The Korean peninsula is surrounded by the sea except on the north. Most marine fish farms are situated in the southern coastal area of the peninsula, followed by western and eastern coastal areas. Since some marine fish culture of yellowtail (Seriola quinqueradiata) started in a pilot scale net cage program on the southern coast of Korea in 1964, aquaculture has developed significantly. Net cage culturing methods also have been used to farm black rockfish (Sebastes schlegeli), sea bass (Lateolabrax japonicus) and red sea bream (Pagrus major).
Olive flounder (Paralichthys olivaceus) is cultured in land-based tank systems. Both olive flounder and black rockfish are the most important marine cultured finfish in Korea, and they have been cultured in land-based tanks and net cages, respectively. Aquaculture techniques have progressed over time and the available area for marine fish culture has increased through the development of culture facilities. In Korea, fish aquaculture began in early 80s. Since then, fish diseases have occurred and caused large economic losses. In early 80s fish diseases occurred at high temperatures over 20°C and they were normally caused by simple infection of bacteria or parasites. During the 1980's, fish disease was not a serious issue. But in the 1990's fish diseases have increased every year along with the increase in production. Main diseases in the 90s were a combination of bacterial and parasitic infections, and viral infections. This pattern was more obvious in late 90s. Parasitic and bacterial infections were main aquacultural diseases in early 90s in Korea. However, the pattern has changed in late 90s. In late 90s, viral infections took main part of aquacultural diseases with complex infections of bacteria and bacteria or bacteria and parasite rather than simple infection of a bacteria or a parasite. In 1996, simple infections caused by one pathogen made up more than 90% of all fish diseases. But every year, complicated infections by more than two pathogens have increased continuously. Especially, in 2001, the complicated infection rate increased to about 30% of the diseases. The total percentage of simple bacterial and parasitic infection was decreased to 61% in 2000, and percentage of viral infection and complex disease increased to 34 %, this percentage is 5 times higher than 5 years ago.
The rock bream, is very popular for Sashimi and thus is very expensive in both Korea and Japan. In Korea, most fish farmers want to culture rock bream. However, since there is a shortage in the supply of fry, mortality is very high. The Vibriosis was dominant disease of all cultured fish. And also, Edwardsiellosis, Streptococcosis, Flavobacterial disease and Scuticociellosis disease are sometimes found as well. In the case of the Rock bream, iridovirus was identified as a predominant pathogen. A diagnostic survey in fish farms with land-based tanks and net cases were conducted in eastern, western, southern and Jeju island of Korea during summer period from 2000 to 2005. Total 2528-fish sample of marine and freshwater fishes was tested for infected diseases during the time period of this survey. The main fish species was olive flounder, fleshy prawn (Fenneropenaeus chinensis), black rockfish (S. schlegeli), rock bream (Oplegnathus fasciatus), red sea bream (P. major), black sea bream (Acanthopagrus schlegeli), sea bass (L. japinicus), gray mullet (Mugil cephalus), rainbow trout (Onchorhynchus mykiss) and others. Infection of pathogens were 75% in 2000, 81.8% in 2001, 86% in 2002, 86.1% in 2003, 77.4% in 2004 and 72.6% in 2005, respectively. Occurrence of complicated infections was very low from 2000 to 2001. However, their occurrence showed high level rather than simple infections from 2002 to 2005. The highest pathogen of bacterial disease was Vibrio. Also, the highest pathogen of parasite disease was protozoa, mainly scuticociliates. The highest pathogen of viral disease was both Iridovirus and VNNV. This project will be still carry out for years by annual surveillance program of pathology team in NFRDI.