Abstract
Upper tidal river area is very important for maintaining population of the Japanese mitten crab Eriocheir japonica, because megalopa larvae settle and metamorphose to the crab stage in this area. I investigated the preference of environmental condition of the megalopa and young crabs of E. japonica by the field experiment in this area. Artificial grass mats (30 cm × 30 cm × 1.5 cm) with three coverage types (stone 25%, stone 75% and stone + reed vegetation 75%) were set in 11 sites in the upper tidal river area of the Saigo River, Fukuoka Prefecture, Japan. Settlement of megalopa concentrated in the rapid flow area right below the fishway, suggesting the positive rheotaxis of megalopa. Thus it was presumed that presence of weir and fishway within tidal river area strongly influences the distribution pattern of young crabs. Young crabs after metamorphosis tended to disperse from the settlement area to the surroundings as they grew larger. Density of crabs was higher under the presence of reed vegetation in the flat flow or pool areas, suggesting that reed vegetation was useful for the nursery ground of young E. japonica. The present result also showed that the method using the artificial grass mats was effective for the survey of E. japonica population.
