Measuring Mantas

  • Rays & Skates
  • Targeted
  • Current
Years funded
  • 2022, 2023
  • Research

D’Arros and St Joseph represent an important aggregation site for the threatened reef manta ray. Body size is a key life history parameter that informs important aspects of species biology. Various techniques and methodologies are available for measuring animals in the wild. Two methods that have been applied to measure reef manta rays include stereo-video measurement and drone measurement. This study presents a critical comparison of the efficacy and applicability of these two established techniques for measuring reef manta rays and aims to establish which technique is most suitable for different research objectives.

Measuring Mantas

Priya Didon

Project leader
About the project leader

Having been born and raised in the Seychelles, I consider myself a proud Seychelloise never ceasing to be amazed by my country’s beauty. From the lush green vegetation, to the crystal clear waters, I always find myself drawn to nature, drawn to its beauty and endless adventures. I view Seychelles as a gem, a gem that needs protecting from natural as well as anthropogenic threats. Growing up, I have participated in various environmental groups and projects including wild life club, eco school, mangrove restoration projects and science fairs at secondary level, just to name a few.
My interest in the...

Project details

Measuring Mantas: comparing efficacy and applicability of techniques for measurement of reef manta rays (Mobula alfredi)

Key objective

The aim of this study is to assess the efficacy and applicability of different methodological approaches for the morphometric measurement of reef manta rays.

Why is this important

Information on body size provides important insights into species biology, population dynamics and reproductive status. As such, obtaining precise measurements of animal body size has important implications for conservation and management, particularly for threatened species. Different methods offer various advantages and disadvantages depending on the research questions. This study will provide useful information as to the strengths and weaknesses of different methodological approaches as well as assessing potential differences in measurement error that could impact the accuracy of resulting measurement estimates.


Reef manta rays are key species in reef ecosystems responsible for transporting vital nutrients from deeper ocean areas. Understanding key population parameters like growth rates, population dynamics and reproductive status are all important in assessing conservation status and estimating population trajectories. Currently size estimates at D’Arros are largely made using visual estimation method. Exploring more refined methods should help to define best practice moving forward.
Photogrammetry is a commonly applied, non-invasive approach to estimate a range of size measurements for a variety of different species. Paired stereo-video camera systems are often used for underwater measurements and produce paired videos or images from two cameras mounted on a bar at a set distance apart. This technique has been shown to produce more accurate estimates of marine species than visual estimation method and has been successfully applied to measure reef manta rays at D’Arros in the past. As technology continues to advance, drones are increasingly being applied to answer a range of scientific questions. Recently, small commercial drones were used to produce measurement estimates for surface-feeding reef manta rays. This involves aerial photogrammetric measurement using drone video of surface-swimming mantas along with a reference marker. Both of these techniques offer advantages and disadvantages, and both are subject to varying degrees of intrinsic measurement error. This study presents a critical comparison of the applicability, strengths and weaknesses of these two techniques by applying both in tandem, measuring the same manta rays simultaneously with both methods and comparing feasibility, measurement precision and ease of processing. Given the value of morphometric measurement in providing important insights into species biology, population dynamics and reproductive status this study should prove timely and informative.

Aims & objectives

The aim of this study is to assess the efficacy and applicability of two established techniques for measuring reef manta rays. The study will also make initial inferences as to the population demography of the reef manta population at D’Arros and St Joseph.
To do this, the study will address several specific objectives:
– estimate sizes of 20-30 individual reef manta rays using stereo-video and drone measurements.
– collect repeat measurements of individual animals with both stereo video and drones.
– assess strengths and weaknesses of stereo and drone video measurement techniques
– use measurement data to provide initial insights into the size and age class distribution of reef manta rays using the D’Arros and St Joseph site.