δ13C and δ15N stable isotopes values were measured along the whiskers of eight harbour seals (Phoca vitulina) and ten grey seals (Halichoerus grypus) captured in the baie de Somme, France, in 2008 and 2012, respectively, to be fitted with FastlocTM GPS/GSM tags (cf GPS data available on http://seamap.env.duke.edu/dataset/2030 for harbour seals and on http://seamap.env.duke.edu/dataset/2026 for grey seals seals). The foraging areas posterior to seal captures are available for all harbour seals and seven on ten grey seals (Planque et al. 2020, https://doi.org/10.1007/s00227-019-3636-8). Seal whiskers provided a temporal integration of isotope compositions during the period of whisker growth, i.e. during months prior to sampling. All whisker segments were analysed with an elemental analyser (Flash 2000, Thermo Scientific, Milan, Italy) coupled to an isotope-ratio mass spectrometer (Delta V Plus with a Conflo IV interface, Thermo Scientific, Bremen, Germany). Isotope compositions are expressed with the usual δ notation in parts per thousands (‰) relative to Vienna PeeDee Belemnite Standard for δ13C and atmospheric N2 for δ15N. Based on replicate measurements of internal laboratory standards, analytical precision is < 0.15 ‰ for both δ13C and δ15N.