This study focused on the species identification, reproduction biology, age and growth analysis of Trichiurus spp. that were collected from 6 different fishing ports in southwestern Taiwan. Trichiurus japonicus is the dominant species and accounted for 72.7% of the total of 8376 samples. Trichiurus nanhaiensis accounted for 21.4%, and Trichiurus lepturus accounted for 3.3%. The other 2.6% of samples could not be identified by appearance. Although Trichiurus nanhaiensis only accounted for 21.4% of the Trichiurus spp. samples, Trichiurus nanhaiensis was more abundant than Trichiurus japonicus off the shallower fishing grounds near the Mituo and Haikou fishing ports. This finding suggests that Trichiurus nanhaiensis inhabits more shallow and more coastal waters than Trichiurus japonicus. L50 values of 248.6 mm and 290.1 mm, respectively, for T. japonicus and T. nanhaiensis individuals with ages <2 yr were determined based on otolith image analysis, and indicated that both species attain maturity within two years after hatching.
The VDR and logbook data suggested that there was a seasonal variation in the abundance of Trichiurus spp., with the species being more abundant from Jul.-Sep. and Oct.-Dec. in 2017. The fishing grounds changed with the seasons, and were mainly distributed at 50-100 m away from Qieding, 100-200 m away from Kaohsiung fishing port 1 and Donggang, and south of Liuqiu Island. We conducted GLMM analysis of resources for the period of 1997-2017 and included factors such as year, season, region, and horsepower of fishing vessels. The results showed a decline in resources in recent years. Geographically, the area with the lowest amount of resources was area D, which is located deeper in the South. In terms of seasonal change, the second quarter exhibited the lowest amount of resources. The results of this study can serve as scientific evidence for the policy-making and fishery management of Trichiurus spp. fishes.