Spread of Neoheterobothrium hirame (Monogenea), a serious pest of olive flounder Paralichthys olivaceus, to Korea

Neoheterobothrium hirame is a large, blood-feeding gill-worm infecting the highly prized olive flounder Paralichthys olivaceus in Japan. There is strong-evidence that this worm is the primary cause of anaemia, a common and serious condition causing losses among both wild and cultured olive flounders, N. hirame was first detected in Japanese waters less than a decade ago, and its population then proliferated and spread throughout most of Japan, except Hokkaido. In neighbouring Korea, olive flounder is the most important species of cultured marine fish, and production currently exceeds that in Japan, However, until now, there have been no reports of any monogeneans or anaemia among olive flounders in Korea. Our survey conducted in 2000 of 100 cultured individuals from 4 provinces revealed 2 immature specimens of N. hirame: 1 from a land-based pond-culture system in southern Cheju Island (off the SW coast of Korea) and the other from a floating net cage near Yosu (in the mid-S part of the peninsula). The geographic range of this pathogen may have been enlarged as a result of introduction(s) of infected broodstock from Japan, but this seems unlikely. (The raising of this species in hatcheries developed in Korea in 1985, 7 years before the earliest detection of the worm in Japan.) Low numbers of flounders were also clearly anaemic, This, and the current rarity of N. hirame in Korean farms, appears to favour the hypothesis of a more recent, natural dispersal of the worm, during migrations of infected flounder across the narrow and shallow Tsushima and Korea Straits, Regardless of route of entry, we expect this pathogen will have an impact on Korean flounder fisheries equally serious to that being experienced in Japan.