〔 水 産 増 殖44巻3号377-381 SUISANZOSHOKU(1996-H8)〕 Recent Fish Disease Problems in Japan Hlsatsugu WAKABAYASHI Department of Fisheries, University of Tokyo, 1-1-1 Yayoi, Bankyo-ku, Tokyo 113, Japan Abstract Aquaculture is becoming more important in the overall fishery sector. It accounted for 16 percent of the gross weight and 29 percent of the value of the total production in 1993, while in 1963, it accounted for only 6 percent and 14 percent, respectively. Although aquaculture is rapidly developing, it is now faced with some new problems, one of which is diseases, especially in intensive culture. The damage caused by diseases in 1991 was estimated at about 18 thousand tons or 26 billion yen. This loss corresponds to about 5 percent of the gross weight and about 7 percent of the value of the total production of intensive culture. At present, many diseases are found in fish farms in Japan. Some pathogens seem to be ubiquitous and cause serious problems as intensive culture develops. Apart from ubiquitous or endemic pathogens, a considerable number of exotic germs and parasites are suspected to have accompanied fish eggs and larvae imported for aquaculture purposes. This paper focusses on such cases that have recently occurred in Japan. The sudden outbreaks of IHN in 1971, BKD in 1973, and coldwater disease around 1985 were thought to have originated from salmon eggs imported from North America. Since 1991, a monogenean parasite, Neobenedeniagirellae, has been found among amberjack fry imported from Hong Kong and Hinan, China. In 1993, a viral disease, causing serious mass mortality in shrimp farms was introduced through Penaeus japonicus fry imported from Fukien, China. There is only a limited system for quarantine of aquaculture seedlings. The Japan Fisheries Resources Conservation Association (JFRCA), a semi-governmental organization, conducts voluntary inspection of pathogens in imported seedlings for aquaculture. It seems that there is no system for providing overall security against exotic pathogens that accompany imported aquaculture seedlings. However, appropriate regulations for trading fish should be established to protect aquaculture from epizootics. (Oncorhynchus masou, etc.) and tilapia (Oreochromis Introduction niloticus) (Table 1). Marine aquaculture accounted In 1993, annual production from aquaculture was for 94 percent of the gross weight and 86 percent of 1.36 million tons valued at 708 billion yen (about 7 the value of total aquacultural production. The princip- billion US dollars), and accounted for 16 percent of al marine culture species included yellowtail (Seriola the gross weight and 29 percent of the value of the tot- quinqueradiata), amberj ack (Seriola purpurascens), red al fishery production. The main culture species in sea bream (Pagrus major), horse mackerel (Trachurus freshwater included common carp (Cyprinus carpio), japonicus), Japanese flounder (Paralichthys olivaceus), eel (Anguilla japonica), Ayu (Plecoglossus altivelis), tiger puffer (Takifugu rubripes), coho salmon rainbow trout (Oncorhynchus mykiss), native trout (Oncorhynchus kisutch), Kuruma shrimp (Penaeus 378 H. Wakabayashi (1996) Table 1. Production of cultured freshwater fishes in cent in Ayu, 6.3 to 7.4 percent in rainbow trout, 7.1 Japan in 1993 to 10.5 percent in yellowtail, 0.9 to 3.7 percent in sea bream, 7.5 to 14.5 percent in Japanese flounder, and 3.6 to 15.5 percent in Kuruma shrimp. The important diseases in freshwater fishes are listed in Table 3 according to the species affected. Principal diseases in marine finfish and shellfish are shown in Table 4. a) Y en: about 100 yen to a dollar in 1993. Table 3. Threatening diseases in freshwater fishes cultured in Japan Table 2. Production of cultured marine fishes in Japan in 1993 a) Y en: about 100 yen to a dollar in 1993. b) Including shell weight . japonicus), oyster (Crassostrea gigas, etc.) and scallop (Patinopecten yessoensis) (Table 2). Aquaculture has been developing rapidly and has become an important part of the fishery industry. However, it is now faced with many problems. One 1) V, virus; B, Bacterium; F, Fungus; P, Parasite; U, unknown agent. problem is the low prices of the products due to over- production in domestic farms and competition with im- Importation of Aquaculture Seedlings ports from foreign countries. Another serious problem and Exotic Pathogens is the damage caused by diseases. Overproduction has caused the prices of cultured Occurrence of Diseases fishes to go down. Partly as a consequence of this, far- Fish and shellfish diseases are estimated to account mers have ventured into new species or inexpensive for a loss of 5 to 7 percent of total aquaculture produc- seedlings from other countries. Apart from ubiquitous tion annually. However, as in all countries, it is very or endemic pathogens, considerable numbers of exotic difficult to get accurate mortality data from fish farms. germs and parasites have been suspected of accom- There is no publication of official statistics