Discrimination of yellowfin tuna from the putative nurseries of the Western Indian Ocean

Stable carbon and oxygen isotope ratios (δ13C and δ18O) were measured in otoliths of young of the year
yellowfin tuna (Thunnus albacares) collected from different nursery areas in the western Indian Ocean. Samples were obtained from February 2009 to May 2010 from three regions that include a variety of physical characteristics and habitat types: Somali waters in the northwestern Indian Ocean (0-10oN), surrounding waters of the Seychelles Islands (0-10oS) and northern Mozambique Channel (13-16oS). Somalia and Seychelles region did no show a significant difference in otolith isotopic signature and thus, fish collected in these regions were pooled together and compared with those collected in Mozambique Channel. Significant differences existed in δ18O values among the two nurseries, with more depleted values in fish collected in Mozambique Channel compared with those collected in Seychelles-Somalia region. Cross-validated classification success, based on quadratic discriminant analysis, was relatively high, with 70% of the fish correctly classified to their respective nursery areas. The ability to discriminate individuals from the putative nurseries in the western Indian Ocean allows estimating the degree of connectivity and natal homing of adult individuals. In this way, otolith δ13C and δ18O of young of the year yellowfin tuna were used as a baseline to predict the origin of juvenile and adolescent individuals captured in Somali waters. Our results reveal that about 80% of yellowfin tuna captured in Somalia region were originated from the same nursery, highlighting the importance of local production. We believed that these comparisons will provide useful information on which nursery areas are the most important for the yellowfin tuna fishery in the Indian Ocean.