From the pool to the sea: Applicable isotope turnover rates and diet to skin discrimination factors for bottlenose dolphins (Tursiops truncatus)

• Isotopic discrimination factors were 1.01 ± 0.37‰ for δ13C and 1.57 ± 0.52‰ for δ15N.
• Carbon half-life turnover rate was estimated to be 24.16 ± 8.19 days.
• Nitrogen half-life turnover rate was estimated to be 47.63 ± 19 days.
• Applicable values are available to assess the diet of free ranging small cetaceans.

One of the most common applications in isotopic ecology is the assessment of animal's assimilated diet through mass-balance mixing models. Its applicability relies on the use of accurate diet to tissue discrimination factors and turnover rates, which are known to vary as a function of several factors including taxon or tissue type. To date, few studies have assessed isotopic discrimination factors and turnover rates in cetacean species under controlled conditions. Previous experimental studies focused on blood, a difficult sample to obtain in the wild, or on a more appropriate tissue, the skin, but assessed in short experimental trials without arriving to the isotopic equilibrium. We carried out the longest controlled feeding experiment available (350 days) in bottlenose dolphins (Tursiops truncatus) in order to assess discrimination factors and turnover rates in skin. Animals' isotopic composition was first stabilized by maintaining individuals under an isotopically constant diet during 172 days. Afterwards, diet was shifted and maintained during 178 days to calculate isotopic discrimination and turnover rates. Estimates for isotopic discrimination factors were 1.01 ± 0.37‰ (mean ± sd) for δ13C and 1.57 ± 0.52‰ for δ15N. Half-life turnover rates were estimated to be 24.16 ± 8.19 days for carbon and 47.63 ± 19 days for nitrogen. This is the first time that applicable values are available to assess the diet of free ranging small cetaceans through stable isotope mixing model analysis.