A Study on an Analysis of an Annual Change of Distribution of Short-necked clam in a Tideland Considering Huge Human Impact

Many short-necked clams inhabit tideland, and it is generally thought that the short-necked clams take a part of a water purification function of the tideland and play important roles of an ecosystem of the tideland. \´SHIO-HIGARI\´ (Shellfish-gathering to catch short-necked clams) is popular leisure for Japanese. In the tideland in the suburbs of a big city in particular, it is anxious about the drying up of short-necked clam resources by SHIO-HIGARI. Indeed, much of adult short-necked clam disappears after the SHIO-HIGARI season at the tideland. We have been investigating quantity of living short-necked clams continuously at "Uminokouen" which is located at south east of Yokohama in Tokyo bay. As a result, the actual situation of human impact of the SHIO-HIGARI on the quantity is becoming clear. On the other hand, the reason why such a spatial distribution of the amount of the short-necked clams in the tideland becomes has not understood yet. Generally it is said that the distribution of short-necked clams depends on emersion time, sea floor topography and geology. In this study, at first, based on our investigation, we discuss relations among the amount and the spatial distribution of the short-necked clams and the emersion time and the sea floor topography. Furthermore, we investigate the relation among the initial spatial distributions of the short-necked clam, the spatial distributions of the amount of the caught the short-necked clam and the visitors\´ loci. Then we predict the human impact on the change of the short-necked clam during the season. Finally we analyze the annual change of the short-necked clam in the tidal land considering such huge human impact, the seabed topography and the emersion time.