on the panying fish eggs and larvae imported for aquaculture losses due to disease. Based on inquiries conducted purposes. annually from 1985 to 1991 by the Fishery Agency, the annual losses in income were 0.5 to 1.0 percent in Infectious hematopoietic necrosis (IHN) common carp, 3.2 to 4.1 percent in eel, 2.7 to 5.6 per- IHN virus was first isolated in the Hokkaido Fish Recent fish disease problems in Japan 379 Table 4. Threatening diseases in marine fishes Coldwater disease cultured in Japan Until recognized in France among farmed rainbow trout in 1986, this disease had been reported only in North America. In Japan, Cytophaga psychrophila, the causative agent of coldwater disease was first isolated from diseased Ayu in Tokushima Prefecture in 19872), and from coho salmon fry in Miyagi and Iwate prefec- tures in 19902). The condition had been recognized since around 1985 in both species. The epizootic has been spreading over Japan among Ayu and salmonids. Although it is not clear whether they carry the bacter- ium, a large number of coho salmon eggs has been im- ported annually from Pacific North America since 1975, due to the lack of natural stocks of coho salmon. Epitheliocystis disease In June of 1984 and 1985, epitheliocystis disease was found on stocks of red sea bream fry one week after they had been imported from Hong Kong3). Another outbreak of epitheliocystis disease was reported in red sea bream (5-8 cm in length and 6-10 g in weight) imported from Hong Kong in 19874). Neobenedenia girellae It was revealed that this parasite of Japanese fishes originated from infected amberjack fry imported from 1) V, virus; B, bacterium; F, Fungus; P, parasite Hong Kong and Hinan, China5). Amberjack is one of the most profitable species due to its higher market value and to the availability of large quantities of inex- Hatchery from diseased kokanee (Oncorhynchusnerka) pensive seedlings from these areas. Bendeniine speci- fry, which originated from eyed eggs imported from mens were collected from imported amberjack and ex- Alaska1). IHN is still one of the most serious disease amined. The incidence of infection per shipment ranged problems in Japanese salmon hatcheries. The hosts in- from 7.7 to 70.0 percent. The specimens had signs of clude kokanee,chum salmon (Oncorhynchusketa), coho vitellogenesis and spermatogenesis, indicating that they salmon, rainbow trout, Yamame (Oncorhynchusmason) were adult parasites. It takes about one week to trans- and other native trout. port live fish from either Hong Kong or Hinan to the final destination within Japan. During challenge tests, Bacterial kidney disease (BKD) it was demonstrated that the oncomiracidia of N. girel- This disease was first recognized among juveniles of lae developed into adults about 10 days after exposure. chinook salmon (Oncorhynchus tshawytscha), kokanee, The possibility that the parasites could have developed pink salmon (Oncorhynchus gorbuscha) and Yamame in to maturity within a few days is remote. Therefore, it the Hokkaido Fish Hatchery and the Hokkaido Salmon is highly unlikely that the infection originated in Hatchery during the years 1973-1974 (2). The chinook Japan. Additional benedeniine specimens collected salmon originated from eyed eggs imported from the directly on amberjack kept in Hong Kong before ship- U. S. ment to Japan were identified as N. girrelae, most of 380 H. Wakabavashi (1996) which contained an egg in the oocyte. It was therefore fectural Fisheries Experimental Station (PFES) , and concluded that the parasite came with imported amber- about 20 of the prefectures have a Fish Disease Con- jack. A diagnostic field survey also revealed that N. trol Center (FDCC) beside PFES. The stations and the girrelae survived and propagated on amberjack, and centers diagnose diseases and provide guidance for that the infection spread to other susceptible hosts, their control. Beginning in 1972, the Japan Fisheries such as yellowtail and Japanese flounder. Resources Conservation Association (JFRCA), a semi- governmental organization, has provided training for Rod-shaped nuclear virus of Penaeus Japonicus prefectural government staff in fish disease diagnosis, (RV-PJ) prevention and control. This training is supported by In 1993, outbreaks of a viral disease in Kuruma funding from the Fisheries Agency (FA). Various shrimp farms caused mass mortalities and forced many courses are offered. The principal course has a three of the farms in Western Japan to be closed. This re- year duration (about two weeks of schooling each sulted in a decrease in the annual production by 6.9 ear). About 25 trainees per year complete the course percent in gross weight and 8.4 percent in value, com- and apply for the Gyorui-boeki-shi (fish health pared to 3,020 tons and 19.7 billion yen for the pre specialist) examination. As of 1995 there was a total vious year. Moribund shrimp exhibited red coloration of 454 Gvorui-boeki-shi. The FA also entrusts the or decoloration and white spots on the body.