Current status and sustainability of cage culture in reservoirs: a case study in China

Cage cutlture has become a significant contributor to fish production in China during the past two decades, particularly due to the rapid expansion in the use of reservoirs. For example, in Ziyang prefecture of Sichuan province, the culture area and fish yields increased from 0.13 ha and 60 tons in 1984 to 20.9 ha and 16,412 tons in 1998, respectively. The productivity and total catch in the open water of reservoirs also increased from 150 kg/ha/yr and 1,133 tons in 1984 to 3,009 kg/ha/yr and 22,700 tons in 1998, respectively. The cultured species in cages includes common carp, grass carp, silver carp, bighead carp, crusian carp, deep crusian carp, prussian carp, largemouth catfish, leiocassis sp., channel catfish, Wuchang Bream, Mandarin fish, soft-shelled turtle, Nile tilapia and pig frog. However, due to high production and economic returns, farmers and some government staff set up excessive number of cages in reservoirs. Cage area has reached 1-3% of the total surface area of reservoirs. As most caged fish are fed with pelleted diets, rapid eutrophication occured in many reservoirs, causing occasional massive mortality of fish in cages and even in open-water in recent years due to deteriorated water quality. This has restricted further development of reservoir fisheries, and hampered multi-uses of reservoirs. To achieve sustainable cage culture in reservoirs, the following measures should be considered: 1. establish and enforce regulations for reservoir fisheries; 2. set-up water quality monitoring systems; 3. determine reservoir carrying capacity for cage culture based on fish production and ratio of cage area to open-water area; 4. optimize the stocking ratios between filter-feeding fish and fed-fish in cage culture, and species composition and density of fish stocked in open water; 5. encourage and promote production and use of environmentally friendly feed such as floating pellets; 6. improve cage design and layout, and collect cage wastes; 7. use mechanical method to oxygenate bottoms and mix reservoir water. The purposes of all these measures are to reduce nutrient loading, recycle nutrients and improve nutrient utilization efficiency, minimize pollution, and thus achieve sustainable development of reservoir fisheries