Natural history of manta rays in the Bird's Head Seascape, Indonesia, with an analysis of the demography and spatial ecology of Mobula alfredi (Elasmobranchii: Mobulidae)

Abstract

The Bird’s Head Seascape (BHS) in West Papua, Indonesia, is widely recognized as the global epicenter of coral reef biodiversity and is protected by an extensive network of 20 marine protected areas (MPAs) totaling over 4.7 million ha. It is home to large populations of both the reef manta ray Mobula alfredi (Krefft, 1868) and the oceanic manta Mobula birostris (Walbaum, 1792). We document the natural history of manta rays in the BHS and describe the demographics and spatial ecology of Mobula alfredi using underwater and aerial observations, a comprehensive photo-ID database, and passive acoustic telemetry. Manta rays were recorded from 127 sites across the BHS, including 70 aggregation sites (cleaning stations and routine feeding aggregations), with the largest feeding aggregation recorded consisting of 112 M. alfredi in the Dampier Strait in the Raja Ampat archipelago. We recorded 4,052 photographically identified M. alfredi sightings of 1,375 individuals between November 2004 and December 2019, with a biased female-to-male sex ratio of 1.58 to 1.0 and 67.4% exhibiting the chevron color morph vs. 32.6% melanistic. Over 85% of sightings came from the two large MPAs (>330,000 ha) of South East Misool and Dampier Strait. Importantly, 16 photo-IDs of somersault-feeding individuals were obtained using a drone, apparently the first report of UAVs used for manta photo-IDs. We resighted 642 individuals (46.7%) at least once during the period, with the two most-resighted individuals registering 67 and 66 resightings over periods of about 12 years. We observed 217 females pregnant at least once, with one having 4 consecutive pregnancies from 2013–16 (and a total of 5 pregnancies in 7 years) and 15 with at least two consecutive-year pregnancies. Four nursery sites were identified with a consistent presence of numerous young-of-the-year (YoY; i.e. ≤2 m disc width) over 3–14 years of observations: we recorded 65 YoYfrom Raja Ampat. The Raja Ampat population is best described as a metapopulation composed of 4–7 subpopulations inhabiting island groups separated by over-water distances of only 20–40 km, but which nonetheless exhibit limited exchange of individuals. We recorded 309 movement events among 7 hypothesized manta subpopulations in Raja Ampat based on photo-IDs between 2004 and 2019 and passive acoustic telemetry between 2013 and 2019, with the longest movement we recorded 296 km minimum distance through water. Importantly, 115 of the identified manta ray sites (90.5%) are distributed within 13 of the 20 BHS MPAs, and 95.9% of sightings (3,887 of 4,052), 89.5% of individuals (1,231 of 1,375) and all 4 identified nursery areas were from within MPAs in Raja Ampat, indicating the Raja Ampat MPA network, and the broader BHS MPA network within which it is nested, are critical for the conservation of manta rays in West Papua.