Dorsal fin spines as a non-invasive alternative calcified structure for microelemental studies in Atlantic bluefin tuna

• We explore the spatial and temporal variability of tracers in bluefin tuna spines using LA-ICPMS.
• The chemical signature of certain tracers is preserved stable into the spines.
• Spines can be used as an alternative structure to otoliths for chemistry studies.

Chemical signatures in calcified structures of fishes represent natural tags of chemical and physical characteristics of the environment. The suitability of dorsal spines of Atlantic bluefin tuna (Thunnus thynnus) was investigated as an alternative non-invasive structure to otoliths. The spatial and temporal variability of trace elements was examined, for the first time, throughout the annual growth increments (i.e. translucent and opaque bands) of Bluefin tuna spines from the Bay of Biscay. Four scanning-ablation line transects were drawn and examined using LA-ICPMS to test for differences in tracer concentrations for each spine sections. Firstly, results confirm the questionable spatial stability of tracers in dorsal fin spines. Secondly, most of the elements analyzed 88Sr, 137Ba, 24Mg, 55Mn, 7Li (biological essential elements), 66Zn and 65Cu (priority pollutants) were consistenly found above the limit of detection (LOD). Strontium and Barium showed a similar pattern throughout annuli with concentrations significantly higher in the translucent bands in the second annulus (i.e. second winter). The concentration of Magnesium displayed an increasing pattern with annulus with no differences between translucent and opaque bands. In contrast, the concentration of Manganese showed a decreasing pattern throughout annuli, with concentrations significantly higher in the opaque bands (i.e. summer bands) regardless of annulus. Lithium, Copper and Zinc showed unclear pattern, although the concentration of Zinc seems to be in sync with the deposition of annuli currently interpreted in the ageing process. Findings indicate that the chemical signature of certain biologically essential elements is preserved stable in the dorsal fin spines strengthening its use as a non-invasive alternative structure for chemistry studies in Atlantic bluefin tuna.