Regional and seasonal feeding by bowhead whales Balaena mysticetus as indicated by stable isotope ratios

The feeding habitats of the Bering - Chukchi- Beaufort stock of bowhead whales Balaena mysticetus during summer and fall migration are generally known, but the relative amounts of food they consume in various parts of the seasonal range are poorly understood. We used natural geographic gradients in stable isotope ratios of carbon and nitrogen within zooplankton (bowhead prey) to measure the relative importance of the eastern Beaufort Sea versus Bering and Chukchi Seas (combined) as feeding habitat for bowheads. The VC content of their prey differs by 3.9%, on a weighted average basis, between the 2 regions. delta(13)C in bowhead muscle changed by only 0.5 %, while bowheads were in the Beaufort Sea, indicating that only ca. 10% of the muscle carbon in the average whale turns over during summer feeding, with 95 % confidence intervals (Cl) of 0 to 23 %. Considering subadult whales only, estimated turnover averaged 14 % with 95 % Cl of 4 to 23 %. For adults, the estimated turnover was 5 % (95% Cl = 0 to 30 %). Overall delta(13)C values in muscle are not significantly different in adult and subadult bowheads, suggesting no major age-related shift in the relative importance of different feeding habitats, and very closely matching Bering- Chukchi-derived food. delta(15)N values for muscles were near 1% higher in whales taken in the 1980s, consistent with the observed long-term declining trends in environmental delta(15)N values as indicated by chronological sampling of baleen plates. In contrast to muscle data, delta(13)C values in baleen, each representing feeding during a shorter time increment, do show seasonal and age-related differences. Subadult whales apparently feed more in the eastern Beaufort Sea than do adults, but nonetheless acquire insufficient food there to significantly alter their bulk carbon isotope ratios.