Body condition of gray whales is correlated with multi-scale ocean variability

Body condition represents an individual’s energy reserves acquired through feeding, which can indicate overall health and reproductive potential. As capital breeders, baleen whales rely on seasonal booms of prey availability, with spatio-temporal variability in productivity regulated by environmental conditions acting at different scales. We use an eight-year dataset (2016-2023) consisting of drone imagery of 110 individuals (423 samples, including replicates) to evaluate the relationships between environmental variables and the body condition of Pacific Coast Feeding Group (PCFG) gray whales during their summer foraging season (May-October) along the Oregon coast, USA. We calculate body condition as Body Area Index (BAI), with associated photogrammetric uncertainty, and use Hierarchical Generalized Additive Models to assess BAI variability relative to demographic unit, annual variability, and environmental variables at three different spatial scales: fine (within local study area), meso (along the Oregon coast), and large (the greater North Pacific region). BAI differed by demographic unit and showed large inter and intra-annual variability, indicating that dynamic environmental conditions regulate foraging conditions. While upwelling (the influx of deep, cold, nutrient rich waters into coastal regions) is an important driver of productivity and prey availability in the nearshore, we demonstrate how increased BAI is related to a balance between upwelling and relaxation (period of no upwelling) at the meso scale. We also describe how elevated hypoxic conditions (dissolved oxygen <2mg/L) is associated with low BAI at the fine scale, underlying the important mechanistic process of relaxation, as upwelled water is low in dissolved oxygen. At the large scale, BAI increased as the Pacific Decadal Oscillation became less negative, toward neutral, reflecting more balanced upwelling-relaxation conditions. Our results highlight previously overlooked environmental variables (relaxation and hypoxia) associated with baleen whale body condition and raise concern for the PCFG population, given intensified upwelling and elevated hypoxia are projected under climate change.