Abstract
The genetic structure of two fluvial forms of the red-spotted masu salmon, amago (ordinary form) and iwame (markless form), collected from two headwater streams, the Kohbaru Stream and the Hakiai Stream, of the Ono River, Oita Prefecture, was examined using mtDNA to identify whether they were indigenous to the streams. The Hakiai Stream consists of two tributaries, Mennotsura-dani and Manryo-dani. The iwame population occurs only in Mennotsura-dani, and since 1970, fishing has been prohibited for the iwame and amago populations in most locations of Mennotsura-dani and Manryo-dani, respectively. One haplotype (Hap-1) was shared by three isolated populations examined: it was fixed (100%) in the iwame population from Mennotsura-dani and dominant (90.5%) in the amago population from the upper Kohbaru Stream, but was infrequent (10.0%) in the amago population from Manryo-dani. Hap-1 was also frequent in the samples of amago fry caught in the Kohbaru Stream. Based on these results and the long persistence of the iwame population in Mennotsura-dani, both the iwame population in Mennotsura-dani and the amago population in the upper Kohbaru Stream are considered to be indigenous, whereas the amago population in Manryo-dani appears to have been mostly replaced by hatchery-reared fish informally stocked in the upper location.
