Tidally Driven Topographic Eddies Exploited by Manta Feeding Aggregations  

The seasonal feeding aggregations of reef manta rays (Mobula alfredi) in Hanifaru Bay, Maldives, are the largest known of their kind, yet the mechanisms facilitating this phenomenon remain poorly understood. Understanding these drivers is crucial for effective conservation and management, as protecting key foraging habitats requires knowledge of the physical processes influencing prey availability. While previous studies highlight the role of zooplankton in attracting manta, the oceanographic mechanisms responsible for concentrating prey within the bay remain hypothesised but unproven. Specifically, the influence of eddy dynamics, tidal forcing, and regional circulation on zooplankton distribution has not been thoroughly examined.
A 2DH hydrodynamic model (Delft3D-FM) was used to simulate ocean circulation patterns and analyse their evolution throughout the tidal cycle. Results reveal that topographic eddies, modulated by tidal flows, create retentive zones within Hanifaru Bay capable of retaining particles. These eddies form midway through the flood tide—aligning with manta aggregation—and dissipate approximately two hours after high tide, when the manta disperse. The combination of this timing and the ability of eddies to retain particles suggests these features are the dominant mechanism driving feeding aggregations in the bay.
By identifying the physical processes that promote prey availability, this study provides critical insight into the environmental conditions underpinning manta foraging dynamics. Given the potential impacts of climate change and coastal development on local hydrodynamics, these results may help inform conservation strategies aimed at preserving key foraging habitats for Mobula alfredi.