Fish tissue lipid‐C:N relationships for correcting δ 13 C values and estimating lipid content in aquatic food‐web studies

Rationale Normalizing δ 13 C values of animal tissue for lipid content is necessary to accurately interpret food‐web relationships from stable isotope analysis. To reduce the effort and expense associated with chemical extraction of lipids, various studies have tested arithmetic mass balance to mathematically normalize δ 13 C values for lipid content; however, the approach assumes that lipid content is related to the tissue C:N ratio. Methods We evaluated two commonly used models for estimating tissue lipid content based on C:N ratio (a mass balance model and a stoichiometric model) by comparing model predictions to measure the lipid content of white muscle tissue. We then determined the effect of lipid model choice on δ 13 C values normalized using arithmetic mass balance. To do so, we used a collection of fish from Lake Superior spanning a wide range in lipid content (5% to 73% lipid). Results We found that the lipid content was positively related to the bulk muscle tissue C:N ratio. The two different lipid models produced similar estimates of lipid content based on tissue C:N, within 6% for tissue C:N values <7. Normalizing δ 13 C values using an arithmetic mass‐balance equation based on either model yielded similar results, with a small bias (<1‰) compared with results based on chemical extraction. Conclusions Among‐species consistency in the relationship between fish muscle tissue C:N ratio and lipid content supports the application of arithmetic mass balance to normalize δ 13 C values for lipid content. The uncertainty associated with both lipid extraction quality and choice of model parameters constrains the achievable precision of normalized δ 13 C values to about ±1.0‰. Published in 2015. This article is a U.S. Government work and is in the public domain in the U.S.A.