Habitat use by endangered Japanese crayfish (<Emphasis Type="Italic">Cambaroides japonicus</Emphasis>) in low-gradient streams of southern Hokkaido,Japan: reach and microhabitat-scale analysis

The Japanese crayfish (Cambaroides japonicus), the only native crayfish in Japan, is endangered and has experienced rapid population declines. We surveyed the habitat requirements of Japanese crayfish at the reach and microhabitat scales in semi-natural low-gradient streams. Habitat use by Japanese crayfish differed between the spatial scales. Reach-scale analysis revealed that the bed slope was the only positive predictor of crayfish density. This finding indicates that relatively high-gradient reaches, such as headwater reaches, are an important habitat for the conservation of Japanese crayfish in low-gradient streams. Microhabitat-scale analysis showed that crayfish density was positively affected by substrate coarseness and the presence of instream vegetation cover (bank vegetation, woody debris, and leaf patches), whereas it was negatively affected by distance from the stream edge. Coarse substrates and vegetation cover may function as shelters from water flows and predators during low flow periods. The use of stream-edge areas may allow quick access to refugia and enable the crayfish to avoid unexpected flood disturbance and predation. These habitat characteristics should therefore be preserved for the conservation of Japanese crayfish, and scale-dependent habitat characteristics should be considered in future conservation plans.