Ecological Modeling Of The East China Sea Shelf Ecosystem

As China's second-largest large marine ecosystem, the East China Sea Shelf has suffered from overfishing, eutrophication and physical disturbance over several decades. The East China Sea Shelf (ECSS) is also the China's most productive and has provided valuable fisheries to coastal communities and regional countries (China, Japan, and Korea) for centuries. In the last 50 years, the ECSS has been heavily exploited, and developments in its commercial fisheries have shifted the exploitative focus from top predators in the ecosystem to more abundant species at lower trophic levels. We observed several signs that such "fishing down the food web" has occurred in the ECSS through prolonged and intensive fishing pressure over this 50-year period. As an overall indicator of fishery impacts on the system, biodiversity declined significantly. At the same time, the composition of fishery landings became increasingly dominated by juveniles and relatively small, lower trophic level species with high turn-over rates. The mean trophic level of landings decreased from 3.5 in 1965 to 2.8 in 1990. Many demersal stocks are fully exploited or overexploited, and some pelagic stocks also show overexploitation trends, although some species are still producing high landings. However, despite these clear indications of ecosystem-level consequences arising from current fishing practices, little ecological modeling work has been done for the ECSS, and to date there is still no quantitative assessment of ecosystem dynamics for this region.To properly manage the fisheries resources of East China Sea and address fishery depletion, studies were undertaken on pollution, fishery and ecology. To support these studies, a large quantity of data was collected on fisheries statistics, population parameters, diet compositions, and physical and chemical variables. However, limited efforts were put into the development of an ecosystem-based fisheries resource management strategy. In this study Ecopath with Ecosim, an ecosystem modeling approach based on food-web, was employed to construct both the static and dynamic models and evaluate the ecosystem structures, functioning, keystone species, succession development and the impacts of fishing in ECSS ecosystem. The mechanism of fishery degradation was revealed. Moreover, the optimal strategies of balancing the economic, social and conservation objectives were discussed. The results and conclusions are described in detail as follows:1. Trophic model of the East China Sea Shelf: analysis of ecosystem structureA trophic mass-balance model of this ecosystem was developed in order to characterize the structure and functioning of its food web, quantify the ecological impacts of fishing that it sustained during the early 2000s. Using a multivariate statistical analysis, thirty-eight functional groups were identified, including fish and invertebrate groups targeted and not targeted by fisheries. Strong benthic-pelagic coupling was found present in this ecosystem. In particular, this study highlighted the inter-dependent relationships that exist among plankton, benthic invertebrates, and detritus. Recent fishing activities were characterized by high gross efficiencies and high exploitation rates for various commercially targeted and non-targeted species, leading to the consumption of much of the ecosystem's fishable production. Overall, the findings give a better explanation of the current problems of eutrophication and fishing-induced evolution in the East China Sea Shelf and highlight the need for developing ecosystem-based fisheries management.2. Interdecadal changes in the East China Sea Shelf ecosystem: a comparison of the 1970s and 2000sI ecosystem models for two time periods (1969-1972 and 1998-2001) constructed and compared them with respect to basic ecological indices, overall system indices, food web and ecosystem structure, energy and nutrient cycling, and trophic flows. The results show that the ecosystem had changed from being dominated by demersal species to a heavily-exploited system dominated by pelagic species with a high turnover rate. Increases in the P/B ratios and fishing mortality levels observed for species groups reveal rapidly intensifying fishery stress over the three decades. Primary production, total biomass, and total throughput decreased throughout this period. Drawing upon Odum's theory of ecosystem maturity, this study shows that the maturity and stability of the East China Sea Shelf ecosystem has decreased. The structured, web-like ecosystem of the 1970s developed into a less complex and the system's maturity fell to its lowest observed level. During this period, the successional development of the system occurred in reverse.3. Evaluating ecosystem development stages and the impacts of fishing in the East China Sea Shelf using EcosimAn ecosystem model representing the continental shelf of the East China Sea was fitted to the available time series data from 1969 to 2000. A process-oriented model was used to explore the extent to which changes in marine reources and the ecosystem were driven by trophic interactions and fishing activities. Fishing effort was used to drive the model, while observed catches were compared with the predicted results. A reduction in the sum of the squared deviations of the observed and predicted value of the catch is used as a metric for calibrating and assessing the fit of the model. Trophodynamic indicators were used to explore the ecosystem's structural and functional changes from 1969 to 2000. The model's predictions match the catch of most functional groups and trophodynamic indicators suggest a degradation pattern over time: both the mean trophic level of community and a modified version of Kempton's index of biodiversity decrease with time, while the total flow to detritus and the loss of production due to fishing increase from 1969 to 2000. Additionally, the demersal/pelagic ration decreases due to an overall increase in the abundance of pelagic fish in the ecosystem. Overfishing is blamed for the degradation of the ecosystem development.4. Evaluating ecosystem-based management strategies of the East China Sea Shelf with EcosimUnderstanding the trade-off relationships between economic, social and ecological objectives is important in designing policies to manage or restore ecosystems. As the most productive sea in China, East China Sea ecosystem supports various commercial, recreational, and artisanal fisheries. Many fisheries are now depleted or had experienced substantial decline in productivity. Ecopath with Ecosim was employed to evaluate how the ecosystem may respond to changes in fisheries activities over a period of 20 years and search for the optimal policy considering the economic, social, and ecological objectives. Four fishery management scenarios, which maximized three independent (fishery profits, employment and ecosystem structure) and the combination of the above three objectives were simulated with the vulnerability value estimated from the time-series simulation. Results suggested that socioeconomically driven policy caused the ecosystem to be vulnerable whereas maximized ecological stability and the composition scenarios. To maximize social and economical criteria, the ecosystem structure was shifted to a simplified state where the high trophic level species became depleted and the low trophic level species gained dominance in the landings with the lowest mean trophic level of catch of 2.66. When an ecosystem structure criterion was considered, the model predicted effort should decreases more than 70% for all fishing sectors. A trade-off analysis indicates that the 'best composition' strategy would be optimal to balance fishery and conservation. This study highlights the conflict between maximizing conservation and social objectives, win-win solutions between conservation and economic objectives may be possible.5. A search for keystones in East China Sea Shelf, using a mass-continuity trophic modelKeystone species play a central role in shaping at least some marine communities in the sense that system-wide phase shifts can be mediated by the presence, absence, or the relative abundance of these key interactions. Identifying keystones is considered crucial for understanding the resilience of ecosystems to exogenous forces because non-linear, or catastrophic, community changes might hinge on the populations of one or a few species. Removal experiments are traditionally the most reliable approach for identifying keystone species, but such experiments can be accomplished only on small scales and with only a few types of species in any given system. I described the results of a series of simulated (functional group) removal experiments using a whole-system food web model of East China Sea Shelf for the period 1998-2001. Four indices were used to rank functional groups, based on the results of these simulated removal experiments: community importance, community longevity support, interaction strength index, and keystoneness index. Marine turtles produced the highest keystone index followed by Seabirds in the present iteration of the Ecopath model of East China Sea Shelf, which would be a good example for the conservational policy searching in the future in East China Sea Shelf ecosystem.