MEPS 522:269-286 (2015)  -  DOI: https://doi.org/10.3354/meps11145

ABSTRACT: 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 (δ¹⁵N, δ¹³C, and δ³⁴S) along continuously growing baleen plates (n = 14) to assess alternative seasonal foraging hypotheses, namely winter fasting vs. year-round foraging. Synchronous δ¹⁵N and δ¹³C cycles, with periods of ¹⁵N enrichment corresponding to foraging on the summer grounds, were inconsistent with standard fasting predictions, although δ¹⁵N cycles could reflect changes in diet-tissue δ¹⁵N discrimination between periods of intense foraging throughout the open-water season and supplemental protein intake during winter/spring. Correlations between δ¹⁵N and δ³⁴S values, potentially meditated through amino acid metabolism, support this interpretation. Reasonable agreement between baleen isotope oscillations and regional baseline δ¹⁵N and δ¹³C 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 δ¹⁵N and δ³⁴S 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.

KEY WORDS: Arctic · Balaena mysticetus · Diet · Fasting · Isoscape · Marine mammal · Stable isotopes · Sulfur · Time series