Abstract
Kirikuchi charr, Salvelinus leucomaenis japonicus, the southernmost population of the genus Salvelinus, is endemic to the Kii Peninsula, central Honshu, Japan. Kirikuchi charr are currently threatened due to anthropogenic disturbances. To conserve Kirikuchi charr, we investigated long-term population dynamics of Kirikuchi charr and red-spotted masu salmon, Oncorhynchus masou ishikawae, which share the same habitat, in the upper part of the drainage in the Totsu River system, Nara Prefecture, from August 1992 to September 2004. Population sizes showed a declining trend for both species; particularly after 2000, the population of Kirikuchi charr was approximately one-third of that in 1992. In 2004, the estimated number of salmon was approximately two times that of Kirikuchi charr. To evaluate the present population structure of both species, their population density and body size structure were investigated in 11 stream sections selected in the drainage. The majority of Kirikuchi charr were distributed in the upper part of the main stream and tributaries, and only a few fish were captured in the lower reaches of the main stream. In contrast, salmon inhabited the entire drainage without any obvious trends. Furthermore, almost all age-0 Kirikuchi charr were captured in the tributaries, whereas age-0 salmon were captured in the entire drainage basin. The average body lengths of Kirikuchi charr at age 0 and >1 were respectively smaller than those of salmon in all stream sections. These results suggest that salmon are competitively dominant over Kirikuchi charr. Based on the ecological evidence, we discuss conservation strategies for Kirikuchi charr.
