Investigating manta ray collective movements via drone surveys

Detailed observational research on free-ranging species of marine megafauna is required to understand their behavioural ecology, including how groups respond to environmental and anthropogenic pressures. New technologies are opening up potential for research on these species in the wild, especially on group-based and collective behaviours. Reef manta rays (Mobula alfredi) are socially interactive elasmobranchs that form groups in coastal reef habitats. Collective behaviours are likely important to their fitness, but may be disturbed by humans. Using small, remotely-piloted drones, we performed aerial observations of manta ray groups in Raja Ampat, West Papua. We empirically quantified patterns of collective movement including relative spatial positions, alignment, speed and leadership positions of conspecifics. We found unique patterns of spatial positioning, alignment and leadership, including differences between sexes, and high levels of local attraction, which were suggestive of distinct collective behaviour states. We suggest that rules of interaction in manta rays vary at the individual level and can shift depending on local environmental and social conditions. Leader-follower behaviour likely has broad importance to cohesive movement and social behaviour in M. alfredi. We suggest that further studies on manta ray movement should consider utilising models of collective motion that capture group-level behavioural processes.