MEPS prepress abstract  -  DOI: https://doi.org/10.3354/meps12971

Validation of the photogrammetric method to assess body condition of an Odontocete, the short finned pilot whale (Globicephala macrorhynchus)

S. R. Noren*, L. Schwarz, K. Chase, K. Aldrich, K. McMahon-Van Oss, J. St. Leger

*Email: snoren@ucsc.edu

ABSTRACT: Validated metrics to monitor body condition of free-ranging animals are critical to track population stability. We tested repeatability and reliability of body width and body length measurements taken from overhead photographs and validated the assumptions upon which the photogrammetric method to monitor cetacean body condition are based. Pilot whales (Globicephala macrorhynchus) served as models. Variability was low for multiple measurements taken from one photograph and across photographs of the same animal; standard deviations represented 1.2 ± 1.2% and 2.5 ± 1.3% of mean estimates, respectively. To account for body length variations across whales, mass, width, and blubber indexes were calculated as the residual values of these variables regressed against length. Across the sites examined (anterior pectoral fin, anterior dorsal fin, and posterior dorsal fin) only photogrammetric body width at the posterior dorsal fin site showed consistent significant positive relationships with measured condition indexes (i.e. width and blubber index slope = 0.10, p = 0.005; width and mass index slope = 18.1, p <0.01). Moreover, only body width body length ratio at this site predicted mass index (slope = 3105, p = 0.05). Thus, changes in photogrammetric measured body width posterior of the dorsal fin at approximately 47% of total body length from the rostrum are related to changes in underlying blubber thickness and body mass. The width length ratio at this site can be used to monitor condition. Additional studies are warranted to determine if body width at this site reliably predicts condition across odontocete species with varying body morphologies.