2023 updates on Pacific saury stock assessment in the North Pacific Ocean using Bayesian state-space production models
Stock assessment for the North Pacific saury was updated based on the specification (2 base cases and 2 sensitivity cases) agreed in the 9th SSC-PS meeting. The basic model employed in the analysis was the state-space surplus production model as has been used since the SSC-PS01 as an interim stock assessment model. The model can account for the process errors in the dynamics 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 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 2023 median depletion level was only 21.0% (80%CI=10.7-34.8%) of the carrying capacity. Furthermore, B-ratio (=B/Bmsy) and F-ratio (=F/Fmsy) in 2022 were 0.337 (80%CI=0.229-0.474) and 0.799 (80%CI=0.517-1.384), respectively. For those three years average values, B-ratio over 2021-2023 and F-ratio over 2020-2022 were respectively 0.336 (80%CI=0.206-0.505) and 1.106 (80%CI=0.750-1.701). In addition, the probability of the stock being in the green Kobe quadrant in 2022 was estimated to be nearly 0%, while the probabilities of being in the yellow and red Kobe quadrants were assessed as 72% and 28%, respectively. On the weight-of-evidence available now, the current Pacific saury stock is determined to be overfished. Note that there is a large difference in the biomass series between the two base cases, while there is little difference in relative quantities such as the B- and F-ratios and depletion level.
Based on the updated results, if we apply the same formula used in TAC calculation in the 2019 Commission meeting, it would be Fmsy*B2023=183,000 (tons). However, considering the current overfished population level and applying a simple discount exploitation rate depending on the current B-ratio, an appropriate catch would be (B2023/Bmsy)*Fmsy*B2023=80,000 (tons).