Stock Assessment of Chub Mackerel (<i>Scomber japonicus</i>) in the Northwest Pacific Using a Multi-Model Approach

Chub mackerel (<i>Scomber japonicus</i>) is a major targeted species in the Northwest Pacific Ocean, fished by China, Japan, and Russia, and predominantly captured with purse seine fishing gear. A formal stock assessment of Chub mackerel in the region has yet to be implemented by the managing authority, that is, the North Pacific Fisheries Commission (NPFC). This study aims to provide a wider choice of potential models for the stock assessment of Chub mackerel in the Northwest Pacific using available data provided by members of the NPFC. The five models tested in the present study are CMSY, BSM, SPiCT, JABBA, and JABBA-Select. Furthermore, the influence of different data types and input parameters on the performance of the different models used was evaluated. These effects for each model are catch time series for CMSY, catch time series and prior of the relative biomass for BSM, prior information for SPiCT, and selectivity coefficients for JABBA-Select. Catch and CPUE (catch per unit effort) data used are derived from NPFC, while some life history information is referred from other references. The results indicate that Chub mackerel stock might be slightly overfished, as indicated by CMSY (<i>B</i>₂₀₂₀/<i>B</i>_MSY = 0.98, <i>F</i>₂₀₂₀/<i>F</i>_MSY = 1.12), BSM (<i>B</i>₂₀₂₀/<i>B</i>_MSY = 0.97, <i>F</i>₂₀₂₀/<i>F</i>_MSY = 1.21), and the base case run for the JABBA-Select (<i>SB</i>₂₀₂₀/<i>SB</i>_MSY = 0.99, <i>H</i>₂₀₂₀/<i>H</i>_MSY = 0.99) models. The results of the models SPiCT (<i>B</i>₂₀₂₀/<i>B</i>_MSY = 2.30, <i>F</i>₂₀₂₀/<i>F</i>_MSY = 0.31) and JABBA (<i>B</i>₂₀₂₀/<i>B</i>_MSY = 1.40, <i>F</i>₂₀₂₀/<i>F</i>_MSY = 0.62) showed that the state of this stock may be healthy. Changes in the catch time series did not affect CMSY results but did affect BSM. The present study confirms that prior information for BSM and SPiCT models is very important in order to obtain reliable results on the stock status. The results of JABBA-Select showed that different selectivity coefficients can affect the stock status of a species, as observed in the present study. Based on the optimistic stock status indicated by the best model, JABBA, a higher catch is allowable, but further projection is required for specific catch limit setting. Results suggested that, as a precautionary measure, management would be directed towards maintaining or slightly reducing the fishing effort for the sustainable harvest of this fish stock, while laying more emphasis on accurately estimating prior input parameters for use in assessment models.