Yellowfin stock structure study in the Western Indian Ocean using trace element analysis in otoliths

Supervisors: Igaratza Fraile, Hilario Murua (AZTI)
The assessment of the yellowfin tuna (Thunnus albacares) population in the Indian Ocean is based on the assumption that it constitutes a single stock. However, there is no sufficient information to confirm accurately this hypothesis and, therefore, the population structure and spatial dynamics could be more complex. By analyzing chemical composition in the otolith signatures of yellowfin tuna collected over a latitudinal gradient of the western Indian Ocean, we aim to investigate the population structure and connectivity between tuna captured in 3 different regions (Mozambique, Seychelles and Somalia) and the spatial dynamics of potential subpopulations. For that, trace element composition (43Ca, 7Li, 25Mg, 55Mn, 57Fe, 59Co, 60Ni, 63Cu, 66Zn, 88Sr and 138Ba) of yellowfin tuna otoliths (N=125) was measured using laser-ablation-inductively-coupled-mass-spectometry (LAICPMS). Otolith material was ablated from the core (signature of the water mass where fish originated) to the edge (signature of the water mass where fish were collected). Significant differences in otolith core and near-core signatures were found in the case of Mg:Ca and Fe:Ca ratios (P<0.05), and these differences were mainly between fish captured in Mozambique and Seychelles while fish collected in Somalia showed intermediate values. No differences were found in the elemental concentrations of the edge. Additionally, PCA applied to the time series of Mg:Ca and Ba:Ca ratios during the first 136 life days of the fish show to differ also among these two regions (MANOVA= 0.045 and 0.035 respectively). These differentiations suggest the occurrence of two subpopulations and migratory contingents in the yellowfin tuna of the western Indian Ocean. Result obtained also highlighted the high mixing potential of the population. Expanding the present analysis across multiple regions and integrating data from several sources (e.g. genetic studies, tagging data) would help to clarify the population structure and movements of yellowfin tuna in the Indian Ocean.