How Robust is <i>Eschrichtius robustus</i> ? A Novel Photographic Index of Body Condition From Boat‐Based Photographs of Gray Whales

ABSTRACT Although the eastern North Pacific gray whale ( Eschrichtius robustus ) population has recovered from population declines associated with whaling, they are closely monitored, especially in association with Unusual Mortality Events. Metrics to improve assessment of the health of individual animals, when averaged, are valuable to inform monitoring and management. We used 30 years of photographic data of a unique gray whale feeding aggregation to develop and test a new methodology for assessing body condition. This group of individuals (20 as of 2021, but 11 in the study group), known as the Sounders, regularly enter Puget Sound, Washington, USA, to forage on ghost shrimp in the spring and early summer. Almost 35,000 images were examined and 729 were selected as being suitable to assess individual body condition. Geometric measuring tools in open‐source image analysis software were used to measure a novel body condition angle, °BC , as a proxy for changes in blubber thickness. This °BC varied significantly with the day of the year, with increases in blubber thickness observed as individuals progressed through the feeding season. Other metrics, such as year and surfacing interval, were not found to have a significant effect on °BC within this dataset. This novel index of body condition illustrates the opportunity to develop new methods to quantitatively assess change in individuals at both seasonal and annual scales. Expansion of this method to the larger eastern North Pacific gray whale population may allow for a more detailed examination of fluctuations in body condition to better understand both environmental and anthropogenic impacts. There is also potential for adaptation of the method to apply to historic photo catalogs of other baleen whale species, creating opportunities for enhanced data‐driven management plans.