Studies On The Feeding Ecology Of Dominant Fishes And Food Web Of Fishes In The Central And Southern Yellow Sea

In the 1990s, studies on the Global Ocean Ecosystems Dynamics (GLOBEC) developed rapidly as an international advanced research field in marine science. In 1995, GLOBEC was selected to be the core project of the International Geosphere-Biosphere Programme (IGBP). The study on the relationship between the food web structure and ecosystem trophodynamics is one of the four fundamental tasks of GLOBEC, with emphasis on the trophodynamic pathways, their variability and the role of nutritional matters in the food web. In October 1999, the Ministry of Science and Technology formally authorized the China-GLOBEC programme entitled "Ecosystem Dynamics and Sustainable Utilization of Living Marine Resources in the East China Sea and Yellow Sea" as the "National Basic Research Programme" (also called 973 Programme). The implementation of this programme has greatly promoted Chinese scientists' capacity in the international advanced research field of GLOBEC.Academician Tang Qisheng proposed that the marine food web and its trophodynamics in China should be studied in accordance with the principles of using the "simplified food web" and connecting the "points" with "facets". Based on the above research backgrounds and the fish stomachs sampled from bottom trawl surveys conducted in the central and southern Yellow Sea from 2000 to 2002, the feeding ecology and food web of fishes were studied in this thesis at the levels of key fish species, dominant fish species and fish communities. The aims of present thesis are to provide some basic data for better understanding of the transfer of nutritional matters, flow of energy and trophic models in the Yellow Sea ecosystem.For the key fish species, the diet composition and the seasonal, ontogenetic and diel variations in the feeding ecology of Pseudosciaena polyactis in the central Yellow Sea were analyzed by employing various statistic methods such as the two-way contingency table test and Kruskal-Wallis nonparametric rank test. The results show that over 30prey species were ingested by P. polyactis. Euphausiids were the most important prey groups to P. polyactis, followed by decapods and fishes. The feeding intensity varied significantly among seasons being the highest in autumn and the lowest in spring and winter, while the feeding intensity didn't vary significantly among different size classes. But the diet composition varied with fish sizes. The proportions of fishes and decapods increased with the increasing fish size, whereas the proportions of euphausiids, copepods and amphipods decreased. Cluster analysis showed that there was an abrupt change in the diet composition at about 110 mm. The feeding intensity showed significantly diel variations in each season with two main peaks occurring at 08:00 h and 24:00 h.For the dominant fish species, the food relationship between three sciaenid fishes {Collichthys niveatus, Johnius belengerii and P. polyactis) in the central and southern Yellow Sea was studied by applying the multivariate statistical techniques, and the morphological differences in the feeding apparatus of three species were also examined. The results indicate that crustaceans were the main food resources of three species, but their dominant prey species were different and their diet compositions differed significantly from each other. Two-way analysis of similarities indicated that diet compositions of three species were all significantly influenced by habitat types and seasons. The intraspecific values for Schoener's dietary overlap index between each size class of C. niveatus and P. polyactis were relatively higher, whereas those of J. belengerii were lower. The values for interspecific dietary overlap index between each size class of three species ranged from 0.50 to 0.56. The principal component analysis (PCA) indicated that the morphological characters of the feeding apparatus were similar between C. niveatus and P. polyactis, whereas these two species showed some differences with J. belengerii.In order to quantify the food relationship between major predators and their preys, the food consumptions of prey groups and dominant prey species by 23 fish species in the central and southern Yellow Sea during autumn were estimated by Egger's model. Results suggest that the total food consumption by these predators in autumn was over 3.09 million tons. Among these predators, Engraulis japonicus and Liparis tanakae wasthe most important pelagic and demersal predator, respectively. Euphausia pacifica, Calanus sinicus, Themisto gracilipes, Crangon affinis and E. japonicus were the five most important prey species in the central and southern Yellow Sea. The total consumption of these preys by 23 predators was over 2.33 million tons. The most important predator consuming fish preys was L. tanakae, followed by Scomber japonicus, Trichiurus lepturus, P. polyactis and Lophius litulon. Among these fish preys, the consumption of E. japonicus by these predators was the highest (over 70 thousand tons), accounting for about 32.4% of the total fish consumption.For the fish communities, the seasonal and spatial variations in the trophic structure of fish communities in the central and southern Yellow Sea were examined by considering the fish community as distinct "ecological unit". The results indicate that planktivores were the dominant trophic guilds in each community. Distributions in the water layer and prey sizes are two main factors distinguishing trophic guilds. Cluster analysis indicated that the Bray-Curtis similarity indices ranged from 24% to 34% in identifying major trophic guilds in each community. Given the generally low dietary overlap between fishes, the potential for strong food competition is relatively low in this ecosystem. There were seasonal and spatial variations in the trophic structures of fish communities and food relationships between fishes, which reflected the differences in the abundance and availability of dominant preys. The trophic levels assigned for major predators were between 3.1 and 4.7, and the weighted mean trophic level for each fish community ranged from 3.3 to 3.4.In summary, the key preys in the Yellow Sea food web have varied greatly compared with the historical data in the mid-1980s. The importance of E. japonicus, Ammodytes personatus and Trachypenaeus curvirostris in the food web has decreased markedly, of which E. japonicus showed the greatest decrease. Currently, E. pacifica, C. sinicus and C. affinis have become the key preys in the Yellow Sea food web.