Population structure of Trichiurus japonicus in northern South China Sea and parameters of its growth, mortality and maturity

Based on the data of bottom trawl fishery survey in northern South China Sea from 2014 to 2015, we investigated the catch rate, population structure, sex ratio, gonadal development, growth and death parameters of Trichiurus japonicus. Results show that: 1) the anal length frequency distribution of hairtail during 2014−2015 was unimodal, and the dominant anal length group was of 160−190 mm. There was no significant difference in the anal length composition between male and female (P>0.05). 2) The proportion of female in spring was relatively higher, the and the ratios of female to male in four seasons were significantly different from 1∶1 (P<0.05). Their gonadal maturity was mainly at Level 2, accounting for 74.2% of the total. The minimum maturity length was 113 mm, and the estimated anal length at 50% sexual maturity (L₅₀) was 241.5 mm. 3) The estimated asymptotic anal length (L_∞) and growth parameter (K) calculated by Electronic Length Frequency Analysis I (ELEFANⅠ) were 585 mm and 0.20 a⁻¹, respectively. The estimated total mortality coefficient (Z) and natural mortality coefficient (M) were 1.172 and 0.475, respectively. Although the exploitation rate of 0.59 indicates that although T. japonicus resources are still in a state of overfishing, the average anal length of the individuals shows signs of recovery.