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

Abstract

Several studies have developed photogrammetric techniques for indirect mass estima- tion of seals. Unfortunately, these techniques are often narrowly delineated for specific field scenar- ios 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 esti- mation of seals under a variety of field scenarios without manipulation of the animal and with mini- mal 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 confi- dence 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.