How to weigh an elephant seal with one finger : a simple three-dimensional photogrammetric application

• Published in: Aquatic Biology
• Main Authors: De Bruyn, P.J. Nico, Bester, Marthan Nieuwoudt, Carlini, Alejandro R., Oosthuizen, Wessel Christiaan
• Format: Article in Journal/Newspaper
• Language: English
• Published: Inter-Research 2009
• Subjects: Mass estimation; Volume estimation; Life history; Phocids; Southern elephant seal > Size; Body size; Photography; Weight (Physics) > Measurement; Photogrammetric pictures; Estimation theory > Computer programs; Elephant Seal; Elephant Seals; Mirounga leonina; Southern Elephant Seal; Southern Elephant Seals
• Online Access: http://hdl.handle.net/2263/12817; https://doi.org/10.3354/ab00135

Several studies have developed photogrammetric techniques for indirect mass estimation of seals. Unfortunately, these techniques are often narrowly delineated for specific field scenarios or species. Many require sophisticated, custom-designed equipment or analytical tools, limiting their applicability. We aimed to devise a photogrammetric technique for accurate volume/mass estimation of seals under a variety of field scenarios without manipulation of the animal and with minimal equipment. We used Photomodeler Pro 3-dimensional modelling software to estimate the mass of 53 weighed southern elephant seals Mirounga leonina. The method is centred on animal volume estimation in relation to the 3-dimensional area around it, rather than features of the animal itself, an approach that liberates limitations associated with earlier studies. No morphometric body measures are required for such volume/mass estimation. We offer predictive equations that allow high confidence in mass estimates relative to measured mass (95% confidence interval of mean deviation from measured mass is from ±1.34 to ±3.83% depending on the field scenario). A single photographer with a measuring stick and non-customised digital photographic equipment can use this technique to determine the mass of an elephant seal anywhere in the field with the push of a button.