Movement Ecology, Population Dynamics, and Conservation of Reef Manta Rays (Mobula alfredi) in Raja Ampat, Indonesia

Abstract

Understanding the population dynamics, movement patterns, and habitat use of animals is critical to evaluate the effectiveness of management measures. However, investigating wide-ranging and highly migratory marine species is challenging. The global populations of reef manta ray Mobula alfredi have declined substantially primarily due to increased fishing pressures. This thesis aimed to understand the population demographics, abundance, movement patterns, and conservation of reef manta rays in Raja Ampat, Indonesia. In brief, manta ray sightings between 2009 and 2019 from two marine protected areas (MPAs) in Raja Ampat: Dampier Strait and South East Misool were analysed using mark recapture modelling to investigate changes in demographic parameters. Management measures implemented in the region were reviewed to assess their potential impact on the manta ray populations. Drones were used to develop a novel method to measure the body size of reef manta rays, and to examine their maturity stage and other biological characteristics. I analysed telemetry data from five satellite and nine acoustic tags to understand residency patterns and habitat use of juvenile reef manta rays in the Wayag lagoon nursery. Finally, 114 acoustic tags and 11 satellite tags were examined using network analysis to explore whether reef manta rays form a metapopulation. Reef manta ray populations in both MPAs demonstrated substantial increases over a decade of study, highlighting the efficacy of long-term implementation of management measures (primarily critical habitat protection and fisheries regulations) combined with the influence of the El Niño–Southern Oscillation. Reef manta rays measured from a drone provided accurate measurements, revealing clear evidence of sexual dimorphism between mature individuals; females were larger than males. Females started to mature at 324 cm disc width (DW) and males at 275 cm DW. Satellite tracked juveniles revealed limited home range sizes in Wayag lagoon, and acoustic tracked individuals demonstrated strong residency patterns and nearly continuous use of the lagoon, providing evidence to confirm Wayag lagoon as a nursery. Conversely, larger tagged individuals in Raja Ampat demonstrated wider home ranges and extended movements around the region. A network analysis based on passive acoustic tracking provided evidence supporting the likelihood that reef manta rays form a metapopulation. While the global populations of reef manta rays show either declining trends or some stability, the reef manta rays in Raja Ampat have increased in abundance. This highlights that conserving large, longlived, and highly migratory species is possible. My thesis has substantially improved our understanding of the differences in habitat use between reef manta rays based on their life stages. Juveniles use nursery areas, like in Wayag lagoon, for extended periods to improve survival in the early stages of their life. Furthermore, I have revealed three spatially discrete subpopulations that mostly likely form a metapopulation throughout Raja Ampat. I have also demonstrated the ability of small drones to collect accurate measurements of the body size of reef manta rays with limited or no impact on the animals. Future research should focus on other regions of importance to manta rays to ensure their conservation throughout Indonesia.