Updates of stock assessment for Pacific saury in the North Pacific Ocean based on indices up to 2019 by using Bayesian state-space production models

Stock assessment for the North Pacific saury was updated based on the specification (2 base cases and 4 sensitivity cases) agreed in the 5th SSC-PS meeting held in November 2019 with additionally available 2019 fishery-dependent indices endorsed in the SSC-PS-06 meeting. The basic model employed in the analysis was the state-space surplus production model as agreed in the SSC-PS01 as an interim stock assessment model. The model can account for process and observation errors in the abundance indices.  Parameters in the models were estimated based on Bayesian framework with a Markov chain Monte Carlo method. The estimation results were diagnosed with respect to shapes of posterior distributions, residual plots, retrospective pattern and predictability of the future population status. The outcomes of stock status and future projection were shown according to the template agreed in the 5th SSC-PS meeting with some modifications to accommodate the data period. 
As for the combined base case stock assessment result, the 2019 median depletion level was only 20.1% (80%CI=12.9-27.5%) of the carrying capacity, declined from 30.7% (80%CI=20.0-42.0%) in 2018. Furthermore, B-ratio (=B/Bmsy) and F-ratio (=F/Fmsy) in 2019 were 0.437 (80%CI=0.294-0.608) and 1.067 (80%CI=0.754-1.514), respectively. For those three years average values between 2017 and 2019, B-ratio and F-ratio were respectively 0.503 (80%CI=0.349-0.677) and 1.428 (80%CI=1.048-1.948). In addition, the probability of the stock being in the green Kobe quadrant in 2019 was estimated to be nearly 0% , while the probability of being in the red Kobe quadrant was assessed to be greater than 60%. On the weight-of-evidence available now, the current Pacific saury stock is determined to be overfished and subject to overfishing. 
For population outlook, population dynamics were projected for some scenarios with respect to several levels of reduction/increase of catch as well as status quo. The results showed that continuation of the current level of catch may cause a severe decline in the population size.