Conservation and Population Ecology of Manta Rays in the Maldives

This multi-decade study on an isolated and unfished population of manta rays (Manta alfredi and M. birostris) in the Maldives used individual-based photo-ID records and behavioural observations to investigate the world’s largest known population of M. alfredi and a previously unstudied population of M. birostris. This research advances knowledge of key life history traits, reproductive strategies, population demographics and habitat use of M. alfredi, and elucidates the feeding and mating behaviour of both manta species. M. alfredi reproductive activity was found to vary considerably among years and appeared related to variability in abundance of the manta’s planktonic food, which in turn may be linked to large-scale weather patterns such as the Indian Ocean Dipole and El Niño-Southern Oscillation. Key to helping improve conservation efforts of M. alfredi was my finding that age at maturity for both females and males, estimated at 15 and 11 years respectively, appears up to 7 – 8 years higher respectively than previously reported. As the fecundity of this species, estimated at one pup every 7.3 years, also appeared two to more than three times lower than estimates from studies with more limited data, my work now marks M. alfredi as one of the world’s least fecund vertebrates. With such low fecundity and long maturation, M. alfredi are extremely vulnerable to overfishing and therefore needs complete protection from exploitation across its entire global range. With similar life history traits assumed for the lesser known M. birostris, adopting the precautionary principle, the same highly protective approach is justified. Through characterising habitat use of M. alfredi, it appears that their use of shallow coral reefs may be a function of behavioural thermoregulation and predator avoidance, with cleaning stations acting as focal gathering points where social behaviour is undertaken, such as courtship and mating. The frequent use of shallow protected lagoons by juvenile M. alfredi suggests these sites may act as nursery areas. The study underscores the importance of long-term research on long-lived species with conservative life history strategies if important ecological and management questions are to be answered.