The influence of a dam having no fishway on the distribution of decapod crustaceans in the Yukinoura river, Nagasaki, Japan.

Samplings of decapod crustaceans were conducted in the Yukinoura River to estimate the influence of a dam having no fishway on the distribution of these animals. Eight decapods were collected from freshwater area. Two of them, Geothelphusa dehaani and Neocaridina denticulata, were landlocked species, and the others were amphidromous or catadromous species. The present fauna can not be compared directly with the fauna before the dam construction because of the absence of comparable studies. Comparing the species composition from respective sampling stations, however, the influence of the dam on their distributions can be estimated as follows: (1) Caridina typus, Macrobrachium japonicum and Macrobrachium formosense could no longer get by the dam into their former habitats. (2) Eriocheir japonicus could reach the dam lake but stopped migrating upstream into its former habitat. (3) Caridina leucosticta had the upper part of its former habitat destroyed by the dam and dam lake. (4) N. denticulata had an expansion of distribution because of the still water environment in the dam lake. (5) G. dehaani was unaffected by the dam. At first glance, Caridina japonica, a resident in the uppermost stream, can be categorized in the fifth category, but its population density seems to have decreased after the completion of dam in 1976.