Ecosystem-based management and models in sustainable management of coastal aquaculture

To be successful, the complex process of ecosystem-based management requires management tools that can integrate physical, chemical and biological processes, modeling tool must function at multiple scales, be easily understood by non-scientist and above all reliably predict management alternatives. It must address specific questions about bays, lagoons or coastal oceans without the ability to predict the cumulative consequences to large marine ecosystems. 　

This presentation discusses the concept of ecosystem modeling and focuses on two examples representing small and medium scale semi-enclosed marine ecosystems using a modified coastal ocean circulation model （Blumberg and Mellor 1987）. Xincun Lagoon in southeast Hainan Island is a 21.97km2 （ ～ 6km X ～ 4km） lagoon with a maximum depth of 10.6m and a 120m-wide outlet to the South China Sea. Xincun City, a major fishing port of ～15,000 people, is on one shore of the lagoon and the other shore is a wildlife reserve. The adjacent lagoon experienced a dramatic growth up to 230ha of fish pen aquaculture in 1996 followed by a catastrophic decline. The natural circulation in the lagoon combined with increased oxygen demand that was created by the fish pens was the likely reason for the disaster. Reducing the number of fish pens （33ha） and the advent of a pearl and macroalgae culture resulted in a more sustainable aquaculture industry and environment. Data indicated that the surface water quality did not violate China’s National Water Quality Standards, but the pens were responsible for an estimated 5,000 tonnes of organic pollutants. Fish pens reduced bottom water and sediment quality. Low quality bottom water also flowed in and out of fish pen area with the tide because of the slow turnover time （up to 90 days）. Further analysis indicated that macroalgae culture on racks and seagrasses act nutrient scrubbers and could play an important role by reducing ecosystem risk of less desirable algal blooms （Rawson, et al. 2002）. 　

The medium scale modeling experiment was conducted on Jiaozhou Bay that is a shallow bay of ～ 400km2 with an average depth of 7m and maximum of 50m. The adjacent city is Qingdao, which is one of China’s largest ports and has a population > 2 million people. During the period of this study the bay cotained three areas of scallop aquaculture pens. Simulation experiments with two-pen stocking densities （12 individual m － 3 and 24 individuals m －3） indicated that scallops dramatically decreased the concentration of phytoplankton in the culture areas. However, the impact that scallop culture had on nutrient concentrations was small （Chen, et. al 1999）. 　A new management model system has been developed with funding from the Georgia Sea Grant Program based on the unstructured grid, finite-volume coastal ocean model （Chen et al., 2003）. This system provides a powerful management tool that allows aquaculture to be integrated into the broader context of coastal and large marine ecosystem management. Fed aquaculture does create pollution, but aquaculture is rarely the only pollution source. We must address the issue of aquaculture’s contribution to pollution and find practical solutions to these complex problems.