Lipid effects on isotopic values in bottlenose dolphins (Tursiops truncatus) and their prey with implications for diet assessment

Abstract

Lipid content may affect δ13C and δ15N values, thus potentially leading to misinterpretation of isotopic compositions. Solutions to this problem may comprise a priori chemical extraction of lipids or a posteriori arithmetic corrections based on the use of the C:N ratio as a surrogate for lipid content (the so-called lipid normalization equations, LNE). We explored the suitability of LNE vs. chemical extractions to account for the effects of lipids in bulk samples of bottlenose dolphins and their prey, as well as their implications in mass-balance mixing model outputs. Chemical extraction of lipids only affects fish δ13C values, with greater isotopic differences between bulk and delipidated tissues than analytical errors. Based on a modeled isotopic scenario, we further demonstrated that the most accurate dietary reconstructions were those obtained when using both species-specific (<0.14% deviation from modeled diet) and general LNE for dietary endpoints (<6.5%). In contrast, deviations of up to 60% from the modeled diet were observed when considering the isotopic composition of the bulk diet, especially when the whole fish was used as a dietary endpoint. To reduce time, work load and economic costs, we recommend the use of species-specific LNE to normalize the isotopic composition of diet prior to dietary quantifications when feasible or a general LNE when dealing with generalist predators consuming a high number of prey species.