Age and growth changes and population dynamics of the black pomfret (Parastromateus niger) and the frigate tuna (Auxis thazard thazard), in the Taiwan Strait
The age and growth and population dynamics of the black pomfret, Parastromateus niger and the frigate tuna, Auxis thazard thazard were studied from specimens collected monthly in the Taiwan Strait from April 2005 to March 2006. With five age groups of each, the dominant black pomfret was one year, while frigate tuna was two years. The total mortality coefficient (Z) and the fishing mortality (F) of the black pomfret were at high levels, the exploitation ratio (E) was over 0.5, and the number of captured juvenile fishes showed that the stock of black pomfret in this area was overexploited and improper fishing gear was utilized. Compared with previous studies, the maximum and mean fork length, body weight and age of the two fishes declined gradually, the populations were younger in age and smaller in size. The growth coefficient k increased and age at the inflexion point of weight t r was younger than before. The smaller change of the fork length at first maturity of the two fishes has indicated a more vulnerable fishery ecosystem in this area. Therefore, in addition to the traditional fishery management approach, such as taking the minimum length into consideration, ecosystem approach to fisheries management (EAF) should be taken to improve the practical fishing management, including conserving the small pelagic fishery population in order to achieve the sustainable fishing of the stocks.