The ayu population in the middle of the Ryukyu Archipelago was recently found to differ considerably from that in the mainland of Japan, and was described as a distinct subspecies (Plecoglossus altivelis ryukyuensis). Little is, however, known about the border between the distributional ranges of nominotypical and the Ryukyuan subspecies. Observation carried out in the northern Ryukyus showed that the ayu also occurs in a few streams with relatively long middle reaches on Yaku Island, but that none inhabit Nakanoshima Island. Variations in meristic characters of specimens from Yaku Island were within those of the nominotypical subspecies. Allozyme analyses by starch gel electrophoresis showed that the ayu from Yaku Island was genetically much closer to the nominotypical subspecies from Kyushu and Honshu than to the Ryukyuan subspecies from Amami-oshima Island. We thus conclude that the ayu on Yaku Island represents the southernmost population of the nominotypical subspecies in the area of the Japan-Ryukyu Archipelago, and that the ayu on Amami-oshima Island forms the northernmost population of the Ryukyuan subspecies. Complete allele substitution detected in five enzyme loci, as well as the absence of any particular genetic similarity, between the populations of Yaku and Amami-oshima Islands, reinforces the previous hypothesis that the two subspecies have been isolated from each other genetically for a long period of evolutionary time.
