Massive occurrences of juvenile clams Ruditapes philippinarum have been appearing every spring at Rokujo tidal flat located in the Toyo River mouth region at the eastern part of Mikawa Bay. These have been collected by licensed fishermen and transplanted into the coastal shallows of Ise and Mikawa Bay. However, the Manila clam population have been showed a tendency to decline sharply from autumn to winter in recent years. To understand this phenomenon, we conducted field observation about its population change, catch effort and the appearance of predators from autumn 2015 to spring 2017. In addition, we continuously monitored water temperature, chlorophyll a and near-bottom currents condition. At first, we examined growth and the survival situation of clams by the usual growth model that assumes water temperature and the quantity of phytoplankton as variables, using data from autumn to winter 2015, when mortality was remarkable. However, we couldn’t represent the physiological death. Then, we improved the model that considered an energy loss of dig caused by wind wave, too. As a result, population changes were correctly expressed. Therefore, we find that an energy loss might have an important role for growth and the survival situation of clams in the shallows where wind wave is remarkable especially in the case of insufficient food condition.

A massive bloom of Alexandrium spp. occurred in Onagawa Bay, in the middle coastal region of Miyagi Prefecture, during spring 2018. Shipments of Japanese scallop Mizuhopecten yessoensis were subsequently regulated for the first time in 38 years, due to paralytic shellfish poison. Therefore, we investigated the growth factors of Alexandrium spp., which cause paralytic shellfish poisoning in the marine environment, and its relationship with a competing species; diatoms. Around the coastal area of Miyagi Prefecture, the influence of warm water became stronger during spring 2018. It was considered that one of the factors of Alexandrium bloom was the development of thermocline due to the influence of warm water, which provided a favorable growth environment for Alexandrium spp. Diatom population composition in the region was also unusual during this period due to the increased water temperature. Thalassiosira nordenskioeldii and Chaetoceros debilis, which are usually the primary constituents of the spring diatom bloom in Onagawa Bay, did not grow. Reduction of these species may have contributed to the failure to suppress the growth of Alexandrium spp. It was considered that the bloom of Alexandrium spp. occurred in Onagawa Bay due to these multiple factors. After the bloom of Alexandrium spp., cyst quantities in Onagawa Bay increased. The cysts may functioned as a “seed population” to re-proliferate when the environment became favorable again. Due to the recent trend of increasing coastal temperatures, it is necessary to pay attention to the occurrence of paralytic shellfish poison in the middle coastal area of Miyagi Prefecture.