Biological reference points and future projections in the 2025 stock assessment for the Northwestern Pacific chub mackerel

We conducted stochastic future projections with candidate base-case stock assessment results to provide scientific information that could be used to draft management advice on the Northwestern Pacific stock of chub mackerel (including the NPFC convention area). The estimated spawning stock biomass at maximum sustainable yield (SSB_MSY) was highly sensitive to input data and model configurations, estimated values were consistently higher than the current stock abundance. We propose using the first to third quartiles of historical SSB as interim reference points which might be used for short- or long-term target and limit, and with future probabilities calculated over these empirical reference points as well as the MSY-based ones. Stochastic future projections showed that, under constant-catch scenarios, unless the annual total catch is kept below 60,000–70,000 tons, there is less than a 90% probability of maintaining SSB above the first quartile, and less than 60% probability of reaching the median five years later (in the 2031 fishing year). Under constant-F scenarios, fishing pressure must be F50%SPR (correspond to about 70,000 tons catch in the 2026 fishing year) or lower to achieve the median SSB with a probability exceeding 50% after five years. Considering the projection results that the stock continues to decline under the current fishing pressure (16-17% SPR) as long as the body weight and maturity rate are kept at a current low level, it is necessary to substantially reduce fishing pressure in order to avoid further decline and facilitate stock recovery. We also argue that development of harvest control rule and target and limit reference points are urgently needed for the long-term sustainable management of this stock.