title: Methods of lipid-normalization for multi-tissue stable isotope analyses in tropical tuna.
abstract: BACKGROUND
  The bias associated with lipid contents in fish tissues is a recalcitrant topic for trophic studies using stable isotopes. Lipids are depleted in the heavy carbon isotope ((13)C) and the lipid content varies considerably among species, tissues and in both time and space. We have applied and assessed different correction methods for tropical tuna tissues.
  
  
  METHODS
  We tested two types of normalization methods to deal with variable lipid content in liver, gonads, and white and red muscles of yellowfin, bigeye and skipjack tuna: a chemical extraction using dichloromethane and a mathematical correction based on three modeling approaches (linear, non-linear and mass balance models). We measured isotopic ratios of bulk and lipid-free tissues and assessed the predictive ability of the correction models with the lipid-free measurements. The parameters of the models were estimated from our dataset and from results from published studies on other species.
  
  
  RESULTS
  Comparison between bulk, lipid-free and lipid-corrected isotopic ratios demonstrated that (1) chemical extraction using dichloromethane did not affect δ(15)N values; (2) the change in δ(13)C values after extraction was tissue-specific; (3) lipid-normalization models using published parameter estimates failed to predict lipid-corrected δ(13)C values; and (4) linear and non-linear models using parameters estimated for each tissue from our dataset provided accurate δ(13)C predictions for all tissues, and mass balance model for white muscle only.
  
  
  CONCLUSIONS
  Models using published estimates for parameters from other species cannot be used. Based on a range of lipid content that do not exceed 45%, we recommend the linear model to correct the bulk δ(13)C values in the investigated tissues but the parameters have to be estimated from a proportion of the original data for which chemical extraction is required and the isotopic values of bulk and lipid-free tissues are measured.
authors.affiliation: IRD, UMR MARine Biodiversity Exploitation and Conservation (MARBEC), Avenue Jean Monnet, Sète, France.|IRD, UMR MARine Biodiversity Exploitation and Conservation (MARBEC), Fishing Port, Victoria, Seychelles.
authors.forename: Fany
authors.initials: F
authors.lastname: Sardenne
authors.affiliation: IRD, Mediterranean Institute of Oceanography (MIO), Aix-Marseille Université/CNRS/IRD/Université de Toulon, 13288, Marseille, France.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Ménard
authors.affiliation: IRD, UMR MARine Biodiversity Exploitation and Conservation (MARBEC), Avenue Jean Monnet, Sète, France.
authors.forename: Maxime
authors.initials: M
authors.lastname: Degroote
authors.affiliation: IRD, UMR MARine Biodiversity Exploitation and Conservation (MARBEC), Avenue Jean Monnet, Sète, France.|INRA, UMR TOXALIM (Research Centre in Food Toxicology), Toulouse, France.
authors.forename: Edwin
authors.initials: E
authors.lastname: Fouché
authors.affiliation: CNRS-Université de la Rochelle, UMR Littoral Environnement et Sociétés (LIENSs), 2 rue Olympe de Gouges, 17000, La Rochelle, France.
authors.forename: Gaël
authors.initials: G
authors.lastname: Guillou
authors.affiliation: CNRS-Université de la Rochelle, UMR Littoral Environnement et Sociétés (LIENSs), 2 rue Olympe de Gouges, 17000, La Rochelle, France.
authors.forename: Benoit
authors.initials: B
authors.lastname: Lebreton
authors.affiliation: Seychelles Fishing Authority (SFA), Fishing Port, Victoria, Seychelles.
authors.forename: Stephanie J
authors.initials: SJ
authors.lastname: Hollanda
authors.affiliation: IRD, UMR MARine Biodiversity Exploitation and Conservation (MARBEC), Fishing Port, Victoria, Seychelles.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Bodin
mesh_terms: D000818:Animals; D002247:Carbon Isotopes / Q000032:analysis*; D005591:Chemical Fractionation; D005260:Female; D029941:Fish Proteins / Q000032:analysis; D019156:Indian Ocean; D008055:Lipids / Q000032:analysis* / Q000302:isolation & purification; D008099:Liver / Q000737:chemistry; D008297:Male; D013058:Mass Spectrometry / Q000379:methods; D008962:Models, Theoretical; D018482:Muscle, Skeletal / Q000737:chemistry; D009587:Nitrogen Isotopes / Q000032:analysis*; D014329:Tropical Climate; D014413:Tuna
chemical_list: D002247:Carbon Isotopes; D029941:Fish Proteins; D008055:Lipids; D009587:Nitrogen Isotopes