Occurrence of a Skin Parasite <i>Argulus coregoni</i> (Branchiura: Argulidae) on Salmonids in Mountain Streams, Central Japan, with Discussion on Its Longitudinal Distribution and Host Utilization in Rivers

Abstract

Argulus coregoni Thorell, 1864 was collected from white-spotted char, Salvelinus leucomaenis (Pallas, 1814), red-spotted masu salmon, Oncorhynchus masou ishikawae Jordan and McGregor, 1925, masu salmon, O. m. masou (Brevoort, 1856), and a hybrid between white-spotted char and masu salmon in mountain streams, Gifu Prefecture, central Japan. The host’s body surface under and near the pectoral fins was the most common infection site for A. coregoni. The infected white-spotted char were caught at high elevations (461–873 m) in the headwater streams. The infected individuals of the two masu salmon subspecies were mostly caught in the upper river reaches, but the elevations where they were caught were lower (237–733 m and 660–707 m, respectively) than those of the white-spotted char. Since A. coregoni is also known as a parasite of ayu, Plecoglossus altivelis altivelis (Temminck and Schlegel, 1846), in the middle and lower river reaches in the prefecture, this parasite utilizes different fish species as its hosts along a river course: white-spotted char and the two masu salmon subspecies serve as the hosts, respectively, in the headwater and the middle to lower sections of the upper reaches, but in the mid- and lower river reaches, ayu is the important host.