Large volume of mussels attaches on the coastal structure in eutrophic area. The dropped and died mussels accelerate the depression of dissolved oxygen in the bottom layer. This study revealed the relationship between fluctuation of salinity and temperature and their attaching activity on the wall. Result of our investigation at Amagasaki Harbor in Osaka Bay showed that the mass of mussels dropped out at the bottom after the salinity and temperature had extensively changed. It was also observed in Komatsushima Harbor that the salinity and temperature jumps initiated the dropout of the mussel. From laboratory experiments in different environmental conditions; salinity and temperature, we cleared the ecological cycle of the mussels in which the changes in salinity and temperature mainly influenced their activities.