Study On The Fishery Biology And Management Strategy Of Larimichthys Polyactis In The Southern Yellow Sea And The East China Sea

Since the 1950s, the small yellow croaker (Larimichthys polyactis) fishery in the southern Yellow Sea and the East China Sea has undergone four phases: prosperity (1950-1960), decline (1961-1976), nearly collapse (1977-1989), and recovery (1990-2008). During the recent 10 years, the nation wide catch landing of small yellow croaker has been reaching a historical peak in China. On the other hand, some distinctive phenomena in their population biology have been observed, which are characterized with individual miniaturization, low age composition and advanced first maturity.In order to elucidate the mechanism underlying the chronological development of this fishery, especially why the fishery resources could remain at a relatively high level even under heavy fishing pressure, we employ population analysis to investigate the changes of the biology and ecology of this fish in the past 50 years, which include their population dynamics, spatial and temporal distribution patterns, reproductive biology, feeding, distribution and environmental characteristics of their spawning grounds, and fishery management strategy etc. The data used for this study are obtained from the fixed-point surveys of fishery resources (Apr., Jun., Sep., Dec.) and the annual monitoring surveys (every months), in the southern Yellow Sea and the East China Sea from April 1999 to September 2008 by East China Sea Fisheries Research Institute, Chinese Academy of Fisheries Research.Results indicate that the small yellow croaker is dominant species which is widely distributed in the southern Yellow Sea and the East China Sea. The catch accounted for 17.01% in weight and 14.17% in number of 588 species of nektons, second to hairtail (Trichiurus japonicus), the No. 1 fishery catch in the sea. In the past 50 years, population changes are characterized with individual miniaturization, domination of low age groups, increases in growth parameters but decreases in approximate length and weight. The parent stocks are characterized with obvious advances in age and size at first maturation.Based on the relationship between environmental factors and fish growth, it is concluded that there are no significant differences in stock-density index between the southern Yellow Sea and the East China Sea in either season except in autumn. Water temperature is the key factor influencing the small yellow croaker resources in these areas. Its growth condition factor varied monthly with growth stages. For the immature fish, it exhibits no significant difference between theses two regions, but the difference for the mature ones is statistically significant, with growth performance index in the East China Sea being superior to that in the southern Yellow Sea.It is found in this study that the spawning grounds of the small yellow croaker have obviously expanded from nearshore in the past to offshore at present. Moreover, the major spawning grounds in the southern Yellow Sea and the East China Sea had different ocean environmental characteristics. Although the relative fecundity has obviously increased compared to the past, their yolk diameter was getting smaller.Results show that the small yellow croaker mainly feed on crustaceans, followed by fishes and cephalopods. A total of food species have been identified in their stomach contents. Among them, the dominant species are Mysidacea sp., Euphausia sp., Benthosema pterotum, Crangon affinis and Trachypenaeus curvirostris. The average trophic level of small yellow croaker is 2.72. Compared to the results of previous studies, the diet composition of small yellow croaker has undergone great changes.Population dynamics and fishery management studies indicate that the size at fishing for small yellow croaker at present is not reasonable and should be modified. At the current resources status, the maximum sustainable yield (MSY), the total allowable catch (TACMSY) and the total allowable catch (TAC0.1) of small yellow croaker in this region are estimated about 12.62×104, 9.52×104 and 7.98×104 tons, respectively. The minimum size at fishing is limited to 150 mm in body length.As for fishery management, the following measurements are suggested: The net mesh size of relevant fishery gears should be restricted. The proportion of the market fish with the size lower than 150 mm in body length should not exceed 20%. Closed fishing grounds should be established in major spawning grounds. The summer closed fishing seasons should be prolonged to June 1-- October, 1. The fishing effort should be restricted with the defined catch quota limited to 8.00×104tons. In addition, economic means should also be implemented to adapt fishery-related measures to regulate small yellow croaker fishery in the southern Yellow Sea and the East China Sea.