Dams, wires and hydraulic drop, in river systems threaten the viability of aquatic organisms through disturbances of life history and meta-population structure. Although influences of large construction have been well studied, these of small construction such as step at the connection between channels have been often ignored. In Japan, river beds are often excavated to keep capacity of discharge, thus steps are formed between the river and the surrounding channels such as draining ditches of rice paddy. Such steps may lead the fragmentation of population of organisms with life history of moving through paddy fields, canals, and rivers. Lethenteron sp. N, a threatened lamprey, spawn in the upper most of streams of rice paddy systems. They move within waterways in the growing phase. Thus, steps in channels will have a negative influence on their population. We investigated the size structure, seasonal usage of habitat and the possibility of movement among nearby channels to clarify the effect of fragmentation for lamprey in Chiba Prefecture, center of Japan.
Results/Conclusions
The distribution of the lamprey changed seasonally, suggesting that they inhabit with moving in the water system. Local conditions of the habitat, such as particle size of substrate, water temperature, quality and velocity of water, were suitable for the species. The abundance and size structure were different between lower and upper of the step, i.e., the abundance was higher in the upper side of the step but the average size was higher in the lower. It suggests large individuals are drop into downstream. We examined 11 channels in the water system whether channel was dis-connected. Nine channels were fragmented by step in the most downstream point. This result suggested that moving of lamprey is strongly prevented. Re-connection of streams can be an effective measure to keep population viability.