Interactive effects of urea and lipid content confound stable isotope analysis in elasmobranch fishes
Stable isotope analysis (SIA) is becoming a commonly used tool to study the ecology of elasmobranchs.
However, the retention of urea by elasmobranchs for osmoregulatory purposes may bias the analysis and
interpretation of SIA data. We examined the effects of removing urea and lipid on the stable isotope
composition of fourteen species of sharks, skates, and rays from the eastern North Pacific Ocean. While
effects were variable across taxa, removal of urea generally increased δ15 N and C:N. Urea removal had
less influence on δ13C, whereas extracting urea and lipid generally increased δ15 N and C:N while also
increasing δ13 C. Because C:N values of non-extracted tissues are often used to infer lipid content and
adjust δ13 C, shifts in C:N following urea extraction will change the inferred lipid content and bias any
mathematical adjustment of δ13 C. These results highlight the importance of urea and lipid extraction and
demonstrate the confounding effects of these compounds, making it impossible to use C:N of non-urea-
extracted samples as a diagnostic tool to estimate and correct for lipid content in elasmobranch tissues.