Infection dynamics of the monogenean Neoheterobothrium hirame among young wild Japanese flounder in the western Sea of Japan

Investigations were made on the infection dynamics of Neoheterobothrium hirame among wild 0-year-old Japanese flounder Paralichthys olivaceus caught in the western Sea of Japan off Tottori Prefecture from spring to autumn in 1999 and 2000. The infection shows the same tendency in both years; flounder became infected in June, a second parasite generation started to appear in early August, and both the prevalence of infection and mean parasite abundance increased thereafter, reaching 100% prevalence in October. One- to two-year-old infected flounder probably acted as the source of infection to the 0-year-old flounder. The parasite distribution showed an increasing overdispersion among 0-year-old fish population from July to August 1999, followed by a continuous decrease in overdispersion in the following months. Furthermore, only a few numbers of fish could be collected in late August to October. These suggest that parasite-induced host mortality occurred among the 0-year-old flounder in this period. In 2000, the parasite showed a similar overdispersed distribution among the 0-year-old flounder populations in late August, but the overdispersion was not clearly decreased afterwards. This is probably because very small-sized parasites were dominant in mid-October. The fish density was extremely reduced from October to November as in 1999. In a survey in August 2000, there was a tendency that severity of anemia was correlated with the intensity of infection. As a conclusion, it is highly probable that N. hirame contributed to the sharp decline in the 0-year-old flounder population in Tottori Prefecture in the autumn of 1999 and 2000.