A study of long-term changes in summer infection levels of Japanese flounder Paralichthys olivaceus with the monogenean Neoheterobothrium hirame in the central Sea of Japan, with an application of a new technique for collecting small parasites from the gill filaments

Neoheterobothrium hirame infection of 0-year wild Japanese flounder Paralichthys olivaceus was investigated. This study was based on annual samples (preserved in formalin or alcohol) totalling 316 fish caught off Igarashi-hama in July and/or August of 1989-1993 and 505 fish caught off Murakami City in August of 1993-1999, Niigata Prefecture. A stirring method was developed to collect the monogenean from the fixed gill filaments. All the parasites were dislodged from the fixed gill filaments by stirring gills of individual hosts in 150 mt of water with a magnet (40 mm long and 8 mm in diameter) at 1150-1200 rpm for 20 min and 30 min for the 0-year fish and 1-year fish, respectively. Adult parasites were macroscopically observed on the buccal cavity wall, whereas immature ones on the gill arches and rakers were detected under a stereomicroscope. No N. hirame was found from 1989 to 1992. The earliest records were from samples caught off Igarashi-hama and Murakami City in 1993, though the prevalence of infection was very low. The parasite occurrence changed annually in the Murakami samples. The annual fluctuations may have been influenced by the infection level of co-existing I-year fish and the population size of 0-year fish recruited each year. It is inconclusive whether or not the monogenean is an indigenous parasite of Japanese flounder in this area.