Normalisation models for accounting for fat content in stable isotope measurements in salmonid muscle tissue

Stable isotope analysis is increasingly used in ecological studies. Because lipid content influences delta(13)C, lipids should be removed from lipid-rich samples before delta(13)C analysis. To account for differences in delta(13)C arising from differences in lipid content, relationships between lipid content, C:N ratio and Delta delta(13)C with lipid removal can be used to normalise lipid content to uniform levels. We investigate these relationships for salmonid muscle and evaluate the suitability of previously published normalisation equations for these fish. Salmonids with a wide range of condition (muscle lipid content = 3-35% of dry weight) were considered. There were no consistent relationships between lipid content or C:N ratio and Delta delta(13)N. There were linear relationships between C:N ratio and lipid content (L = -16.53 + 6.27 x C:N); C:N ratio and Delta delta(13)C (Delta delta(13)C = -1.87 + 0.65 x C:N); and lipid content and Delta delta(13)C (Delta delta(13)C = 0.01 + 0.10 x L), which should be used on salmonid stable isotope studies.