Biological reference points and future projections with the results of stock assessment for the Pacific chub mackerel
The stock assessment of chub mackerel in the Northwest Pacific was conducted by the Technical Working Group on Chub Mackerel Stock Assessment (TWG CMSA). This document introduces methods for calculating biological reference points and stochastic future projections and shows the results based on the parameters estimated in the stock assessment to provide scientific information that could be used to draft management advice on this stock. We also evaluated the historical spawning potential of this stock by calculating unexploited spawning biomass (SSB) per recruit (SPR0) based on annually changing biological parameters of weight and maturity. The calculated SPR0 shows that the spawning potential of this stock has been drastically changed and significantly decreased to half of the historical average during the most recent 7 years. The varying and decreasing SPR0 results in varying and decreasing estimates of unexploited spawning biomass (SSB0) and SSB achieving maximum sustainable yield (SSBMSY). In addition, because the stock-recruitment relationship estimated in the stock assessment model indicates very weak density dependence in the ranges historically observed with low steepness, the estimated SSB0 and SSBMSY were extreme extrapolations, which may have resulted in these estimates being somewhat uncertain and unrealistic. In light of these findings, stock status and management recommendations are summarized focusing on the points that 1) current SSB is below and fishing mortality rates are above the MSY reference points although the extent is uncertain, 2) future stochastic simulations assuming constant-catch shows future catch amount around 100,000 metric tons per year maintain the stock above the current spawning biomass (2022) with high probability (~90%), 3) development of harvest control rule and target and limit reference points are urgently needed for the long-term sustainable management of this stock.