Simple simulations for determining the way for aging and defining fishing year in the stock assessment of chub mackerel

In this document, simple simulations were conducted to explore the appropriate method for defining fishing year and timing for aging (timing to add one year to one’s age) in chub mackerel stock assessment. Using the biological parameters and fisheries scenarios similar to those of chub mackerel in the Pacific, I observe how and to what extent potential biases can occur in the abundance estimation when different fisheries use different definitions of fishing year and timing for aging in creating annual catch-at-age data. Based on the results, I recommend that a consistent definition of fishing year and timing for aging be used across fisheries to avoid potential biases in abundance estimation. In addition, because average annual weight at age should differ among fisheries due to different fishing seasons even though the weight-age relationships of the population are consistent, the single weight-at-age representing the population when evaluating total abundance should be used while fishery-specific weight-at-age could be used in calculating total catch weight by fishery.