The potential fish species pool of the Kubo, Tochikura, and Ichinono river systems (tributaries of the Iwai River, Kitakami River drainage), Iwate Prefecture, northern Japan, was compared with the observed ichthyofauna by using historical records and new field surveys. Based on the literature survey, the potential species pool comprised 24 species/subspecies but only 20, including 7 non-native taxa, were recorded during the fieldwork. The absence during the survey of 11 species/subspecies from the potential species pool suggested either that sampling effort was insufficient, or that accurate determination of the potential species pool was hindered by lack of biogeographic data and ecological data related to the habitat use of the species. With respect to freshwater fish conservation in the area, Lethenteron reissneri, Carassius auratus buergeri, Pseudorasbora pumila, Tachysurus tokiensis, Oryzias latipes, and Cottus nozawae are regarded as priority species, and Cyprinus rubrofuscus, Pseudorasbora parva, and Micropterus salmoides as targets for removal.