Investigating natal origin and trans-oceanic migrations of Albacore tuna (Thunnus alalunga) from the West Indian Ocean using otolith chemistry

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Labonne, Maylis | Darnaude, Audrey | Petit, Cécile | Médieu, Anais | Pernak, Marianne | Nikolic, Natacha | Clear, Naomi | Eveson, J | Farley, Jessica | Marsac, Francis | Davies, Campbell

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International audience. Natal origin of 92 Albacore tuna (Thunnus alalunga) from the West Indian Ocean was investigated using trace elements in otoliths. Otoliths were collected from (i) 46 juveniles caught in 2 locations in South Africa (SA-North and SA-South) in 2018 and Tasmania (SWTS) in 2019, which are both known to be nurseries areas for the species, and (ii) 46 older fish caught in the Reunion island (SWI) during 3 sampling events in 2018. Otolithsconcentrations in seven chemical elements (B, Mg, P, Cu, Zn, Sr and Ba) were analysed by LA-ICP-MS at two points, near the core and at the edge, providing an elemental signal from the material deposited while the fish were close to their spawning grounds and from material deposited while they were in (or close) to their capture areas, respectively. Core and edge signatures for the same otolith were significantly different for most elements.At the edge, PCA identified different signatures (in Sr, Ba, and B) between the 3 main capture areas. Multi-elemental signatures for the Reunion samples were stable over the 3 sampling events. Those for South Africa samples split into 2 groups, the group of juveniles sampled in the Atlantic (SA- North) exhibiting signatures in B statistically different from those of all the juveniles from the Indian Ocean (SA-South and SWTS).The PCA on core signatures (from West Indian ocean and Atlantic ocean samples) identified some variation in spawning origin according to fish capture location. Based on the most relevant number of clusters, 2 putative spawning origins (SpO) with distinct chemical signatures in B and P were identified. All two SpO apparently contribute to the 3 sampling events investigated, but in varied proportions. Interestingly, although someoverlap was found between SA-North and SA-South signatures, the two juvenile sampling locations mostly had different spawning origin. SpO-2 was identified as the main spawning source for the albacore analysed (52%), principally in SA-S (76%), SWI-May18 (62%) and SWI-Dec18 (70%). SpO-1 apparently provides 47% of the fishsampled, principally in SA-N (69%) and SWI-Feb18 (85%). The samples collected do not allow to prove that these different spawning origins correspond to spatially discrete zones in the Indian or Atlantic Ocean. Nonetheless, these results bring evidence for trans-oceanic migrations in T. alalunga, which has strong implications for the management of its stocks

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