MEPS 574:75-84 (2017)  -  DOI: https://doi.org/10.3354/meps12190

ABSTRACT: Non-intrusive and non-destructive acquisition of length measurements for marine megafauna is increasingly valuable given growing threats to many species’ long-term survival. Stereo videography provides a means of obtaining length data with minimal impact on the organism and minimal observer bias. However, for many researchers, there are still significant financial barriers to employing stereo videography. Small-action cameras have reduced costs, but camera calibration still requires significant investment in software and equipment. Here, we trial open source calibration procedures using the R package StereoMorph and a simple 2D checkerboard as a calibration object to test if this approach yields accurate length data. We used a stereo-video system comprising 2 GoPro™ cameras to estimate the lengths of known targets in a pool and the lengths of oceanic whitetip sharks Carcharhinus longimanus in situ. Sharks were restrained in water alongside a boat and measured with a tape measure for comparison with the stereo-video lengths. Both pool and field trials yielded accurate results comparable to previous studies using 3D calibration cubes. Stereo-video measurements of lengths >1 m had proportional errors of <1% in the pool and <3.0% (64.2 mm) in comparison with tape-measured lengths of sharks in the field. Our open source calibration methods and affordable GoPro™-based stereo-video system yielded measurements that are comparable to other systems that use 3D calibration techniques and more expensive cameras. Additionally, we improve on previous attempts of 2D calibration approaches.

KEY WORDS: Stereo-video · 2D calibration · Shark · Length estimation · Carcharhinus longimanus · Non-intrusive · Population demography · Dimensions