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Discrimination of yellowfin tuna (Thunnus albacares) from nursery areas in the Indian Ocean using otolith chemistry

Iraide Artetxe-Arrate*, Igaratza Fraile, Naomi Clear, Audrey M. Darnaude, David L. Dettman, Christophe P├ęcheyran, Jessica Farley, Hilario Murua

*Corresponding author:

ABSTRACT: Yellowfin tuna (Thunnus albacares) is a highly exploited species in the Indian Ocean. Yet, their stock structure is still not well understood, hindering to assess the stock at a suitable spatial scale for management. Here, young-of-the-year (<4 months) yellowfin tuna otoliths were collected in 2018 and 2019, from four major nursery areas in the Indian Ocean: Madagascar, Seychelles-Somalia, Maldives, and Sumatra. First, direct age estimates were made in a subset of otoliths by visually counting microincrements to identify the portion of the otolith corresponding to the larval stage. Then, we developed two dimensional maps of trace element concentrations to examine spatial distribution of elements across otolith transverse sections. Different distribution patterns were observed among the elements analysed; Li, Sr and Ba were enriched in the portion of the otolith representing early life, whereas Mn and Mg concentrations were heterogeneous across growth bands. Last, we analysed inter-annual and regional variation in otolith chemical composition using both trace elements (Li, Mg, Sr, Ba and Mn) and stable isotopes (δ13C and δ18O). Significant regional variation in otolith chemical signatures was detected among nurseries, except for between Madagascar and Seychelles-Somalia. Otolith δ13C and δ18O were important drivers of differentiation between western (Madagascar and Seychelles-Somalia), Maldives and Sumatra nurseries, whereas the elemental signatures were cohort specific. Overall nursery assignment accuracies were 69-71%. The present study demonstrates that baseline chemical signatures in the otoliths of yellowfin tuna are distinct and can be used as a natural tag to investigate the nursery origin of older individuals in the Indian Ocean.