Investigating stage-specific changes in nutritive condition during the migratory period and their implications for reproductive success in Eastern North Pacific gray whales (Eschrichtius robustus)

Eastern North Pacific (ENP) gray whales (Eschrichtius robustus) undergo annual migrations, during which they accumulate energy reserves on their foraging grounds to support reproduction and other metabolic needs during extended fasting periods. Whale body condition serves as a valuable indicator for predicting trends in both individual and population-level reproduction and survival. This study examines the stage-specific changes in nutritive condition during the migratory period of ENP gray whales and proposes a threshold of female body condition necessary for successful reproduction. We utilized a dataset collected by the National Oceanic and Atmospheric Administration (NOAA) from 1987 to 2019 and Bayesian modeling to transform and standardize body condition metrics and explore the minimum body condition of females necessary for successful reproduction. Our results revealed a minimum threshold of 0.145 (maximum width divided by the total length (MaxWL)) and interannual variability in body condition, closely linked to prey availability in the Arctic feeding grounds. The findings demonstrate the potential of a threshold to provide early warnings of potential reproductive failures in the future. Understanding the relationship between body condition, female energetics, and reproductive success provides important insights for managing and protecting the ENP gray whale population amid environmental changes and challenges.