The development of robust morphometric indices from accurate and precise measurements of free-swimming whale sharks using laser photogrammetry

Abstract

To enable the study of population dynamics of wild animals the determination of the age, growth rate and maturity status of a sample of the individuals present is required; consequently, obtaining repeated accurate and precise total length (TL) measurements for individuals over time can be especially valuable. However, there are limited easily applied methods to ascertain the TL of large free-swimming fish, especially the largest extant species of fish, the whale shark (Rhincodon typus). This study expands on previous work and presents the results of a robust laser photogrammetry system developed to achieve accurate TL, pre-caudal length (PCL) and further morphometric measurements of whale sharks observed between 2009 and 2011 in seasonal feeding aggregations located in the Seychelles and Djibouti. Calculations for repeatability (r) indicated a high level of precision for the system with r approaching 1 for both TL and PCL, increasing further with the use of morphometric measurements. TL measurements of 'straight sample sharks' also provided geometric mean linear regression equations to enable the prediction of TL from defined morphological indices. Continuous validation of the system against objects of a fixed length also indicated a high level of accuracy for the method of measurement. We concluded that the laser photogrammetry system can be confidently employed to obtain accurate in-water TL, PCL and morphometric measurements for R. typus, with wide ranging implications and applications for the study of R. typus, and other large marine fauna.

Testing parallel laser image scaling for remotely measuring body dimensions on mantled howling monkeys (Alouatta palliata)

“…Photogrammatic methods, which involve taking measurements of body dimensions from photographs, have been tested in whale sharks [Graham & Roberts, 2007;Jeffreys et al, 2013;Rohner et al, 2011], marine mammals [Durban & Parsons, 2006; conflict of interest : None Ireland et al, 2006;Lourie et al, 2014;Perryman & Lynn, 1993;Perryman and Westlake, 1998;Webster et al, 2010], large terrestrial mammals such as elephants [Lee & Moss, 1995;Morgan & Lee, 2003;Shrader et al, 2006;Wijeyamohan et al, 2012] and artiodactyls [Bergeron, 2007;Berger, 2012;Willisch et al, 2013], terrestrial primates such as baboons [Domb & Pagel, 2001], macaques [Kurita et al, 2012], gorillas [Breuer et al, 2007;Caillaud et al, 2008], and chimpanzees [Deschner et al, 2004;Emery & Whitten, 2003;], as well as one arboreal primate-red colobus monkeys [Rothman et al, 2008]. Several techniques have been developed, and they primarily differ regarding how the scale of the objects in the photograph is determined.…”

Growth trajectories in wild geladas (Theropithecus gelada)

“…Because the lasers are separated by a known distance [Barrickman et al, ; Bergeron, ; Rothman et al, ], the distance between the lasers provides a convenient a scale within every photo, allowing researchers to easily obtain linear measurements from an animal a short distance away. Indeed, parallel lasers have emerged as an accurate, simple, and relatively cheap tool to measure body and limb length in wild animals [Bergeron et al, ; Jeffreys et al, ; Packer, ; Rowe & Dawson, ; Webster et al, ], providing an unprecedented opportunity to assess growth and life history in wild primates.…”

Peer Review #1 of "Laser photogrammetry improves size and demographic estimates for whale sharks (v0.1)"

“…Measuring only a portion of the body, such as PCL or B P1 , enhances precision as it excludes the caudal fin or the whole posterior part of the body which can be flexed when the shark is swimming and results in out of plane (foreshortened) images. We used B P1 to scale TL in preference over the distance from the spiracle to the 5 th gill slit (A1 in Jeffreys et al, 2012). This was because sharks in our study were mostly surface feeding, which resulted in a dorso-ventral flexion of the head that precluded an assessment of the A1 metric.…”