Effects of lipid and urea extraction on stable isotope values (δ 13 C and δ 15 N) of two batoids: A call for more species‐specific investigations

The presence of lipids and urea in elasmobranch tissues can affect carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope values, introducing bias in food web interpretations. Information on how lipids and urea affect δ 13 C/δ 15 N is only available for < 5% of ~ 1150 described elasmobranch species and results are highly variable among existing studies. Here, we investigate the effects of lipid and urea extraction on δ 13 C, δ 15 N, and C:N ratios in muscle and blood plasma of two batoids ( Pastinachus ater and Himantura australis ) and examine the influence of lipid/urea extraction on isotopic niche size and overlap. Lipid extraction (LE) did not affect δ 13 C, δ 15 N, or C:N ratios, suggesting low lipid content. Urea extraction (UE), however, increased δ 15 N and C:N in muscle. The ~ 1‰ δ 15 N increase represents a shift of ~ 0.5 trophic levels relative to bulk samples, highlighting the importance of UE to accurately assess trophic positions. Although there was no effect of any treatment on niche size, the probability of P. ater occurring within the niche of H. australis increased following UE. Overall, results suggest that urea should be removed from muscle prior to analysis, but LE is not required. Given the interspecific variability in the effects of lipid/urea on elasmobranch δ 13 C/δ 15 N, more studies are needed to assess the effects of lipid and urea on a broader range of species to produce a generalized understanding. Where no species‐specific data are available, we recommend pilot samples are analyzed to determine if LE is needed prior to preparation of the overall sample set.