Christine L Dudgeon

Understanding the drivers of megafauna aggregations is critical for managing threatened species in a changing climate. We analysed an 18-year dataset (2007–2024) of reef manta ray (Mobula alfredi) abundance and zooplankton density at Hanifaru Bay, Maldives (5.1733°N, 73.145°E) —the world's largest known manta ray aggregation. Generalised Linear Models were used to evaluate sightings against local environmental variables and broad-scale climatic indices. Results reveal that interannual abundance was modulated by the Indian Ocean Dipole, with sightings and prey density reaching decadal highs during the record Negative Indian Ocean Dipole and La Niña of 2021–2022. Locally, zooplankton density was the strongest predictor of M. alfredi abundance, explaining > 60% of the observed variation. Sightings were 25x higher during peak prey availability than during prey absence. Aggregations were timed with spring tides (peaking 2.7 hours post-high tide), new and full moon phases, and 25–35 km/h north-northwest winds. We also identified a critical thermal threshold at 28.6°C, beyond which sightings declined by 15.7% per degree of sea surface temperature warming. This sequence confirms that the local bathymetric retention mechanisms are dependent on specific regional oceanographic conditions. Our findings suggest that while Hanifaru Bay consistently concentrates zooplankton biomass, its functional utility as a foraging hotspot is vulnerable to climate-driven shifts in the Indian Ocean Dipole and rising sea surface temperatures, which may decouple the predator from its primary nutrient supply.