Occurrence of parasitic copepods <i>Ergasilus hypomesi</i> Yamaguti, 1936 and <i>E. wilsoni</i> Markewitsch, 1933 (Copepoda, Ergasilidae) on early stage Japanese smelt (<i>Hypomesus nipponensis</i> McAllister, 1963

The occurrence of adult females of two euryhaline ergasilid species of parasitic copepods—Ergasilus hypomesi and E. wilsoni on larval and juvenile Japanese smelt (Hypomesus nipponensis) was investigated in Lake Abashiri, a brackish lake in eastern Hokkaido, Japan, from May to August 2016. The first E. hypomesi infection in Japanese smelt was observed in late June, and the prevalence of parasitism (infection rate) increased to 13.9% in middle July and 35.7% in early August. The mean parasitism intensity of E. hypomesi was mostly stable (1.2 parasites/fish) in July and August. Meanwhile, the first infection by E. wilsoni occurred in mid-July, three weeks after the first E. hypomesi infection, and the prevalence of parasitism was 3.0% in July and 5.6% in August. The mean parasitism intensity by E. wilsoni was 1.1–1.3 parasites/fish. The prevalence of parasitism by E. hypomesi was higher than that of E. wilsoni, likely because of the low salinities in the lake. The timing of ergasilid infections may be affected by the morphological development of hosts. Most individuals of E. hypomesi attached to the fins and body surface of hosts, corresponding to the development of fins in these young fish. Meanwhile, because E. wilsoni attached to the gills mainly and the intensity of parasitism by E. wilsoni increased with the growth of host fish, their infection might be delayed until the complete development of gills.