Experimental validation confirms a carbon stable isotope lipid normalization procedure for Pacific salmon

Abstract

Carbon stable isotope analysis is an important tool in studies of fish ecology. Studies using this tool must account for the effect of lipids present in fish tissue on carbon stable isotope (δ13C) values. Simple correction equations have been developed to correct for this effect. For taxa with high fat content, choosing an accurate correction equation can have a significant impact on results. Pacific salmon (Oncorhynchus spp.) are a lipid-rich genus. Their broad marine distribution and ecological, cultural, and commercial importance make them prime candidates for stable isotope analysis. To determine both an accurate lipid correction equation for Pacific salmon δ13C values, and the effect of lipid extraction on bulk nitrogen isotope (δ15N) values, we performed pairwise isotope analysis on lipid-extracted and untreated muscle samples from 68 Chinook salmon O. tshawytscha spanning a size range of 165-750 mm and C:N values of 2.96-14.25. We compared the fit of existing δ13C lipid correction equations from 3 previously published models to our data, and optimized the top performing model using a leave-one-out cross validation. The model of Kiljunen et al. (2006; https://doi.org/10.1111/j.1365-2664.2006.01224.x) performed the best (mean squared error: 0.22, r2: 0.91), while the optimized model only slightly improved on it (MSE: 0.20, r2: 0.93). For δ15N, we determined that Chinook δ15N values significantly increased by 0.6‰ following lipid extraction. Our results confirm a lipid normalization procedure that is broadly applicable to Pacific salmon, and supports streamlined analysis of δ13C and δ15N from a single untreated muscle tissue sample.