Abstract
Eastern North Pacific gray whales ( Eschrichtius robustus ) rely on energy obtained from Arctic feeding grounds to complete their 15,000–20,000 km annual round‐trip migration between feeding and breeding areas. However, quantitative estimates of their food requirements remain scarce. Using an age‐structured bioenergetics model incorporating life cycle stages and migration timings, we estimated daily energy needs across life stages. We found females need to supply nursing calves with 24–35 L of milk per day for 9–10 months. After weaning, juveniles require ~400 kg per day of Arctic amphipods (~5.6% of body weight), while the largest adults need a minimum of ~1000 kg per day (~5.4%) during the 154‐day summer foraging period. In contrast, pregnant females need 1600–1900 kg of prey per day (~9%–10%) to support fetal growth and store sufficient energy to produce milk while fasting and traveling northbound after calving in Mexico. Upon returning to their Arctic feeding grounds, lactating females require 1300–1600 kg of prey per day (7.9%–8.6%) for another 3.6 months to support the high costs of milk production. These estimates provide a quantitative framework for assessing how changes in Arctic prey availability can affect gray whale health, fitness, reproductive success, and population trends.