δ13C and δ15N stable isotope compositions of the whisker of 24 harbour seals (Phoca vitulina) and 12 grey seals (Halichoerus grypus) captured in the baie de Somme, France, in 2019 and in 2022, for telemetry tracking

δ13C and δ15N stable isotopes values were measured along the whiskers of 24 harbour seals (Phoca vitulina) and 12 grey seals (Halichoerus grypus) captured in the baie de Somme, France, to be fitted with GPS/GSM tags (15 harbour seals and 12 grey seals captured in May and October 2019 ; telemetry tracking data of these individuals are available on https://doi.org/10.17882/89715) and with DTags (9 harbour seals captured in May 2022). The present data are complementary to those obtained previously on 8 harbour seals and 10 grey seals captured in 2008 and in 2012 in the baie de Somme (data available on https://doi.org/10.17882/76528 ; data published as part of Planque et al. (2021), https://doi.org/10.1002/ece3.7739, and Planque (2021), http://dx.doi.org/10.13140/RG.2.2.35986.12480). 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.