Abstract: The effect of intensive pen fish farming on surrounding aquatic vegetation is studied in East Taihu Lake.The affected area is larger than 1200 m in diameter.In the central area, Potamogelon maackianus is replaced by Hydrilla verticillata,Ceratophyllum demersum and Myriophyllum slicatum.spirogyra sp.becomes the dominant species. and its plants lost their value as fish food.Pollutant from the pens is the main cause of vegetation degeneration because the load of nitrogen and phosphorus is as high as 242 and 27.4 g·m^-2·yr^-1respectively in the pens (14.3 hm²). The obstacle-effect of the pens on windwave,water current and plant removal is another important cause of the degeneration of aquatic vegetation in the surrounding area. The total area of pen fish farming should be limited in the lake,and the pen should be small,well-distributed and moved its position every two or three years to prevent regional or whole lake eutrophication and vegetation degeneration.

Key words: Pen fish farming, Pollution effect, Aquatic vegetation, East Taihu Lake