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This report presents a preliminary stock assessment for Indian Ocean yellowfin tuna (Thunnus albacares) using Stock Synthesis 3 (SS3). The assessment uses an age-structured and spatially-explicit population model and is fitted to catch rate indices, length-composition data, and tagging data. The assessment covers 1950 – 2020 and represents an update of the previous assessment model, taking into account progress and improvements made since the previous assessment. The assessment assumes that the Indian Ocean yellowfin tuna constitute a single spawning stock, modelled as spatially disaggregated four regions, with 21 fisheries. Standardised CPUE series from the main longline fleets 1975 – 2020 were included in the models as the relative abundance index of exploitable biomass in each region. The CPUE indices from EU Purse seine sets on free schools were included in a subset of models with the spatial and fleet structure revised to better accommodate the distribution and size structure of the purse seine fisheries. A new index based on associative dynamics of yellowfin tuna with floating objects and an additional index from the Maldivian pole and line fishery were also available, and the utility of these indices was examined in the assessment. Tag release and recovery data from the RTTP-IO program were included in the model to inform abundance, movement, and mortality rates.
A range of exploratory models is presented to address issues in observational datasets, improve the stability of the assessment model, and explore the effects of alternative model assumptions. The proposed final assessment model options correspond to a combination of model configurations, including alternative assumptions about the spatial structure (2 options), longline CPUE catchability (2 options on the effect of piracy), weighting of the tagging dataset (lambda =0.1 or 1), steepness values (0.7, 0.8, and 0.9), natural mortality values (2 options), and growth parameters (2 options). The model ensemble (a total of 96 models) encompasses a range of stock trajectories. Estimates of stock status were combined across from the 96 models and incorporated uncertainty estimates from individual models as well as across the model ensemble. Overall stock status estimates do not differ substantially from the previous assessment. Biomass is estimated to have been declining in recent years, and since the previous assessment. Spawning biomass in 2020 was estimated to be 78% of the level that supports the maximum sustainable yield (SB2020/ SBMSY = 0.78). Current fishing mortality is estimated to be 27% higher than FMSY (F2020/ F40%SB = 1.27). The probability of the stock being currently in the red Kobe quadrant is estimated to be 67%. The catches in the last five years have been higher than the estimated MSY. Considering the quantified uncertainty, the stock is considered be overfished and is subject to overfishing in 2020. The estimated stock status is summarized as below:
