Using laser metrics to measure wild bottlenose dolphins

Photo-identification is a powerful tool for estimating key population parameters. However there is often a lack of information on individual co-variates that constrains assessments of many important life history traits. In this study we adapted a new laser metric technique, recently used to measure killer whale dorsal fin height, to remotely measure individuals from the resident bottlenose dolphin population on the east coast of Scotland. Long-term research on this population has discovered much about individual ranging patterns, associations and reproductive rates. Here, our aim was to develop techniques to explore variation in these patterns in relation to estimates of body size (that might provide correlates of age or sex) and information on body condition and growth. A simple portable aparatus was designed with two Beamshot 4mW laser sights fixed at 10cm apart and attached to the tripod mount of a Canon EF 70-200 f/2.8L USM lens. During photo-ID surveys the laser dots were projected onto the dorsal fin or body of the dolphin and photographs were taken, providing digital images with a known scale. Our aim was to determine the distance from blowhole to dorsal fin which previous studies indicate can be used to estimate total length, and to measure dorsal fin size as a potential indicator of sex. Laser dots were visible between 5 and 20m. Calibration of known length objects resulted in accurate estimates with average errors of 0.6% at 5m and 3% at 20m. Of 65 individuals recorded on 4 photo-ID trips in 2007 we used Image Tool 3.0 to estimate the total length of 11 individuals (ranging from 260cm to 323cm). We describe the development and use of this technique for bottlenose dolphins and suggest it can be incorporated into standard photo-ID surveys to provide important additional information on the characteristics of recognisable individuals.