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.