Decadal Bowhead Whale Baleen Stable Isotope and Hormone Chronologies (1985–2010) Provide Insight Into Arctic Ecosystem Change

ABSTRACT Arctic sea ice loss is altering ecosystem dynamics and threatening ice‐associated species such as the bowhead whale ( Balaena mysticetus ), the only Arctic‐endemic baleen whale. Sea ice‐mediated changes in primary productivity, zooplankton assemblages, and killer whale range expansion in the eastern Canadian Arctic may alter bowhead prey availability and predation pressure. We analysed stable carbon (δ 13 C) and nitrogen isotope ratios (δ 15 N) and concentrations of the adrenal glucocorticoid corticosterone and the thyroid hormone triiodothyronine (T3) along baleen plates of 19 subsistence‐hunted bowhead whales from the Eastern Canada‐West Greenland (EC‐WG) population. Individual stable isotope (SI) and hormone profiles were combined into chronologies spanning 25 years (1985–2010), partitioned into annual and seasonal (summer and winter) components using SI cycles. Linear mixed‐effects models revealed significant long‐term declines in both δ 13 C and δ 15 N, with only weak statistical support for seasonally different rates for δ 13 C (−0.024‰ year −1 vs. −0.033‰ year −1 ) and δ 15 N (−0.033‰ year −1 in summer vs. −0.024‰ year −1 in winter). Declining δ 13 C and δ 15 N potentially reflect localized declines in primary productivity and/or changes in marine biogeochemistry, superimposed on global SI trends due to anthropogenic marine C and N inputs, rather than a gradual (i.e., linear) change in trophic level. Weakly declining corticosterone and T3 concentrations (−0.023 and −0.042 ng g −1 year −1 , respectively; no interaction with season) remained within presumed‐normal ranges, interpreted here to indicate no substantive shifts in adrenal activity or energetic demands over the same timeframe. The Arctic has continued to change rapidly since 2010, and these baleen‐derived chronologies can serve as a benchmark for continued monitoring of impacts on the EC‐WG bowhead population.