A photogrammetric method to estimate total length of the largest mammal, the blue whale ( Balaenoptera musculus )

Length measurement of individuals provides useful information for biological and ecological studies and is instructive in estimating and parameterization of population dynamics and to identify changes in population structure. This study presents a photogrammetric method to estimate the total length of blue whales from a boat, using sequential images taken to measure the exposed body flank of the whale, from the blowhole to the dorsal fin, while simultaneously measuring the perpendicular distance between the whale and the camera to determine the scale of the image. The photo sequences were joined and measured with specialized software. Total length was predicted from blowhole–dorsal fin length using body proportion, mainly from whaling data. A Bayesian model calculated uncertainty in predicted total length due to errors in the photo-sequence process and variability in body proportion. Nearly all the uncertainty in total length was due to variability in the photo-sequence measurements of blowhole–dorsal fin length rather than in body proportion. The precision of estimates of total length depended on the number of replicate photo-sequences and size of whale but generally were in the range of 2–3%. A comparison of the total lengths of four individuals simultaneously measured by replicated photo-sequences and aerial photogrammetry showed that the photo-sequence method was unbiased. The photo-sequence method applied to 169 individuals sighted during 2006–2015 in the Gulf of California revealed an average length of 20.49 m and a range of 7.45–29.01 m, which falls within the known length range for Northeast Pacific blue whales. The photo-sequence method can be carried out in conjunction with other field studies of the species as it does not modify sampling logistics, nor add to the cost of the study. It also has the advantage of being adaptable to other cetaceans that show natural flank marking.