Disturbance of Korean Lake Ecosystems by Aquaculture and Their Rehabilitation

The many artificial, multipurpose reservoirs in the Republic of Korea (henceforth, Korea) have been used for water storage, flood control, hydroelectricity, sightseeing, fishing, aquaculture, and other activities. In 1978, cultured fish contributed 1,831 metric tons (t) of product per year to the inland fishery. By 1992, the cultured-fish contribution had drastically increased to 20,049 t per year. Aquaculture in reservoirs contributes most of this production. Most cultured fishes are alien fishes. Concomitant with the increase in the aquaculture industry, many ecological, limnological, and biological problems emerged. Among them, reservoir eutrophication and the spread of alien-fish species into Korean waterways are severe problems. Lake Soyang, the largest and the most important reservoir in Korea, was constructed in 1973 and has a water-holding capacity of 2.9 billion tons. From 1978 through 1997, the lake supported many fish farms for the production of alien species such as mirror carp (Israeli carp) (Cyprinus carpio), channel catfish (Ictalurus puntatus), and rainbow trout (Oncorhynchus mykiss). These fishes were raised with fish feed, which has a high phosphorus concentration. Prior to the introduction of aquaculture, the lake was very clean; Secchi-disk transparency was 9 m, and phytoplankton density was low. However, after 10 years of aquaculture, symptoms of eutrophication appeared, such as an anoxic zone in the hypolimnion, low dissolved oxygen layers in the metalimnion, increased Anabaena spp. density, and high phosphate concentration in the hypolimnion. It was believed that the eutrophication was due to an increase of phosphorus from the fish feed. In 1990, the annual total phosphorus (TP) loading into the lake from the watershed was 104 kg=yr, and that from the fish farms was 48 kg=yr. The critical TP loading for eutrophication of Lake Soyang is 87 kg=yr. The fish farms were thus believed to be a major cause of eutrophication. All of the fish farms were closed and removed between 1997 and 1999. The water quality of Lake Soyang should improve sooner or later. Also a new and different ecosystem will be established. Alien fishes also predominate in Lake Paldang, which is the major source of drinking water for 20 million people. Bluegill (Lepomis macrochirus), imported Crucian carp (Carassius cuvieri), largemouth bass (Micropterus salmoides), and other alien fishes released from fish farms are replacing domestic fish species. In 1996, more than half of the fish caught were alien fishes, which implies negative effects for the native species community. Among the severe problems generated by aquaculture in Korean lakes, eutrophication and the presence of alien fishes are the most important. Fortunately, the former problem can be solved, with government and citizen efforts, by clearing the fish farms; on the other hand, the latter problem cannot yet be solved.