Effects of Lipid Extraction and Lipid Normalization on Stable Carbon and Nitrogen Isotope Ratios in Double-Crested Cormorants: Implications for Food Web Studies

Cormorants are desirable subjects for food web studies using stable isotopes (C and N) because of global fisheries conflicts, but no validated lipid-normalization procedures are currently available for any cormorant species. Accordingly, the effects of chloroform-methanol and petroleum-ether lipid extractions and three published lipid-normalization models on stable C and N isotope signatures in Double-crested Cormorant (Phalacrocorax auritus) muscle and liver tissues were investigated. The presence of lipids in cormorant muscle and liver decreased δ13C values by approximately 1–2‰, more so than has been reported in other birds. Cormorants showed large variation in the relationship between the C:N ratio of bulk tissue and the change in δ13C values after lipid extraction, violating a major assumption of published lipid-normalization models. Despite this violation, two of the three tested models performed reasonably well for correcting δ13C values. The circumstances under which these models might fail are unknown, so caution is warranted when applying them to new species. Petroleum-ether lipid extractions did not reduce the C:N ratio of tissue samples to those of pure proteins (4.0 or below; over half of the samples ranged from 4.38 to 5.27); thus, lipid extraction using chloroform-methanol is recommended to ensure the greatest accuracy of carbon isotope analyses of cormorant tissues.