Trophic ecology of bowhead whales (Balaena mysticetus) compared with that of other arctic marine biota as interpreted from carbon-, nitrogen-, and sulfur-isotope signatures

Abstract

In this study, stable carbon (δ 13 C), nitrogen (δ 15 N), and sulfur (δ 34 S) isotope ratios were measured in muscle tissue from the Bering-Chukchi-Beaufort Sea population of bowhead whales (Balaena mysticetus; n = 84) and various marine biota between 1997 and 2000. In previous investigations, stable carbon and nitrogen isotope ratios in baleen from this population have been used to elucidate age, migratory behaviour, and feeding ecology. However, information on δ 13 C, δ 15 N, and δ 34 S isotope patterns in bowhead whale muscle tissue and variability within the Bering Sea population is limited. Stable sulfur isotope values did not vary with δ 13 C enrichment for three consecutive seasons (n = 53) and this suggests that habitat selection by bowhead whales was consistent over the sampling period. We found that in contrast to other studies, seasonal differences (spring versus fall) in δ 13 C values were not associated with seasonal changes in δ 15 N values, suggesting either that bowhead whales maintain a consistently lower trophic position relative to other marine mammals or that stable carbon and nitrogen isotope fractionation is tissue-dependent and (or) isotopedependent within this species. Seasonal fluctuation in δ 13 C values was consistent for all age classes of bowhead whales and suggests that the Bering and Beaufort seas are both important regions for feeding.

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

“…The conclusions reached by Hoekstra et al (2002) differ from ours and are discussed in detail in the second subsection of 'Discussion'.…”

“…This should allow a more direct approach to estimation of feeding, as evidenced by the turnover of carbon in muscle tissue. Hoekstra et al (2002) determined that the 10 whales taken in fall at Kaktovik during 1997 to 1999 (the years of our sampling) had an average muscle δ 13 C of -21.6 ‰ and that 28 whales taken in spring at Barrow during 1998 to 2000 averaged -19.9 ‰, i.e. a seasonal shift of -1.7 ‰.…”

“…A recent paper by Hoekstra et al (2002) contrasts isotope ratios of bowhead whales taken within the same years in spring and fall. This should allow a more direct approach to estimation of feeding, as evidenced by the turnover of carbon in muscle tissue.…”

“…In addition, a recent paper by Hoekstra et al (2002) using similar isotopic techniques indicated that bowhead whale feeding in the eastern Beaufort Sea is important in the annual food budget for these animals. The conclusions reached by Hoekstra et al (2002) differ from ours and are discussed in detail in the second subsection of 'Discussion'.…”

Seasonal and Ontogenetic Variation in Subcutaneous Adipose Of the Bowhead Whale (Balaena mysticetus)

“…Histological and biochemical blubber parameters are related to environmental conditions to formulate hypotheses about developmental and environmental correlates of morphology. Blubber fatty acid composition and structure has also been used to infer population distribution, phylogeny, and feeding ecology in a variety of species including bowheads (Lockyer et al, 1984;Hoekstra et al, 2002;Budge et al, 2004;Samuel and Worthy, 2004;Budge et al, 2008;Cooper et al, 2009;Loseto et al, 2009). …”