Do Lipids Matter in Stable Isotope Analysis? Assessing the Need to Account for Lipids in Stable Isotope Analysis of Fish Tissues

Stable isotope analysis (SIA) is a powerful tool to examine diet and food web dynamics.  SIA assumes “you are what you eat” relative to carbon (C) and nitrogen (N).  However, fractionation of δ¹³C during lipid synthesis violates this assumption and high-lipid tissues should be lipid-corrected to avoid biasing δ¹³C. The purpose of this study is to (1) assess the effect of chemical lipid extraction on δ¹³C of fish white muscle and liver tissue and (2) determine if published mathematical lipid normalization models accurately predict δ¹³C of lipid-free samples.  Lipid extraction does not have a biologically significant effect on δ¹³C in muscle with low initial lipid content, but enriches δ¹³C in fish liver by more than 1‰.  The Post et al. (2007) lipid normalization model accurately predicts δ¹³C of lipid-extracted fish liver with C:N ratios less than 7.  Our results indicate accounting for lipids is unnecessary in tissues with C:N ratios below 3.5, however, tissues with C:N ratios greater than 3.5 contain sufficient lipid content to bias δ¹³C.  When tissue C:N ratios are between 3.5 and 7, δ¹³C can be corrected mathematically with the Post et al. (2007) model.  Tissue with C:N greater than 7 should be lipid extracted before SIA.