Variations In Fish Community Structure And Biodiversity In The Northern East China Sea

Fisheries science and management are progressively switching their attention from single species to ecosystems, increasing the need for measuring the structure and functioning changes of fish community. In the study, we detect the changes of structure and biodiversity of fish community in the northern East China Sea during the recent decades. And we expected that the study could provide scientific basis for the theory and practice of fishery resources management. The graduation thesis consists of three chapters as follows:Since negative ecological impacts caused by overfishing precedes all other pervasive human disturbance to coastal ecosystems, including pollution, degradation of water quality, and anthropogenic climate change, in the chapter I, the methods for evaluating the fishing effects on fish are reviewed. Previous studies showed that community descriptors might be used as indicators of the impact of fishing. The criteria used for the evaluation of these indicators are meaning, expected effect of fishing, exclusiveness to fishing effects, and measurability. Ecologists have developed five main classes of community metrics for such tasks: diversity, aggregate indicator, indicators based on communities'functions, multivariate analyses and metrics derived from ecosystem models. Each class is explained briefly. Most of methods are sensitive to specific aspects of structural change in a community and each has particular advantages and disadvantages. No one measure is adequate to capture all community properties that may be interest, so it's better use a suite of community metrics to explore a comprehensive view of community structural changes.Based on the data collected from bottom trawl surveys conducted in January of 1991 and 2004-2006, the temporal changes of fish community structure in the northern East China Sea in winter were analyzed in the chapter II. The changes were described in two taxonomic levels: one is in species level; the other is in functional group level. A total of eight functional groups were identified within this system, which are based upon trophic levels and adult mobility. They are planktivores, planktivores/benthivores, benthivores, benthivores/piscivores, omnivores, mobile piscivores, elasmobranch, roving piscivores, respectively. The results indicated that there was a significant and positive relationship between species and functional diversity. Species and functional diversity undergone little significant changes through more than ten years, while the slope of the linear model explained the relationship between species and functional diversity became steeper from 0.547 to 0.627 and the average number of species within most functional group became smaller except planktivores and benthivores. All these indicated that the level of redundancy of the fish assemble in the northern East China Sea was lower. Multivariate analyses indicated that there were significant changes in both species and functional group composition for the fish assemblage between the different periods. The chief discriminating species were Trichiurus japonicus, Harpadon nehereus ,Pseudosciaena polyactis , Setipinna taty and Pampus cinereus, while elasmobranch and planktivores/ benthivores were the main discriminating functional groups. The percentage of the total catch for planktivores/ benthivores, benthivores, benthivores/piscivores, omnivores, elasmobranch and roving piscivores decreased significantly in 2004-2006 compared to that in 1991, while the percentage of the total catch for the two other functional groups obviously increased. In summary, the fish community in the north of the East China Sea underwent the structural and functional changes during the recent decade. To protect the fishery resources effectively in the area, some measures must be implanted as follows: releasing the fishing pressure and reducing the environment pollution caused by human activities.The midsummer fishing moratorium system practiced annually by China government aims to protect the fishery resource and restore the marine ecosystem. To evaluate the effects of the measure on fish community, temporal changes in the structure and function of the fish community in the northern East China Sea were examined based on data from bottom trawl surveys in spring over the period 2000-2007 in the chapter III. The results revealed that some stocks such as small yellow croaker Pseudosciaena polyactis, Anglerfish Lophius Litulon, grey goblinfish Minous monodactylus restored following the implantation of the fishing moratorium system, while some other stocks, including butterfish Pampus argenteus, firefly-fish Acropoma japonicum, decreased. All of these should account to the directional changes of the fish community in term of the relative biomass composition. Although the relative species composition undergone directional changes, the fish community was still moderately disturbed. None of kinds of species diversity indexes(H', J', N1, N2) exhibited consistent changes during the study period, meanwhile the indictors related to function of fish community, including the slopes and intercepts of size spectrum, showed no consistent temporal variations, which reflected the productivity and turnover rate of the study community didn't restore in the recent decade. The midsummer moratorium system didn't play an obviously positive role in restoration of ecosystem in the study area. To maintain the ecosystem health, a comprehensive set of restrictions must be imposed on the fishery in the area.