef habitats. Therefore, understanding shark response to monsoonal shift is crucial for revealing the environmental forces that shape their spatial ecology and their long-term resilience.

The aim of this study is to examine how the seasonal monsoon cycle in the Indian Ocean influences the habitat use and spatial distribution of reef-associated sharks at D’Arros Island and St Joseph Atoll. This will be achieved by:

• Quantifying seasonal and interannual patterns in habitat use, residency and spatial distribution of grey reef, blacktip reef and lemon sharks, using 10 years of acoustic telemetry data.
• Characterising seasonal variability in wind and surface current associated with the north-west and south-east monsoon phases, using long-term oceanographic datasets.
• Examining relationships between monsoon-driven wind and current patterns and shark habitat use across distinct reef zones at D’Arros Island and St Joseph Atoll.
• Assessing long-term trends over the decade to identify potential shifts in shark habitat use in relation to changes in monsoon characteristics, providing insight into possible climate-related influences on the spatial ecology of reef sharks.