title: Accounting for the effects of lipids in stable isotope (δ13C and δ15N values) analysis of skin and blubber of balaenopterid whales.
abstract: BACKGROUND
  Stable isotope values (δ(13)C and δ(15)N) of darted skin and blubber biopsies can shed light on habitat use and diet of cetaceans, which are otherwise difficult to study. Non-dietary factors affect isotopic variability, chiefly the depletion of (13)C due to the presence of (12)C-rich lipids. The efficacy of post hoc lipid-correction models (normalization) must be tested.
  
  
  METHODS
  For tissues with high natural lipid content (e.g., whale skin and blubber), chemical lipid extraction or normalization is necessary. C:N ratios, δ(13)C values and δ(15)N values were determined for duplicate control and lipid-extracted skin and blubber of fin (Balaenoptera physalus), humpback (Megaptera novaeangliae) and minke whales (B. acutorostrata) by continuous-flow elemental analysis isotope ratio mass spectrometry (CF-EA-IRMS). Six different normalization models were tested to correct δ(13)C values for the presence of lipids.
  
  
  RESULTS
  Following lipid extraction, significant increases in δ(13)C values were observed for both tissues in the three species. Significant increases were also found for δ(15)N values in minke whale skin and fin whale blubber. In fin whale skin, the δ(15)N values decreased, with no change observed in humpback whale skin. Non-linear models generally out-performed linear models and the suitability of models varied by species and tissue, indicating the need for high model specificity, even among these closely related taxa.
  
  
  CONCLUSIONS
  Given the poor predictive power of the models to estimate lipid-free δ(13)C values, and the unpredictable changes in δ(15)N values due to lipid-extraction, we recommend against arithmetical normalization in accounting for lipid effects on δ(13)C values for balaenopterid skin or blubber samples. Rather, we recommend that duplicate analysis of lipid-extracted (δ(13)C values) and non-treated tissues (δ(15)N values) be used.
authors.affiliation: Marine and Freshwater Research Centre, Department of Life Science, Galway-Mayo Institute of Technology Galway, Dublin Road, Galway, Ireland. conor.ryan@iwdg.ie
authors.forename: Conor
authors.initials: C
authors.lastname: Ryan
authors.forename: Brendan
authors.initials: B
authors.lastname: McHugh
authors.forename: Clive N
authors.initials: CN
authors.lastname: Trueman
authors.forename: Chris
authors.initials: C
authors.lastname: Harrod
authors.forename: Simon D
authors.initials: SD
authors.lastname: Berrow
authors.forename: Ian
authors.initials: I
authors.lastname: O'Connor
mesh_terms: D000273:Adipose Tissue / Q000737:chemistry*; D000465:Algorithms; D000818:Animals; D002247:Carbon Isotopes / Q000032:analysis*; D008055:Lipids / Q000302:isolation & purification*; D013058:Mass Spectrometry; D009587:Nitrogen Isotopes / Q000032:analysis*; D012867:Skin / Q000737:chemistry*; D014907:Whales / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D008055:Lipids; D009587:Nitrogen Isotopes