Brood discrimination, maturation, and population structure of the hairtail,Trichiurus Haumela (Pisces, Trichiuridae) in northern East China Sea

Based upon analysis of the seasonal fluctuation of GSI (gonado-somatic index), the spawning period of the hairtail extends from April to October. The maximum time difference between the early and the late brood individuals is about half a year. It is possible to distinguish the early brood and the late brood on the basis of the diameter of the first ring, setting 1.32 mm as a border line. Therefore, the identification of the first ring was solved in dispute. The maturation stages can be identified on the basis of the macroscopical and microscopical characteristics of the ovary; i.e., immature (II-A, B), maturing (III-C, D, E, IV-F), mature (IV-G, V-H, VI-I) and recovering (VII-J). Maturation is completed in the course of one year from different brood seasons. The smallest size at first maturity in female was about 170–180 mm in length and 80–100 grams in weight. There were two discrete groups in autumn which shown the same population unit in which those individuals may separate temporarily into different groups of biological activities in certain period of their life. Fishing of hairtail has been greatly intensified, the population structure has been subject to a series of change. But the hairtail has relatively strong adaptability, so that its population can maintain a reasonable size despite the heavy fishing.