Fisheries biology of Nemipterus bathybius and its optimum catchable size in northern South China Sea
南海北部深水金线鱼生物学及最适开捕体长

Based on the investigation data in 1997-1999 and 1964-1965, this paper studied the population structure, stock density distribution and its seasonal variation, growth and mortality, as well as the feeding behaviour of Nemipterus bathybius (Snyber) in northern South China Sea. The results indicated that the body length of the samples was 4.2-21.5 cm, and body weight was 2.9-241 g, with the length of 9.6-14.0 cm and weight of 27 -59 g as predominant. The stock density was the highest (9.92 kg km(-2)) in spring and the lowest (5.53 kg x km(-2)) in summer, and had an obvious zonal distribution. Catch rate increased with the water depth in 60-150 m water zone. Macrura, fishes and cephalopods were the main foods of Nemipterus bathybius. The growth parameters of von Bertalanffy formula estimated with ELEFAN I in FiSAT II package were Linfinity = 22.39 cm, k = 0.44 a(-1), and t0 = -0.63 a. The natural mortality calculated with Pauly's empirical equation was M = 0.94, and the total mortality estimated by length-converted catch curve was Z = 2.65. Assessment with Beverton-Holt model indicated that the optimum catchable age and body length were 1.1 a and 12 cm, respectively. The stock was over-exploited, mainly due to the capture of under-size juveniles. It was suggested that the legal body length of Nemipterus bathybiu at its first capture in northern South China Sea should be 12 cm.