Abstract
We investigated seasonal changes in the abundance of Pseudodactylogyrus spp. (Pd) in Japanese eel Anguilla japonica in culture ponds. For this purpose, the number of parasite eggs floating in the rearing water was counted as an indicator. In spring, the average number of floating eggs was 40.3 (SD= 53.2) per liter of rearing water, which was maintained at 28-30ºC by heating. In summer, the water temperature increased up to 30ºC or higher without heating. During this period, the number of floating eggs was high at 30ºC and was low at ≥ 32ºC. The number of floating eggs was the highest (maximum number of eggs, 1,678 per liter) in autumn when the temperature ranged from 23ºC to 30ºC. In winter, the number of eggs was again low. On the basis of these results, we reared eels infected with Pd in fresh water at 29ºC, 32ºC and 35ºC for 20 days to control this parasitic disease. Parasites did not decrease in number at 29ºC, but clearly decreased at 32ºC, and almost disappeared at 35ºC within 5 days. When heavily infected eels were maintained at 35ºC for 5 days in a greenhouse pond, mortality due to the infection clearly decreased.
