Abstract
Natural infections of adult Neoheterobothrium hirame, a haematophagous monogenean parasite, on Japanese flounder Paralichthys olivaceus in the central Seto Inland Sea off Kagawa Prefecture, Japan, were monitored from 1997 to 2010. N. hirame was found for the first time in the survey area in 1998, and the prevalence rapidly increased annually, reaching a peak level of 52.4% in 2000, then remained at the high level until 2005, and subsequenty suffered a sharp and constant decline until 2010. Occurrences of severely anemic fish were rare and condition factors of the affected fish were not related to N. hirame infection. Whereas the prevalence and intensity of the parasite were statistically irrelevant to the sex or age of the host, they were significantly higher in winter and spring than in summer and fall, indicating a seasonal fluctuation. The prevalence and intensity in the survey area were lower than those reported in the western sea of Japan, which might be affected by environmental factors and biological features of Japanese flounder in this area, including drastic seasonal changes of the water temperature, higher than 25℃ in summer and lower than 10℃ in winter, and differences in their habitat in the Seto Inland Sea between 0-year-old naïve fish and one- to two-year-old infected fish.
