Odontocete strandings from space: Accurately counting individuals with very high-resolution optical and synthetic aperture radar satellite imagery

Cetacean strandings provide early warning of emerging concerns regarding animal, ocean, and human health and allow insights into local cetacean population, density, distribution and mortality, and thus offer significant conservation value for the assessment of ecosystems. However stranding networks (programmes for monitoring and coordinating a ground response and investigation of local strandings) are infrequent to non-existent within minimally populated areas, coastal areas with limited economic resources, geographically remote areas, complex coastlines and areas of geopolitical unrest. Very high-resolution (VHR) satellite imagery offers the prospect of improving monitoring in these regions. While VHR optical satellite imagery is able to detect large baleen whale strandings (> 12 m), mass strandings (MSE, strandings of two or more animals excluding mother calf pairs) are predominantly of smaller-sized odontocetes (∼1–6 m). Detecting odontocetes is therefore crucial for VHR satellites to be useful for monitoring strandings globally. Here, we analyse MSEs involving large (∼11–18 m) and small odontocetes to; (1) investigate whether stranded animals can be detected and counted in VHR optical satellite imagery, (2) determine the minimum spatial resolution of VHR optical satellite imagery necessary for detecting and counting stranded animals, and (3) explore the utility of synthetic aperture radar (SAR) to detect and count stranded animals. We successfully detected and counted large and small stranded odontocetes in optical and SAR satellite imagery. With optical imagery, we found that 0.5 m spatial resolution is suitable for accurately counting large odontocetes (100% accuracy, one image) while 0.3 m resolution is most suitable for counting smaller odontocetes (82–100% accuracy, four images). With SAR, two or more observers detected a total of 108 small odontocetes with high certainty (‘likely’ or ‘definite’ detections) compared to 111 in a VHR optical satellite image collected five hours later. These positive results show the potential for stranding networks to expand their monitoring capacity both in terms of scale and speed of detection.