Photogrammetric determination of body length and body mass in Galápagos sea lions (Zalophus wollebaeki)

Measuring body morphometrics of large mammals is difficult, time consuming and requires a lot of equipment. Photogrammetric measurements are less intrusive, faster and only a single researcher with a bag full of equipment is necessary. In the present study a photogrammetric method to determine the body length and mass of Galápagos sea lions (Zalophus wollebaeki) is developed and evaluated. A photogrammetric method developed for body length calculations of whales (Jaquet 2006) is used, extended and improved. Pictures of animals are taken and the distance between camera and animal is used for scaling. For calibration of the camera and validation of the method some animals are captured and body parameters are measured manually. Accuracy (R², s.d.) and precision (mean deviation of manual and photogrammetric values) of the method are evaluated. The application of this photogrammetric method derives highly accurate (adult length R² = 0.777, s.d. = 3.71; mass R² = 0.867, s.d. = 6.26) and precise (mean deviations: length = 2.02, mass = 5.29) calculations of body length and body mass for adult and offspring Galápagos sea lion. Correlations between manually measured and photogrammetrically calculated values are found to be highly significant (p-values: length: p = 0.001, mass: p = 0.0001). Additionally the impact of several factors (while taking the picture, while analyzing or position of the animal in the picture) influencing the accuracy of this method are analyzed. Based on these analyses, recommendations are given for the application of the photogrammetric method to determine body length and mass of sea lions