Arithmetic normalisation models for the effects of lipid on carbon stable isotope values in silver carp (Hypophthalmichthys molitrix) tissue.

Abstract

Invasive silver carp (Hypophthalmichthys molitrix) threaten Mississippi River basin ecosystems due to their ability to outcompete native species. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analysis has been used to study how silver carp impact native ecosystems, but lipids in fish tissues commonly bias their δ¹³C values. Chemical lipid extraction and mathematical equations that normalise δ¹³C values for lipid content can account for this bias, but have not been assessed for silver carp. We examined δ¹³C, δ¹⁵N, and C:N ratios before and after chemical lipid extraction using 2:1 chloroform:methanol in silver carp muscle and whole fish collected along the Mississippi River. We used linear and natural log models to estimate lipid-extracted δ¹³C values in silver carp muscle and whole fish samples based on their non-lipid-extracted δ¹³C values and elemental C:N ratios. Arithmetic models were evaluated for best fit, parsimony, and accuracy between mathematically normalised and chemically lipid-extracted δ¹³C values. Chemical lipid extraction increased silver carp δ¹³C values and decreased C:N ratio in muscle and whole fish, and increased δ¹⁵N values in whole fish but not fish muscle. While both linear and natural log models accurately estimated lipid extracted δ¹³C values, natural log models provided better fit and parsimony throughout a wide range of C:N ratios. These results confirmed the need to account for lipid effects on δ¹³C values in silver carp. Moreover, our study will allow researchers to conduct isotopic analysis without the added time and cost of chemical lipid extraction and facilitate the comparison of silver carp muscle and whole fish isotopic values across studies.