How Big Is That Manta Ray? A Novel and Non-Invasive Method for Measuring Reef Manta Rays Using Small Drones

Ben C Stevenson,Mark V Erdmann,Edy Setyawan,Rochelle Constantine,Muhamad Izuan

doi:10.3390/drones6030063

Ben C Stevenson, Mark V Erdmann + Show 3 more

Open Access

https://doi.org/10.3390/drones6030063

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Journal: Drones	Publication Date: Feb 28, 2022
Citations: 17	License type: CC BY 4.0

Affiliation: University of Auckland, Indonesian Coral Reef Foundation

Abstract

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 surface-feeding 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.

Highlights

Introduction published maps and institutional affilBody size is a life-history parameter used to provide insights into key aspects of species and individual biology, including the maturity stage, reproductive status, population demographics, and environmental and habitat conditions to which individuals are exposed [1–4]
While the aforementioned methods have been used with increasing accuracy to measure the body size of M. alfredi, all three methods require a diver or snorkeler to be in the water with the manta rays to visually estimate their size or operate the camera setups, which can be time-consuming and costly and can potentially lead to negative behavioral responses from the manta rays being measured [22]
The smallest animal was an unsexed individual estimated at 206.8 cm disc width (DW), while the largest was a female estimated at 375.6 cm DW

Summary

Introduction

Body size is a life-history parameter used to provide insights into key aspects of species and individual biology, including the maturity stage, reproductive status, population demographics, and environmental and habitat conditions to which individuals are exposed [1–4]. Obtaining accurate measurements of animal body size is crucial for the conservation and management of most animal species [2]. Measuring the morphometric features of large animals, which can be difficult or dangerous to approach, can be problematic [3,5]. Most methods require close contact with or restraint of the animals, which has the potential to impact negatively upon them [6]. Photogrammetry, a non-invasive approach, allows us to accurately obtain measurements of large animals without physical contact [2,7]. Photogrammetry has been used to estimate the length, body weight, and other size measurements of a wide variety of animal species, from sheep to sharks to dolphins [8–10]

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