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Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 Analysis of spatio-temporal fluctuations of East China Sea fishery resources using GIS F. Su, C. Zhou, Q. Shao, Y. Du & S. Wang State Key Lab. of Resource & Environmental Information System, Institute of Geography, Chinese Academy of Sciences, China Abstract Catch data from the East China Sea from 1987 through 1997, and other environmental factors, were overlain using Geographic Information System (GIS) technology. Maps of the spatio-temporal density of the demersal and pelagic fishery resources were produced. After analyzing the density distribution, we analyzed relationships between spatio-temporal distributions and environmental factors. The Kuroshio Current plays a major role in determining the fishery density distribution, especially for the demersal fishery. The South Yellow Sea Water Mass and the expansion of the Chang) iang Diluted Water (ECDW) are two other main environmental factors. The demersal fishery resource is mainly distributed between the 200-m isobath and the central route of Kuroshio Current. Density shifts of the demersal and pelagic fisheries were analyzed by calculation of centers of gravity. Trend analyses showed that the fishery resources generally decreased in the study area during the 11-year period. 1 Introduction In our research, we used GIS to analyze the spatio-temporal fluctuation of fishery resources in the East China Sea. The East China Sea is a marginal sea which lies between the Asian continent and the Pacific Ocean. Its current system is composed of littoral currents and the Kuroshio Current system. Therefore, water in this region is composed of littoral diluted water and water from the far sea. The littoral diluted water has low salinity, low transparency, and great interannual variability in temperature. The temperature and salinity of the far sea water are high, so that gradients of temperature and salinity in the East China Sea Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 250 Environmental Coastal Regions HI are steep. Biomass in this region is low compared with other regions in the world, only 3.92 tonnes per square kilometer. Approximately 11 percent of production in this region is pelagic fishes and 40 percent is demersal fishes [1]. This region is the most important fishing area for China, generating 40 percent of China's total fishery production [2]. Weizhong Chen analyzed the change of fishery resources in the East China Sea based on the biographic characteristics of sample fish, total catch production, and production of the main species[3]. Weihai Liu and Benyi Zhan calculated the correlation between CPUE and catch effort, and analyzed the utilization of fishery resources using two surplus production models, the Schaefer Model and the Fox Model. [2]However, none of these analyses contains a spatial component. GIS is still not widely used in fisheries management, as the typical stock assessment models do not include spatial information. There are several areas in which the use of GIS in management of marine fisheries is emerging [4-5]. One area is for Pacific salmon in the USA. Olivier et al. [6] analyzed the relationship between fishery resource fluctuations and environment factors, including fishing effort. Several studies have analyzed spatial and temporal variation of groundfish assemblages off the east coast of North America [7-13]. Also, GIS is now being used in for modeling fish habitat in U.S. estuaries [14]. 2 Data Sample data for 1987-1997 were obtained from four of the biggest fishery companies in China. The catch of these four companies totals 12 percent of the total catch from the region by all Chinese companies. The East China Sea was divided into regular grid with 10' x 10' each cell (Figure 1). Each grid cell has one catch record per day for each gear type. Trawl catches were used as a measure of demersal fish density; catches in purse seines were used as a measure of the density of pelagic fishes. There was a 54 percent probability that two companies were fishing in the same cell on the same day, with an over 80% overlap if the low-production records are deleted. The results are only slightly changed if the records of any one company are deleted, which demonstrates the robustness of the sample data set. 3 Analysis 3.1 Fishery resource distributions and environment in the East China Sea 3.1.1 Demersal fish distribution Demersal fishes are caught primarily on the shelf (Figure 1). There are three areas where the density of demersal fishes is high: Changjiangkou Zhoushan, Kuroshio and Dasha. The Kuroshio area is divided into four subareas in Figure 1. Densities in the majority of cells in the Changjiangkou Zhoushan and Kuroshio areas is 0.5-2 tonnes per tow, with some over 2 tonnes Per tow. In the Kuroshio area, high density occurs strictly between the central line of the Kuroshio