Analysis of identification method for gonadal maturity and MAA calculation methods on Chub mackerel in China

In order to maintain consistency in the standards of gonadal maturity, both China and Japan use gonadal development stage IV as the mature individual for chub mackerel by visual inspection method. At the same time, age identification is conducted for each sample. Both 0-year-old and 1-year-old individuals are immature, and 2-year-old individuals are also almost immature, with only very few mature individual during studying samples. Individuals aged 4 years or older are considered mature individuals. Three years old fish is the age that marks the boundary between mature and immature gonadal development. The maturity fluctuation range of a 3-year-old individual with the gonad stage is 0.15-0.3. Considering that visual methods may have some errors in determining the developmental level of ovarian fish eggs, the GSI index is used for correction and reference. As defined GSI greater than 1.6 as matured individuals based on the result of NPFC-2025-TWG CMSA11-WP10. The range of sexual maturity ratio for 3-year-old individuals with GSI≥1.6 is 0.03-0.375 (mean=0.12), and 2-year-old individuals also have an average ratio of 0.057. If we consider including 2-year-old sexually mature individuals as 3 years old, the proportion of sexual maturity at 3 years old reaches 0.19, but this is still lower than the research results of Japanese colleagues (0.3). KG=3 as a criterion (represents stricter standards than GSI) for determining sexual maturity is not suitable for the Chub Mackerel population in the open sea.

Overall, it indicates that the gonadal development of chub mackerel in the open sea area is relatively slow, which may be due to the fact that this area is not a spawning ground. We adjusted the MMA table based on the threshold of gonadal index, as shown in Table 5. Considering the criteria for mature eggs and the comprehensive factors of GSI, we suggest using the simplify the average value MMA values in Table 5 to be closer to the true values. Therefore, the preliminary conclusion is that the maturity of chub mackerel in the CA sea area should generally be lower than that of individuals near Japanese EEZ area.