Three-dimensional photogrammetry as a tool for assessing morphometrics and estimating body mass of Steller sea lions

Abstract

A technique was developed to indirectly assess morphometrics and to estimate body mass of Steller sea lions (Eumetopias jubatus using three- dimensional (3D) photogrammetry. 3D computer wireframes of thirty-five Steller sea lions of various age classes were generated from multiple time-synchronous digital photos. Overall average estimates of standard length and axillary girth were within ±2.8% and ±4.2% of physically measured dimensions, respectively. Average estimates of standard length and axillary girth from wireframes based on ideal body postures were within ±1.7% and ±3. 1% of physically measured dimensions, respectively. These measurements were used to estimate body mass by applying previously existing regression equations. Regressions of physically measured mass on photogrammetrically estimated body volume yielded a predictive model. Body mass estimates using this model were on average within 10.5% (with a 95% confidence interval of ±2.35%) of the physically measured mass. The use of 3D photogrammetry decreases many of the problems associated with camera and body position encountered with two-dimensional photogrammetric techniques. This technique can be used to estimate the body mass of free-ranging, active sea lions, eliminating the need for sedation, heavy weighing equipment, and animal disturbance.