Interactive effects of urea and lipid content confound stable isotope analysis in elasmobranch fishes

Abstract

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 14 species of sharks, skates, and rays from the eastern North Pacific Ocean. While effects were variable across taxa, removal of urea generally increased δ 15 N and C:N. Urea removal had less influence on δ 13 C, whereas extracting urea and lipid generally increased δ 15 N, C:N, and δ 13 C. Because C:N values of nonextracted tissues are often used to infer lipid content and adjust δ 13 C, shifts in C:N following urea extraction will change the inferred lipid content and bias any mathematical adjustment of δ 13 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.