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Rationale: Stable isotope analysis of growth layers in sperm whale teeth dentine can provide valuable insight into individual long‐distance displacements and diet. Although treating teeth half‐sections with formic acid and rubbing their surface with a graphite pencil improves growth layer visibility and reduces sampling error, previous studies mostly used untreated half‐sections as the effect that this treatment may have on stable isotope ratios in dentine is unknown. The present study investigates the treatment effect on stable C and N isotope ratios in sperm whale teeth dentine. Methods: In the teeth of 30 sperm whales, we analysed and compared samples of powdered dentine obtained from (a) untreated half‐sections, (b) half‐sections etched with formic acid and rubbed with a graphite pencil and (c) half‐sections etched with formic acid from which the graphite pencil rubbing had been cleansed off. δ13$$ {\delta}^{13} $$C and δ15$$ {\delta}^{15} $$N values were compared between the three sample groups. Results: We found significant differences in values of both elements between untreated and etched samples, with a mean increase of 0.2% in δ13C and δ15N values in the etched samples. No significant differences were found between etched samples with graphite rubbing versus those without it. Significant linear regression models were calculated to predict untreated δ13C and δ15N values from the values obtained in the etched half‐sections with limited precision. Conclusions: We show for the first time that formic acid etching has a clear effect on δ13$$ {\delta}^{13} $$C and δ15$$ {\delta}^{15} $$N values in sperm whale teeth dentine. The developed models permit the estimation of untreated values from etched half‐sections, thus enabling the use of the latter in stable isotope analysis. However, as treatment procedures may vary between studies, it is advisable that similar predictive models are developed case‐by‐case to ensure comparability of results. [ABSTRACT FROM AUTHOR]

Werth et al. (Werth et al. Polar Biol 43:707–723, 2020) propose that in all mysticete whales the growth rate of baleen varies with the position they occupy in the mouth. If this were true, comparison between individuals and studies of results of chemical tracers in baleen would be compromised. Here we compile results from previous research conducted in balaenopterids using stable isotope ratios as tracers as well as images of baleen racks showing growth ridges and conclude that, at least in the Balaenopteridae family, the growth rate of baleen is constant along the whole baleen rack irrespectively of the position in the mouth, thus ensuring reliability of comparisons. [ABSTRACT FROM AUTHOR]

Highly migratory marine species pose a challenge for the identification of management units due to the absence of clear oceanographic barriers. The population structure of North Atlantic fin whales has been investigated since the start of whaling operations but is still the subject of an ongoing scientific debate. Here we measured stable isotopes of carbon, nitrogen and oxygen in skin samples collected from 151 individuals from western Iceland, Galicia (NW Spain), the Azores archipelago and the Strait of Gibraltar (SoG). We found spatiotemporal differences in stable isotope ratios suggesting that fin whales sampled in these four areas may share a common feeding ground within the Northeast Atlantic at different times during the year. Our results also suggest that SoG whales use this common feeding ground in summer but exploit Mediterranean resources during the winter months, further supporting the existence of a limited but current exchange of individuals between these two basins. • δ 13C, δ 15N and δ 18O values were measured in fin whale skin from four North Atlantic areas. • Spatiotemporal differences were found in the stable isotope profiles. • All whales may share a common Atlantic feeding ground at different times during the year. • Gibraltar whales use this common feeding ground in summer but the Mediterranean in winter. [ABSTRACT FROM AUTHOR]