Seasonal foraging behaviour of Eastern Canada-West Greenland bowhead whales: an assessment of isotopic cycles along baleen

Eastern Canada-West Greenland (EC-WG) bowhead whales Balaena mysticetus migrate seasonally between northwestern Hudson Bay/Foxe Basin and Gulf of Boothia in summer and Hudson and Davis Straits in winter. Despite recent advances in knowledge of summer diet composition, determining seasonal variation in foraging behaviour of EC-WG bowhead whales remains a priority for understanding how annual metabolic requirements are met, as well as identifying factors driving seasonal habitat selection. We measured stable nitrogen, carbon, and sulfur isotope composition (delta N-15, delta C-13, and delta S-34) along continuously growing baleen plates (n = 14) to assess alternative seasonal foraging hypotheses, namely winter fasting vs. year-round foraging. Synchronous delta N-15 and delta C-13 cycles, with periods of N-15 enrichment corresponding to foraging on the summer grounds, were inconsistent with standard fasting predictions, although delta N-15 cycles could reflect changes in diet-tissue delta N-15 discrimination between periods of intense foraging throughout the open-water season and supplemental protein intake during winter/spring. Correlations between delta N-15 and delta S-34 values, potentially meditated through amino acid metabolism, support this interpretation. Reasonable agreement between baleen isotope oscillations and regional baseline delta N-15 and delta C-13 variation also indicated foraging occurs within isotopically distinct food webs across the summer and winter ranges. We conclude that EC-WG bowhead whales forage throughout their distribution, and conservatively interpret delta N-15 and delta S-34 cycles to reflect reduced food consumption during winter. Foraging outside of periods of peak productivity likely contributes to annual metabolic requirements and winter habitat selection.