Laser-photogrammetry is a non-invasive technique used for size estimation in applications where traditional methods to determine length are either impractical or unreliable. Laser photogrammetry provides greater precision for measuring free-swimming whale sharks when compared to previous methods that compare whale sharks to objects of known size such as boat length, diver height, and rope length. These estimates are subject to the effects of water movement and visual distortion from refraction. Laser-photogrammetry uses a known reference distance projected onto a surface using pre-calibrated lasers to standardize measurements and utilizes ratios of pixel-counts to calculate a length estimate. Laser-photogrammetry has been demonstrated as a useful technique for determining body length estimates in free-swimming whale sharks, (Rohner et al. 2011), but measures of variance have been lacking because repeat measurements on the same individual animals are rarely obtained. Georgia Aquarium houses 4 juvenile whale sharks that can be unequivocally identified and photographed repeatedly, and therefore provided an opportunity to refine the techniques described by Rohner. We then applied them to an aggregation of whale sharks at St. Helena Island. Using a similar laser-photogrammetry apparatus and body proportion equations for determining total length prescribed by Rohner, we observed relatively high variance on repeated measurements made on the same animal, such that confidence intervals for any given individual measurement estimate were unacceptably high. Using measures of central tendency from multiple measurements, rather than relying on single images, provided measurement estimates with acceptable confidence intervals for determining total length. We applied the central tendency technique to characterize lengths in a previously unstudied whale shark aggregation in the southern Atlantic Ocean off of St. Helena Island in 2015 and 2016. We used the mean of 4–27 repeated measurements to determine total length estimates of the individuals photographed in St. Helena during these time periods, instead of relying on single-point photos to determine length estimates for the study. At 8.4–10.6 m total length, St. Helena animals are larger than those found in juvenile-dominated coastal aggregation sites like Yucatan, Mexico, but smaller than the large solitary females found in Galapagos. The central tendency approach represents a significant improvement to the photogrammetry method, but it increases workload significantly, so there is still a need to develop non-invasive measurement methods that are accurate, precise, and quick.


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