Context . The ringed seal (Phoca hispida), a small phocid seal with a circumpolar Arctic distribution and a strong association with sea ice, occurs at the southern limit of its range in Hudson and James Bays: an area that experiences complete ice cover in winter and complete open water in summer. Because of the high seasonal variability in environmental conditions, it is expected that ringed seals experience seasonal changes in diet and foraging habitat, which will be reflected in body condition and biomarkers of stable isotopes and fatty acids.

Aims . The purpose of the present study was to investigate intra-annual variation in the feeding habits and body condition of the ringed seal.

Methods . Tissue samples and morphological measurements from south-eastern Hudson Bay ringed seals were obtained every month during the Inuit subsistence hunt from November 2009 to May 2011 (n = 192). Muscle samples were used for δ¹⁵N and δ¹³C stable isotope analysis, blubber was used for analysis of fatty acid composition, bodyweight and sculp weight were used to estimate percentage blubber, and lower right canines were used to determine age.

Key results . Fatty acid composition, δ¹⁵N, and δ¹³C varied significantly by season, suggesting seasonal changes in foraging habitat and diet. Variation in percentage blubber indicated that poorest body condition occurs following the molting and fasting period, followed by a gradual increase from late summer through fall, with the highest body condition occurring in time for freeze-up in December.

Key conclusions . Patterns of δ¹³C indicate pelagic feeding during the open-water season (August–December) when fat and energy stores are replenished, increased benthic foraging during the period of ice cover (January–May), followed by a period of fasting during the spring molt (June–July). Fatty acid composition suggested seasonal changes in diet that could include increased importance of pelagic fish in the fall during the period of positive energy balance.

Implications . The first continuous collection of ringed seal tissue samples provided a comprehensive seasonal pattern of biomarker composition, which is baseline data that have important applications for short-term management and ecology studies as well as long-term conservation and monitoring programs.