The Arctic marine ecosystem has experienced extensive changes in sea ice dynamics, with significant impacts on ice-dependent species such as polar bears (Ursus maritimus). We used abundance estimates, age/sex structure, and body condition data to estimate population energy density and storage energy in Western Hudson Bay polar bears from 1985 to 2018. We examined intra-population variation in energetic patterns, temporal energetic trends, and the relationship between population energetics and sea ice conditions. Energy metrics for most demographic classes declined over time in relation to earlier sea ice breakup, most significantly for solitary adult females and yearlings, demonstrating their vulnerability to nutritional stress. Population energy metrics declined significantly over time in relation to earlier breakup and longer lagged open water periods, suggesting multi-year effects of sea ice decline. This study provides insights into ecological mechanisms linking population responses to sea ice decline and highlights the utility of long-term bioenergetics research.