All skin and prey samples were lipid-extracted, lyophilized, and homogenized by grinding them into a fine powder; as noted above the small set of subsamples that were analyzed to test the effects of lipid-extraction were not lipid-extracted (bulk tissue). Baleen plates were cleaned with a solution of 2:1 chloroform:methanol to remove surface contaminants. Sub-samples of keratin powder were collected with a Dremel rotatory drill fitted to a flexible engraving shaft at 1 cm intervals along the outer edge of each baleen, starting at the proximal section inserted in the gum (which represents the newest tissue) (Fig 2B ). Baleen grows uniformly on the transverse perspective at a constant (but unknown) rate; thus our sampling strategy would yield equal time intervals between adjacent sub-samples [37 (link),42 –44 ,55 ,57 ,82 ]. Previous studies have confirmed the consistency of isotope values along the length of two adjacent baleen plates of a gray whale (Eschrichtius robusutu) [82 ] and two plates from opposing sides of the mouth of a bowhead whale (Balaena mysticetus) [43 ]. Consequently, we assumed that each baleen provides a consistent record of the past foraging history for each blue whale. Lastly, we compiled δ13C and δ15N data from the literature of blue whale prey from foraging zones in the northeast Pacific (S2 Table ).
Approximately 0.5–0.6 mg of each tissue sample (dried skin, baleen, and prey) was weighed into a tin capsule. Carbon (δ13C) and nitrogen (δ15N) isotope values were measured with a Costech 4010 elemental analyzer coupled to Thermo Scientific Delta V isotope ratio mass spectrometer at the Center for Stable Isotopes at the University of New Mexico (Albuquerque, NM). Isotope data are reported as delta δ values, δ13C or δ15N = 1000 [(Rsample / Rstandard)—1], where R = 13C/12C or 15N/14N ratio of sample and standard [83 ]. Values are in units of parts per thousand or per mil (‰) and the internationally accepted standards are atmospheric N2 for δ15N and Vienna-Pee Dee Belemnite limestone (V-PDB) for δ13C [83 ]. Within-run analytical precision was estimated via analysis of two proteinaceous internal reference materials, which was ±0.2‰ for both δ13C and δ15N values. We also measured the weight percent carbon and nitrogen concentration of each sample and used the C/N ratio as a proxy of lipid content [84 (link)].
Approximately 0.5–0.6 mg of each tissue sample (dried skin, baleen, and prey) was weighed into a tin capsule. Carbon (δ13C) and nitrogen (δ15N) isotope values were measured with a Costech 4010 elemental analyzer coupled to Thermo Scientific Delta V isotope ratio mass spectrometer at the Center for Stable Isotopes at the University of New Mexico (Albuquerque, NM). Isotope data are reported as delta δ values, δ13C or δ15N = 1000 [(Rsample / Rstandard)—1], where R = 13C/12C or 15N/14N ratio of sample and standard [83 ]. Values are in units of parts per thousand or per mil (‰) and the internationally accepted standards are atmospheric N2 for δ15N and Vienna-Pee Dee Belemnite limestone (V-PDB) for δ13C [83 ]. Within-run analytical precision was estimated via analysis of two proteinaceous internal reference materials, which was ±0.2‰ for both δ13C and δ15N values. We also measured the weight percent carbon and nitrogen concentration of each sample and used the C/N ratio as a proxy of lipid content [84 (link)].
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