Abstract
We investigated the infection dynamics of adult Neoheterobothrium hirame in young Japanese flounder Paralichthys olivaceus (20-50 cm in total length) in the Joban Sea, eastern Japan. The prevalence and abundance of the parasite, which were statistically irrelevant to host factors namely both wild/released and male/female, showed seasonal fluctuations and peaked in winter. The oldest record of the parasite in the Joban Sea was found in 1997. Although the infection level was high from 1998 to 2002, it decreased dramatically in 2003. The factors influencing the adult parasite infection level were unclear, but one of the possible factors would be the continuous low water temperature from August 2002 to February 2004, especially in the summer of 2003, when new infections should have occurred. The low temperature seems to have decreased the egg-laying rate of N. hirame and consequently lowered its infection level.
