Spy in the sky: a method to identify pregnant small cetaceans

Data on sex ratios, age classes, reproductive success and health status are key metrics to manage populations yet can be difficult to collect in wild cetacean populations. Long-term individual-based studies provide a unique opportunity to apply unoccupied aerial system (UAS) photogrammetry to non-invasively measure body morphometrics of individuals with known life history information. The aims of this study were 1) to compare length measurements from UAS photogrammetry with laser photogrammetry and 2) to explore whether UAS measurements of body width could be used to remotely determine pregnancy status, sex or age class in a well-studied bottlenose dolphin population in Scotland. We carried out five boat-based surveys in July and August 2017, with concurrent photo-identification, UAS, and laser photogrammetry. Photographs were measured using bespoke programmes, MorphMetriX for UAS photos and a Zooniverse project for laser photos. In total 64 dolphins were identified using photo-ID, 54 of which had concurrent UAS body length and 47 with laser body length measurements. We also measured body widths at 10% increments from 10-90% of body length for 48 individuals of known sex, age class and/or pregnancy status. There was no significant difference in the length of individuals measured with UAS and laser photogrammetry. Discriminant analyses of the body width-length ratios expected to change during pregnancy, correctly assigned pregnancy status for 14 of the 15 females of known pregnancy status. Only one pregnant female was incorrectly assigned as not pregnant. However, our results showed that length and body width cannot accurately allocate these bottlenose dolphins to sex or age class using photogrammetry techniques alone. The present study illustrates that UAS and laser photogrammetry measurements are comparable for small cetaceans and demonstrates that UAS measurements of body width-length ratio can accurately assign pregnancy status in bottlenose dolphins.

种群性别比例、年龄组、繁殖成功率与健康状况数据是野生动物种群管理的核心指标，但在野生鲸类种群中却难以获取。长期基于个体的研究为应用无人航空系统（Unoccupied Aerial System，UAS）摄影测量法无创测量已知生活史个体的身体形态参数提供了独特契机。本研究的目标有二：其一，对比无人航空系统摄影测量与激光摄影测量所得的体长数据；其二，探究在苏格兰一处已被充分研究的宽吻海豚种群中，是否可通过无人航空系统测得的体宽数据远程判断个体的妊娠状态、性别与年龄组。我们于2017年7月至8月开展了5次船基调查，同步进行照片识别法（photo-ID）、无人航空系统摄影测量与激光摄影测量工作。照片测量采用定制程序：针对无人航空系统拍摄照片的MorphMetriX软件，以及针对激光摄影照片的Zooniverse项目。本次研究共通过照片识别法确定了64头海豚，其中54头同时获取了无人航空系统测得的体长数据，47头获取了激光摄影测量的体长数据。我们还对48头已知性别、年龄组及/或妊娠状态的个体，在其体长的10%至90%区间内以10%为间隔测量了体宽。无人航空系统与激光摄影测量所得的个体体长数据无显著差异。对妊娠期间会发生变化的体宽-体长比进行判别分析后，在15头已知妊娠状态的雌性个体中，有14头的妊娠状态被正确判别，仅1头妊娠个体被误判为未妊娠。但研究结果表明，仅依靠摄影测量技术，无法通过体长与体宽数据准确区分该种群宽吻海豚的性别与年龄组。本研究表明，针对小型鲸类，无人航空系统与激光摄影测量所得数据具有可比性；同时证实，通过无人航空系统测量体宽-体长比，可准确判别宽吻海豚的妊娠状态。