The distribution and abundance of an endangered subspecies of the ayu Plecoglossus altivelis ryukyuensis was determined on the three major islands, Amami-oshima, Tokunoshima, and Okinawa Islands, in 1986. The fish was observed only in five streams in the southern part of Amami-oshima Island. Among these five streams, the majority of fish were found in the Yakugachi River. While the number of fish in this particular stream was approximately 3.5×104, in all other streams the densities were much lower, and we therefore conclude that this subspecies is threatened and some attempts should be made for conservation of this fish.
