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This report presents a preliminary stock assessment for Indian Ocean bigeye tuna (Thunnus albacares) using Stock Synthesis 3 (SS3). The assessment uses a spatially structured, age-based model that integrates catch rate indices, length-compositions, and tagging data. The assessment model covers the period 1975–2021 and represents an update and revision of the 2019 assessment model, taking into account newly available information since the previous assessment. The assessment assumes that the Indian Ocean bigeye tuna constitute a single stock, modelled as spatially disaggregated four regions, with 12 fisheries. Standardised CPUE series from the main longline fleets 1975 – 2021 were included in the models as the relative abundance index of exploitable biomass in each region. Standardised indices of bigeye tuna caught by European purse seiners from sets on associated tuna schools (2010 – 2021) in the western tropical region were also included. Tag release and recovery data from the Indian Ocean regional tuna tagging program were included in the model to inform abundance, movement, and mortality rates
A range of sensitivity models is presented to explore the impact of key data sets and to assess the effects of alternative model assumptions, including the utility of CPUE and length compositions, data weighting, and spatial and temporal model structures. The proposed final assessment model options correspond to a combination of model configurations, including alternative assumptions about key biological parameters. The final models involved running a combination of options on selectivity configurations for the main longline fisheries in region 2 and 3 (2 scenario), steepness (3 values), growth (2 scenario), natural morality (2 levels). These models encompass a wide range of stock trajectories. Estimates of stock status were combined across from the 24 models and incorporated uncertainty estimates from both within and across the model ensemble.
The overall stock status estimate is more pessimistic than the previous assessment. Considering the quantified uncertainty, spawning stock biomass in 2021 was estimated to be 25% of the unfished levels and 90% of the level that can support MSY (SSB2021/SSBMSY = 0.90), current fishing mortality was estimated to be higher than FMSY (F2021/FMSY = 1.43). The probability of the stock being currently in the red Kobe quadrant is estimated to be 79%. Therefore, the stock is considered be overfished and is subject to overfishing in 2021. However, the recent catches have been lower than the estimated MSY and the spawning biomass has been increasing in the last three years. Considering the quantified uncertainty, the stock is considered be overfished and is subject to overfishing in 2021. The estimated stock status is summarized as below:
