Comment are off
How Big Is That Manta Ray? A Novel and Non-Invasive Method for Measuring Reef Manta Rays Using Small Drones
This study explores the application of small, commercially available drones to determine
morphometric the measurements and record key demographic parameters of reef manta rays (Mobula
alfredi) in Raja Ampat, Indonesia. DJI Mavic 2 Pro drones were used to obtain videos of surfacefeeding
M. alfredi with a floating, known-length PVC pipe as a reference scale—thus avoiding the need
to utilize altitude readings, which are known to be unreliable in small drones, in our photogrammetry
approach. Three dimensions (disc length (DL), disc width (DW), and cranial width (CW)) from
86 different individuals were measured. A hierarchical multivariate model was used to estimate the
true measurements of these three dimensions and their population-level multivariate distributions.
The estimated true measurements of these dimensions were highly accurate and precise, with the
measurement of CW more accurate than that of DL and, especially, of DW. Each pairing of these
dimensions exhibited strong linear relationships, with estimated correlation coefficients ranging from
0.98–0.99. Given these, our model allows us to accurately calculate DW (as the standard measure of
body size for mobulid rays) using the more accurate CW and DL measurements. We estimate that
the smallest mature M. alfredi of each sex we measured were 274.8 cm (males, n = 30) and 323.5 cm
DW (females, n = 8). We conclude that small drones are useful for providing an accurate “snapshot”
of the size distribution of surface-feeding M. alfredi aggregations and for determining the sex and
maturity of larger individuals, all with minimal impact on this vulnerable species.
morphometric the measurements and record key demographic parameters of reef manta rays (Mobula
alfredi) in Raja Ampat, Indonesia. DJI Mavic 2 Pro drones were used to obtain videos of surfacefeeding
M. alfredi with a floating, known-length PVC pipe as a reference scale—thus avoiding the need
to utilize altitude readings, which are known to be unreliable in small drones, in our photogrammetry
approach. Three dimensions (disc length (DL), disc width (DW), and cranial width (CW)) from
86 different individuals were measured. A hierarchical multivariate model was used to estimate the
true measurements of these three dimensions and their population-level multivariate distributions.
The estimated true measurements of these dimensions were highly accurate and precise, with the
measurement of CW more accurate than that of DL and, especially, of DW. Each pairing of these
dimensions exhibited strong linear relationships, with estimated correlation coefficients ranging from
0.98–0.99. Given these, our model allows us to accurately calculate DW (as the standard measure of
body size for mobulid rays) using the more accurate CW and DL measurements. We estimate that
the smallest mature M. alfredi of each sex we measured were 274.8 cm (males, n = 30) and 323.5 cm
DW (females, n = 8). We conclude that small drones are useful for providing an accurate “snapshot”
of the size distribution of surface-feeding M. alfredi aggregations and for determining the sex and
maturity of larger individuals, all with minimal impact on this vulnerable species.