Current Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 Environmental Coastal Regions HI 251 and the 200-m isobath, resembling a belt along the Kuroshio. The Chang) iangkou Zhoushan area lies between the 50-m and 100-m isobaths. In half of the cells in the Dasha area, the density of the demersal fishery is in the 0.5-2 tonne range. The last two areas resemble two slices of a pie . Figure 1: Density distribution of demersal fishes 3.1.2 Pelagic fish distribution The pelagic fishes occur mainly near the coast (Figure 2). There are four areas where the density of pelagic fishes is high: Dasha, Chang] iangkou Zhoushan, Wentaimindong, and Duima. The density of pelagic fishes per cell in the Dasha area ranges from 2-50 tonnes per purse seine. One-third of these cells has densities between 2 and 10 tonnes per purse seine, the middle third has densities of 10-20 tonnes per purse seine, while the highest third has densities over 20 tonnes per purse seine. Densities in the other three areas are between 2 and 10 tonnes per purse seine. 3.1.3 Relationships between fish density and the environment Figure 2: Density distribution of pelagic The fishing ground for demersal fishes in the fishes Kuroshio area occurs in the updraft current, which lies to the left of the Kuroshio Current. The bottom drops steeply east of the 200-m isobath, which enhances the intensity of the ascending current. These factors have important influences over the demersal fishing grounds. The fishing grounds in Changjiangkou Zhoushan area lie in the Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 252 Environmental Coastal Regions III temperature-front zones between the Changjiang Diluted Water (ECDW), the Yellow Sea Cold Water Mass, and the Taiwan Warm Current. There are two temperature fronts in the upper waters of the Dasha area. One, on the west, is formed by the Yellow Sea Warm Current and the Yellow Sea Matrix Water. The other, to the east, is formed by the littoral water of Korea and the Duima Warm WaterMass[15-16]. The Kuroshio Current plays a major role in determining the density of the bottom-fishery resource. High biomass areas typically lie to the left of major currents in the Northern Hemisphere [17]. The catch data indicates that fishery productivity is high where high primary productivity occurs. Xuechuan Wong, Fenqi Li and Beiwei Lu's [18-19] have shown that chlorophyll-a concentrations are high in the middle of the East China Sea's shelf, and distributed discontinuously in a belt along the 200-m isobath as the same as the ditribution of high fish density areas(Figure 1). 3.2 Changes in fishing ground location Changes in the location of fishing grounds over the 1987-1997 period were evaluated by calculation of the annual "center of gravity" for each fishing ground [20]. The following formula was used to calculate the center of gravity. xc = — xw(x, .y)(Wy, .yc = -— M M D is the area being analyzed, (xc, yc) are the coordinates of the center of gravity, M is the total biomass, and u (x, y) is the density at the point of (x, y). For use with the sample data, we re-arranged the above two formulas as follows. In these formulas, d is the area of the fishing ground and P (x, y) is the density offish in the cell (x, y). 3.2.1. Demersal resources For the demersal resource, centers of gravity were calculated for the four fishing grounds shown in Figure 1. The position shifts of each fishing ground are shown in Figure 3. The center of the Dasha fishing ground shifted to the northwest almost continuously during the study period. The net distance shifted was 56 nautical miles to the west and 28 nautical miles to the north. The center of Changjiangku Zhoushan fishing ground shifted primarily to the south. The net distance moved was 22 nautical miles. The center of the first Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 Environmental Coastal Regions III 253 Kuoshio fishing ground drifted erratically, with a net change of 24 nautical miles to Dasha Fishing Groun< the west and 15 nautical miles to the north. The center of the second Kuoshio fishing 1987 ground also drifted erratically. The net distance shifted was 29 nautical miles to the south and 10 nautical miles to the east. 3.2.2 Pelagic resources For the pelagic resource, Second Kuroshio 1997 centers of gravity were Fishing Ground calculated for the four fishing 1987 grounds shown in Figure 2. 1987 First Kuroshio The position shifts of each Fishing Ground fishing ground are shown in 1997 Figure 4. The center of the 90 Dasha Fishing Ground shifted Nautical Mile to the west continuously from 1987 to 1997, with a net FigureS:Locations of the centers of gravity change of 31 nautical miles. for the East China Sea demersal fishing Over the entire 1987-1997 grounds period, the center of the Changjiangkou fishing ground shifted to the south slightly from 1987 to 1997, but the inter-annual shifts were considerable, about 20 nautical miles. The center of the Zhoushan fishing ground shifted about 30 nautical miles inter-annually, but without a discernable pattern. The center of the Wentaimingdong fishing ground shifted to the northeast, with a net position change of 30 nautical miles to the east and 15 nautical miles to the north. 