A common freshwater goby, Rhinogobius brunneus, consisting of diadromous and landlocked races, is widely distributed throughout the Japanese Islands. Using body color and color pattern as taxonomic characters, MlZUOKA (1974) and MlZUNO (1976) recently reported that the diadromous fish can be classified into 6 different types. During the course of our limnological (N.I. and S.M.) and histo-endocrinological (Y.H.) studies, we noticed these polymorphic variations in this fish caught in Niigata districts. Thus, based on the color pattern, we intend to elucidate the distribution and intraspecific differentiation of this species occurring in Niigata area. The first report of this series of researches deals with the results obtained in Sado Island on the Japan Sea, one of the largest islands attached to the Main Island of Japan (Honshu), lying 32 miles west of Niigata City. From May to December, 1977, 1345 specimens of fish consisting of both sexes were collected by us from 69 places of 34 rivers and streamlets. Four types were discriminated among the specimens; cobalt-, cross-band-, orange- and dark types. Cross-band type was the commonest race inhabiting the lower reaches of the rivers. Orange type seemed to be the lacustrine race which was found exclusively in Kuninaka Plain located in the central part of the Island. Cobalt type was widely distributed in the mountain streams, and black type was confined only to three streams. Distribution of black type in Sado Island located in rather high latitude is noteworthy, since this type is regarded as stenothermal and warm-water fish, and the records of collection were restricted in the limited region of southern Japan.
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