Lipid extraction effects on stable isotope values (δ13C and δ15N) of elasmobranch muscle tissue

Given the known effect of lipid content on delta C-13 values and the potential effect of urea on ON values, examining the effects of lipid extraction, which can potentially extract both, is of particular importance for elasmobranch isotope ecology. Through analysing paired delta C-13, total %C, delta N-15, total %N and C:N values of non-lipid extracted (BULK) and lipid extracted (LE) muscle samples from twenty-one elasmobranch species, we assessed whether lipid extraction was required: (i) to remove lipids given reported low lipid content and, (ii) to determine if delta N-15 values were affected and whether this relates to the retention of isotopically light urea by elasmobranchs. The mean (+/- SD) delta C-13 values of eight out of twenty-one species significantly increased following lipid extraction with two species, the Greenland (Somniosus microcephalus) and whale (Rhincodon typus) shark, showing a marked increase (5.0 +/- 0.4 parts per thousand and 3.3 parts per thousand, respectively). The mean (+/- SD) and maximum increase in delta C-13 values were 0.6 +/- 1.2 parts per thousand and 5.9 parts per thousand, respectively. For delta N-15 data, thirteen species showed a significant increase following lipid extraction and a concomitant reduction in total percent nitrogen (%N). The C:N ratio for these species also increased from unexpectedly low values of <3.0 to similar to 3.0, the value expected for pure protein. The mean and maximum observed increase in delta N-15 values were 0.6 +/- 0.6 parts per thousand and 2.3 parts per thousand, respectively. There was no effect of increasing animal size on delta C-13 and delta N-15 difference (LE-BULK) for the two species examined. Field sampled animals (sampled immediately upon capture in the marine environment) showed a greater delta N-15 difference than animals sampled in the laboratory (sampled several hours after capture in the marine environment) (1.0 +/- 0.5 parts per thousand and 0.4 +/- 0.4 parts per thousand respectively), while estuarine sampled animals (sampled immediately) showed the smallest difference (0.1 +/- 0.6 parts per thousand). The delta C-13 data demonstrate that lipid extraction is required to remove lipids from elasmobranch muscle tissue given both intra- and inter- species variability. In addition, the increase in delta N-15 values, decrease in %N and increase in C:N ratio indicate that lipid extraction is removing soluble urea. Given lower delta N-15 diet-tissue discrimination factors for large marine predators, removal of urea is required to elucidate accurate trophic position estimates and relative food web position of elasmobranchs and for diet reconstruction. It is recommended that investigators undertake lipid extraction trials on elasmobranch muscle tissue to determine effects on delta C-13 and delta N-15 values on a species-by-species basis. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.