Abstract
The Western Japan was visited by a heavy drought in summer 1973. In Lake Shinji which communicates with the Sea of Japan by a canal through lake Naka-umi, chlorinity of the waters increased rapidly in July from 23‰ as in an ordinary year to 79‰ and this high chlorinity continued till October. This caused an outbreak of a copepod parasite on crusian carp, Carassius auratus C. The parasite was identified as Caligus orientalis GUSSEV, 1951, which was described on the specimens found on estuarin fishes of the genera Mugil, Limanda Hyprohamphus, Hexagrammus, Sebastodes, and Leuciscus. Infection of this parasite on Carassius auratus was not reported before as far as the present author knows.
Experiments on the tolerance of this species for various chlorinity of water from 0‰ to 18.4‰ revealed that the optimum chlorinity was about 6.4‰ Cl at 2224°C. From this, it is evident that an ascent of chlorinity owing to the drought made this parasite possible to invade Lake Shinji from sea and to infest on a new host, C. auratus, and that exhaustion of the host fish through hypertonic condition make the damage serious.
