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Variation in otolith elemental fingerprints was investigated in the swordfish Xiphias gladius to complement genetic data obtained by next generation sequencing in the framework of a collaborative project on population stock structure of tuna, billfish and sharks of the Indian Ocean (PSTBS-IO). Swordfish specimens for this work were sampled in the southwest (SWI), west central (WCI) and southeast (SEI) regions of the Indian Ocean. A total of 70 otoliths (30 from SWI and 20 from each WCI and SEI) were selected and the elemental signatures of their cores were analysed by LA-ICP-MS to investigate potential differences in spawning origin among regions. Among the 15 chemical elements analysed, only Mg, P, Sr, Ba and B were above detection limits and significantly contributed to the variation in otolith core composition. Based on differences in these five elements, three groups of distinct multi-elemental signatures, denoting potentially discrete spawning origins (SpO), were identified using hierarchical clustering based on Euclidian distances. All SpO identified apparently contributed to the swordfish stocks of the three regions sampled, but in different proportions. SpO-1 was the most common spawning source among the fish sampled (49%); it probably corresponds to the swordfish spawning ground located between northeast Australia and Indonesia. SpO-3 was found to provide 34% of the total fish analysed, but mainly in SWI (53%) and WCI (35%). Therefore, it could correspond to a potential spawning ground reported for the species in the western Indian Ocean. Lastly, SpO-2, which contributed to only 17% of the total fish analysed (mainly in SEI and WCI), may correspond to minor spawning grounds previously reported in the central and northeast Indian Ocean. The contrast in otolith core fingerprints between SWI and SEI otoliths suggests differences in main spawning origin, at least for the swordfish captured in these two regions of the Indian Ocean. However, our results also show some degree of mixing among stocks, which might prevent genetic differentiation among them. Additional analyses are needed to consolidate these results, as well as information on the spatiotemporal distribution of chemical tracers in the water masses of the Indian Ocean to assign regions to otolith elemental signatures.
