Distribution Of Fish Populations And Structure Of Fish Communities In The Coastal Water Of Central Taiwan Strait And Their Relationships With The Variation Of Water Masses

To detect the relationship between horizontal distributions of fish assembles and water masses or between seasonal distributions of fish assembles and water masses, three fish trawl surveys were conducted in the southwest waters of the Nanri Islands in December 2009 and in April and August 2010. Density and distribution pattern of fish assemblages, body length, body weight, juvenile ratio, dominant species, index of relative importance, and contribution to the total number of dominant species were analyzed in the study. The results showed that fish densities in terms of both weight and number were higher in summer(2599.60 kg·km-2, 22.6×103 ind·km-2) than those in spring(601.26 kg·km-2, 17.30×103 ind·km-2) and winter(730.36 kg·km-2, 22.60×103 ind·km-2). The average body length of fish was the highest in spring(14.95 cm), while the average body weight was the highest in winter(49.76 g). The proportion of juvenile was the highest in summer(as high as 81.72%). The variation in distribution pattern and biological parameters of fish assembles was probably associated with spawning and forging activities of fish. For example, it was mainly composed of spawning adult assembles in spring, while foraging juveniles were predominant in summer. Distribution pattern of fish assemblages was related to the seasonal variation in water mass and food availability in the coast water of Fujian. Fish densities were higher on the offshore side of Zhejiang-Fujian coastal water and Taiwan Strait warm current water. While in summer fish was more abundant in the nearshore region where food was particularly abundant. Changes in fish density is mainly affected by the change of dominant species, e.g.,the predominant species Setipinna tenuifilis exerted the largest contributions to weight and mantissa in spring(0.50 and 0.59); the predominant species in summer Polydactylus sextarius has significantly contributed to variation in the weight and no. of fish in winter, predominant species Harpadon nehereus contribute the most to mantissa(0.78), while Dasyatis zugei exerted the greatest contribution to weight(0.69). The distribution pattern of fish assemblages was in consistent with the distribution pattern of dominant fish species. And this can be used as one of the important basis for the management of fisheries.In the meantime, the species composition and ecological groups in costal water of central Taiwan Strait were discussed by use nonlinear multidimensional scaling and analysis of similarity. The results show that a total of 40 species were identified in winter, eco-groups dominated by warm-water-species. But the ratios of warm-temperature-species were the highest(30%) among three seasons, and accounting for 17.65% and 16.96% in the biomass and individual density respectively. There are 39 species of fish appeared in the spring, and eco-groups are still dominated by warm-water-species. The biomass and individual density of the warm-water-species accounted for 86.48% and 94.52% of the total amount respectively. In summer, fish species accounting to 61, and warm-water-species appeared in large number by 55, accounting for 97.17% and 99.35%in the biomass and individual density respectively. It concluded that the seasonal variations of eco-groups and species compositions in the costal water of central Taiwan Strait were affected by the changes of water masses. Effected by Zhejiang-Fujian coastal water and Taiwan Strait warm current water alternately, species turnover rate and the number and density of warm water species were increased gradually, and accompanied by the estuarine species replaced by coast of species from winter to summer. These phenomenon were relating to fish spawning, feeding and wintering migration, when Zhejiang-Fujian coastal water and Taiwan Strait warm current water occupied this area.As for the division of fish communities in investigation area, Bray-Curtis cluster was used to classify the fish communities in the survey area. On the basis of community division, the species number, density, ecological groups and diversity characteristics of fish species were analyzed.And the effects of the change of the water masses on the fish communities’ classification and diversity are discussed in detail. The result indicates that the fish communities can be divided into two different groups in spring: the northern coastal community and southern nearshore community. The species numbers, density and value of individual diversity index(H ’) of the northern coastal community are 27, 7.47×103ind·km-2 and 2.71. And the corresponding values of the southern nearshore community are 32, 24.34×103ind·km-2 and 1.97 respectively. The species number and density of the southern offshore communities are both higher than the northern offshore ones, but the warm temperate species hold the opposite situation. The summer fish communities are divided into western coastal community and eastern nearshore community, which is different from the situation in spring. The values of species number, density and H ’ of western coastal community are 55, 329.24×103ind·km-2and 1.97, while species numbers, density and H’ are 27, 106.47×103ind·km-2 and 1.97 and 1.62 in eastern nearshore community. For the western coastal communities, both of the individual density of coastal species and nearshore species of the western coastal communities are higher than those of the eastern nearshore community. In winter, the demarcation and traits of the fish communities are similar to those of the spring, which can be divided into northern coastal community and southern nearshore community. The species numbers, density and value of H ’ are 25, 9.96×103ind·km-2 and 2.71. And the values of species number, density and H ’ are 32, 28.85×103ind·km-2 and 1.97 respectively. The individual density of warm water species and warm temperate species in southern coastal community is higher than those in the northern nearshore community. We concluded that the demarcation and characteristics of the fish community is closely related to the seasonal change of water masses. In winter and spring, the number of the fish species, individual density, diversity and the individual density of the warm water species in the area controlled by Taiwan Strait warm water are both higher than those of which dominated by Fujian-Zhejiang coastal waters. In summer, with the influenced of Taiwan Strait warm water and overland runoff, the corresponding values of the fish community in the western side are higher than those in the eastern side.