Polar compounds preclude mathematical lipid correction of carbon stable isotopes in deep-water sharks

Lipids affect stable isotope values generated for marine fishes, however these effects remain poorly described for many extant shark taxa, especially deep-sea species. Here, we report the effects of lipid extraction (LE) on δ¹³C, δ¹⁵N, and C:N values of seven deep-sea sharks, generate novel mathematical normalizations for δ¹³C based on the relationship between bulk and lipid extracted values (δ¹³C_Bulk and δ¹³C_LE) and examine whether common normalized correction models provide a robust method for addressing lipid-biasing effects in two species, the Cuban dogfish (Squalus cubensis; n = 20), and Greenland shark (Somniosus microcephalus; n = 24). LE generally resulted in enrichment of ¹³C and ¹⁵N, but produced variable effects on C:N across all species. Novel mathematical normalizations for δ¹³C were derived from the pooled shark community, and a single species-specific correction was generated for the Cuban dogfish, but could not be determined for the Greenland shark. Four common lipid correction models used for teleosts, failed to accurately predict δ¹³C values statistically similar to δ¹³C_LE, in both Cuban dogfish and Greenland sharks, likely due to the confounding effects of lipids and urea on C:N. These observations suggest that chemical lipid extraction should be a mandatory procedure prior to interpreting stable isotope data for deep-sea sharks, at least for those species where lipid effects are large.