An evaluation of lipid extraction techniques for interpretation of carbon and nitrogen isotope values in bottlenose dolphin (Tursiops truncatus) skin tissue

We studied the effects of two common chemical extraction techniques on bottlenose dolphin (Tursiops truncatus) skin tissues with the intent to develop a mathematical lipid correction for dolphin skin δ¹³C. One method employs a hot solvent mixture (chloroform and methanol) while the other method requires washing the samples with cold solvent followed by water. The water wash method resulted in significant alteration of tissue δ¹⁵N. We found no correlation between change in sample mass and C/N or between change in sample mass and the change in δ¹³C (Δδ¹³C) following lipid extraction. Although Δδ¹³C was positive following lipid extraction (mean = 1.6‰ and 1.2‰, for the two methods), there was no correlation between C/N and Δδ¹³C for either method. Cumulatively, these results prevented us from applying a mathematical lipid normalization. Based on our findings and consideration of previously reported results, we suggest that applying these extraction techniques to dolphin skin with C/N < 4.5 introduces greater uncertainty than is warranted. We recommend against lipid correction for dolphin skins with C/N < 4.5, but stress that the resulting uncertainty in δ¹³C needs to be accounted for when implementing isotope mixing models to assess diet or organic matter sources.