Otolith stable isotopes highlight the importance of local nursery areas as the origin of recruits to yellowfin tuna (Thunnus albacares) fisheries in the western Indian Ocean

Yellowfin tuna (Thunnus albacares) supports the second largest tuna fishery worldwide, and in the Indian Ocean, it is overfished and subject to overfishing. This situation presents a significant challenge to fisheries management, requiring effective measures to rebuild and then maintain the stock at sustainable levels. A single stock of yellowfin is currently assumed by the Indian Ocean Tuna Commission (IOTC) for stock assessments in the Indian Ocean. However, the relative contribution of different spawning components to the total catches, and the degree of mixing rates of yellowfin tuna in the Indian Ocean by individuals from different production zones, are still unknown. This study uses otolith core oxygen and carbon stable isotope composition (δ¹⁸O and δ¹³C) of young-of-the-year yellowfin tuna from nursery areas located in the western (FAO Area 51) and eastern (FAO area 57) Indian Ocean, either side of 80 ºE, to establish a reference baseline of isotopic signatures. Then, a mixed population program (HISEA) and Random Forest (RF) assignment approaches were used to predict the most likely origin (west or east) of sub-adult and adult yellowfin tuna captured from four fishery areas of the western Indian Ocean (offshore Pakistan, Seychelles, Reunion, and South Africa) by comparing their otolith core values to that of the baseline. Both approaches show that the western Indian Ocean fisheries are mainly composed of west origin fish (> 95 %). We also found some individuals with an otolith isotopic signature that was not characteristic of either of the samples available in the baseline. We simulated an alternative baseline group formed by individuals with mean and standard deviation δ¹³C and δ¹⁸O values above the maximum ranges of the original baseline. We then used RF to infer again the most likely origin of fish in the mixed sample considering 3 possible sources (west, east, alternative). About one third of the samples were assigned to the alternative group, possibly indicating that they differ in geographical or temporal terms with the origins represented in the original baseline. Findings of otolith stable isotope composition of yellowfin tuna in the western Indian Ocean can provide a more comprehensive understanding of the species’ spatial structure and connectivity beyond the current assumption of an ocean basin single stock.