A report on potential effects of an ectoparasite Argulus coregoni (Crustacea: Branchiura) on ayu under rearing condition

The ayu Plecoglossus altivelis altivelis, a popular fishery resource in Japan, harbors a branchiuran parasite Argulus coregoni in wild rivers. This parasite has so far been investigated in its direct and indirect pathogenicity to freshwater salmonids, which are considered as the preferred hosts, but potential impacts on other host fishes including ayu have not been emphasized. We here evaluate the negative effect of this parasite on ayu, based on experimental infections using parasite specimens obtained from the same host species in the Oouchiyama River of Mie Prefecture, Japan. After 16 days from the release of A. coregoni (n = 14) into a tank rearing 68 host ayus, 9 ayus were dead and 16 developed abnormalities (i.e. erosion, ulcerative lesion, appearance of cloudy areas or haemorrhages on the body surface and basal-part erythema of the pectoral fins), whereas no abnormality was found in the control tank which also reared 68 host individuals. In addition, the infected group exhibited a decrease in body condition compared to the control group. The same pathogenicities were also recorded in an additional replication. These findings suggest that A. coregoni is responsible for biological and economic losses in the stock enhancement. Infection ecology for ayu, therefore, deserves further attention. •Brief experiments were conducted to infect ayu with an ectoparasite, Argulus.•Host’ death and abnormalities were found in the parasite-released tanks.•The parasite may negatively affect the stock enhancement in ayu.