'han&jiangK 90 Dasha Fishing Ground Wentaimingdong Nautical Mile Figure 4: Locations of the centers of gravity for the East China Sea pelagic fishing grounds. Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 254 Environmental Coastal Regions HI 3.3 Spatio-temporal analysis of the fishery-stock fluctuations Temporal trends in density over the study period were analyzed by grid cell using least-squares linear regression. The data were organized as in Table 1. The regression equation was as follows: Y = ax+b, where Y is the density in a given cell and year, x is the number of years from 1987 (e.g., the value of x for 1991 is 1991-1987 = 4), and a and b are the coefficients. The value of "a" is the slope of the line, and indicates the temporal trend in the cell. A positive slope would indicate an increasing trend in catch, while a negative slope would indicate a decreasing trend. The absolute value of the slope indicates the magnitude of the trend. Table 1. Density of demersal fishes by grid cell and year. Year Cell No 15727 87 88 89 90 92 91 93 94 95 96 97 34 64 45 58 50 14 30 24 7 15728 44 81 56 32 73 24 17 26 6 31 26 16426 16427 51 70 36 62 103 20 58 19 19 106 46 51 46 157 40 28 27 108 3 42 18 64 H-4- Miles Nautical Figure5 Change of density of demersal fishes 9 27 The results of the trend analyses were divided in seven categories: high decrease , medium decrease, low decrease , unchanged, low increase , medium increase, and high increase (Figure 5). Most of the cells show decreases for the demersal fishery, especially in the third and fourth Kuroshio fishing grounds. In the southeast of the second Kuroshio fishing ground, there are some cells in the low increase category, located southeast of the Kuroshio Current. In the Dasha fishing ground, Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 Environmental Coastal Regions III 255 most of the cells are also in the low increase category, and some are in the medium increase category. In the Changjiangkou Zhousha fishing ground, some cells are in the low increase category. In the first Kuroshio fishing ground, there are some cells along the Taiwan Warm Current in which density has increased slightly. In the whole East China Sea, there are some cells in the high decrease category, but no cells in the high increase category. These patterns are similar to the changes in location discussed in Section 3.2.1. Figure6 Change of density of pelagic Density of the pelagic fisheries decreased in fishes two-thirds of the cells in the East China Sea during the study period (Figure 6). In the Dasha fishing ground, there are some cells in the high decrease category, and the number cells in the low decrease and low increase categories is the same. In the Duima fishing ground, most of the cells are in the medium decrease category. In the Wentaimingdong fishing ground, the density in half of the cells are in the low increase category , while that in the other cells are in the medium category. These patterns are similar to the changes in location discussed in Section 3.2.2. 4 Conclusion There are three areas in which the density of demersal fishes is high. The Kuroshio area resembles a belt between the 200-m isobath and the center of the Kuroshio Current. The other two areas lie in the Dasha and Changjiankou Zhoushan areas, respectively. Fish distributions in the East China Sea are influenced by the Kuroshio Current, the Yellow Sea Water Mass, and the expansion of the Changjiang Diluted Water (ECDW). Over the 1987-1997 study period, the Changjiangkou Zhoushan fishing ground shifted to the south slightly, the Dasha and the first Kuroshio fishing grounds shifted to northwest, and the Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 256 Environmental Coastal Regions HI second Kuroshio fishing ground shifted to the southeast. The trend analysis showed that the demersal fish resources in the East China Sea have decreased. There are four areas in which the density of pelagic fishes is high: Dasha, Changjiangkou Zhoushan, Wentaimindong, and Duima. Overall, the Dasha fishing ground shifted to the west; the Changjiangkou fishing ground shifted to the south slightly, but with inter-annual variability; the Zhoushan fishing ground did not have a substantial net change in location, but shifted long distances interannually; and the Wentaimingdong fishing ground shifted to the northeast. The trend analysis showed that the pelagic fish resources in the East China Sea have decreased slightly. Acknowledgements We wish to thank Shixian Han, Weizhou Chen and Vincent Lyne for discussions about this work. 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