Preliminary Indian Ocean Swordfish Stock Assessment 1950-2018 (Stock Synthesis)

This report presents a preliminary stock assessment for Indian Ocean swordfish (Xiphias gladius) using Stock Synthesis 3 (SS3). The assessment uses a spatially disaggregated, sex explicit, and age structured model that integrates several sources of fisheries and biological data. The assessment model covers the period 1950–2018 and represents an update and revision of the 2017 assessment model with the inclusion of updated longline CPUE indices, and a revised fleet structure. A range of sensitivity models are also presented to explore the impact of key data sets and model assumptions. 

The assessment assumed the Indian Ocean swordfish constitute a single spawning stock partitioned into 4 spatial areas (NW, NE, SW, and SE), to account for differential abundance and depletion levels among regions. The assessment model defined 15 fisheries, based on fleet and region. Standardised CPUE series (as relative abundance indices) are available from the Japanese, Taiwanese，Portuguese, Spanish, South African, and Indonesia longline fleets. The final model options focused pm the Japanese (for 4 regions), Portuguese (SW), and South African (SW) indices, assuming that they are proportional to the regional abundance of swordfish. The utility of CPUE indices from other fleets were assessed in sensitivity models. Length compositions data are available for 14 fisheries. Separate selectivities were estimated for each fleet but were assumed to be common for all regions.  

The final assessment model options correspond to a combination of model configurations, including alternative assumptions on growth (otolith or spine based age estimates), alternative values of SRR steepness (0.7, 0.8, or 0.9), recruitment variability σ_R (0.2 or 0.4), and the alternative sample size of length composition data (20 or 5). The model ensemble (a total of 24 models) encompass a 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 estimates do not differ substantially from the previous assessment. Fishing mortality rates have remained relatively stable in recent years and are well below the F_MSY. Biomass was estimated to have been increasing since 2010. Spawning stock biomass in 2018 was estimated to be 42% of the unfished levels and 170% of the level that can support MSY (SSB₂₀₁₈/SSB_MSY = 1.75).  With high likelihood, current fishing mortality was estimated to be lower than F_MSY (F₂₀₁₈/F_MSY = 0.6). The probability of the stock being currently in the green Kobe quadrant is estimated to over 95%. Considering the quantified uncertainty, the stock is considered not to be overfished and is not subject to overfishing in 2018. The retrospective analysis provided some confidence on the robustness of the model with respect to recent data, yet the uncertainly on levels of most recent recruitment may undermine the predictive capabilities of the model. Annual catches since the 1990s have been within the range of the estimated MSY and the current catch is projected to be sustainable over the longer term. the estimated stock status is summarized as below: