Update on natural mortality estimators for chub mackerel in the Northwest Pacific Ocean

The Technical Working Group (TWG) on Chub Mackerel Stock Assessment (CMSA) has determined to update the calculation of natural mortality coefficients (M) because age-specific M at age 0 was not available in the previous document (NPFC-2019-TWG CMSA02-WP01). Since a recent paper estimated life-history parameters in relation to the individual growth by adding new data to NPFC-2019-TWG CMSA02-WP01, we here updated both age-common and age-specific M values obtained from life-history parameters. The growth coefficient (K) and asymptotic fork length (L∞) were estimated to be higher and lower, respectively, than those in the previous estimates because the newly added data had smaller fork lengths for five years of age and older than the previous ones. The updated M values obtained from equations such as Pauly and Jensen were necessarily higher than those in the previous document because of the higher growth coefficient; accordingly, the median value of M among various estimators changed from 0.41 to 0.53. We also estimated age-specific M including age 0 using Gislason estimators. We suggest application of the median of updated estimators and the updated Gislason estimates as the age-common M and as the age-specific M, respectively, for rerunning candidate stock assessment models under the scenarios of operating models in CMSA.