Residency and Use of an Important Nursery Habitat, Raja Ampat's Wayag Lagoon, by Juvenile Reef Manta Rays (Mobula alfredi)

The behaviour and spatial use patterns of juvenile manta rays within their critical nursery habitats remain largely undocumented. Here, we report on the horizontal movements and residency of juvenile reef manta rays (Mobula alfredi) at a recently discovered nursery site in the Wayag lagoon, Raja Ampat, Indonesia. Using a multi-disciplinary approach, we provide further corroborative evidence that the lagoon serves as an important M. alfredi nursery. A total of 34 juvenile rays were photo-identified from 47 sightings in the sheltered nursery between 2013-2021. Five (14.7%) of these individuals were resighted for at least 486 days (~1.3 years), including two juveniles resighted after 641 and 649 days (~1.7 years), still using the nursery. Visually estimated (n=34) disc widths (DW) of juveniles using the nursery site ranged from 150-240 cm (mean +/- SD: 199 +/- 19), and the DW of two juveniles measured using drones were 218 and 219 cm. Five juveniles were tracked using GPS-enabled satellite transmitters for 12-69 days (mean +/- SD: 37 +/- 22) in 2015 and 2017, and nine juveniles were tracked using passive acoustic transmitters for 69-439 days (mean +/- SD: 182 +/- 109) from May 2019-September 2021. Satellite-tracked individuals exhibited restricted movements within Wayag lagoon. The minimum core activity space (50% Utilisation Distribution-UD) estimated for these five individuals ranged from 1.1-181.8 km(2) and the extent of activity space (95% UD) between 5.3-1,195.4 km(2) in area. All acoustically tagged individuals displayed high residency within the nursery area, with no acoustic detections recorded outside the lagoon in the broader Raja Ampat region. These juveniles were detected by receivers in the lagoon throughout the 24 h diel cycle, with more detections recorded at night and different patterns of spatial use of the lagoon between day and night. The observed long-term residency of juvenile M. alfredi provides further compelling evidence that the Wayag lagoon is an important nursery area for this globally vulnerable species. These important findings have been used to underpin the formulation of management strategies to specifically protect the Wayag lagoon, which will be instrumental for the survival and recovery of M. alfredi populations in Raja Ampat region.

Automated summary

Juvenile reef manta rays show long-term residency and specific spatial use patterns in the Wayag lagoon, serving as an important nursery for this vulnerable species. These findings have been used to develop management strategies to protect the lagoon and contribute to the survival and recovery of the manta ray populations in the Raja Ampat region.