Ocean acoustic tomography, which has developed as a three-dimensional mapping tool of deep-sea temperature and current, is now applied to coastal sea measurement. Long-term measurement of temperature and current fields in the coastal sea is still difficult, espcially in the sea around Japan, which is chracterized by heavy shipping traffic and fisheries activity. This difficulty can poptentially be overcome through the application of coastal acoustic tomogrpahy (CAT). It is expected that CAT will promote progress in the coastal oceanography of Japan in the near future.

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Ariake Sound, southern Japan, has a huge area of tidal flats where the clam Ruditapes philippinarum is one of the major fishery products. This is also true of Kumamoto Prefecture bounding the Sound. In the past, annual catch yields of the clam in Kumamoto occupied over the half of the total catch yields of the clam in Japan. Although it reachied a peak of 65.5×10³ ton in fiscal year (FY) 1977, the catch drastically decreased in FY 2000 to about 1% of the maximum catch year. This is also true of the other clams important commercially to the sound. Total catch yields of R. philippinarum in Japan reached a peak of 140-160×10³ ton per year in FY 1975-1987, and then showed drastic decreases of the yields. Preliminary analyses of annual catch yield statistics of the clam in all Japanese regions indicate that the cause of decreasing yields may be different depending on fisheries regions, that is, the causes in Kumamoto may be shared with those of other prefectures bordering Ariake Sound, though the causes for R. philippinarum within the Sound may be different from those for other clams important commercially to the Sound. On the bases of analyses of historical data on annual catch yields of R. philippinarum and environmental conditions in the Sound and on several studies dealing with larval recruitment processes as determinants of benthic population size of the clam, one of the causes may be higher larval mortality, rather than decrease of spawning stocks due to overfishing. Short- and long-term studies are proposed in order to identify the causes for R. philippinarum in the Sound and also to make clear mechanisms by which the clam populations may be maintained.

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The method of Argo float termination is discussed from legal, technical and scientific points of view. According to our domestic law, a label specifying the ownership of the float is necessary, and having a plan to retrieve the float after the mission is preferable in order to avoid a claim of ocean dumping. In this context, we are to terminate the mission of the float so as to allow it to drift on the sea surface after a certain times of dive. For a better retrieval plan, the behavior of the float after the mission is estimated by using a global surface drifting buoy database prepared by the Atlantic Oceanographic and Meteorological Laboratory, which shows 17% and 7% of terminated buoys will be stranded and picked up, respectively, within a year. In the Pacific Ocean, the stranding rate of terminated Argo float will be low in the subtropical gyre, high in the subarctic gyre, and middle in the equatorial area, where the floats tend to be stranded on islands near the equator, New Guinea, and the Philippines within a year. An example shows that retrieving an Argo float is practicable, and suggests that attachment of fouling organisms to the float is possible to sink it within a year after termination. However, positive retrieval should be encouraged because it is very useful for checking the sensor stability and for data quality control.

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In early March 1992, a steep temperature rise of about 5.5°C for 5.5 days was observed in the eastern part of the mouth of Suruga Bay and the western part of the bay head. The warm water in the eastern part of the bay mouth was influenced by an intrusion of the Kuroshio warm water. The intruded warm water moved cyclonically along the bay coast. The propagation speed of the warm water estimated from the time lag of the steep temperature rise and the distance between two mooring stations is about 0.79 m s^-1. The estimated speed is similar to that of the coastal density current in a rotating fluid, 0.93 m s^-1, theoretically derived by the same method as Kubokawa and Hanawa (1984). As a similar event that previously occurred in Sagami Bay was called kyucho, the steep temperature rise in this case is also called kyucho. The intrusion of the Kuroshio warm water into Suruga Bay as kyucho is related to a northward shift of the Kuroshio path. This northward shift seems to be related to the occurrence of a small scale meander of the Kuroshio off Irosaki. The temperature change induced by kyucho, has a little effect on the sea level change in the bay.

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In considering the equilibrium heat balance in the Japan Sea, net heat loss through the sea surface is accomplished under the following two conditions; 1) a closed deep basin, and 2) lateral heat inflow transport to the southern upper layer, i.e., inflow of the Tsushima Warm Current (TWC). This is realized by heat transport through abyssal waters, because the downward heat supply from the TWC to the abyssal water must be balanced by heat loss from the sea surface somewhere in the northern Japan Sea. Therefore, the northern deeper waters continuously transfer heat to upper waters and both waters are easily mixed in the vertical direction. Such a heat condition would produce a significant thermal front as the northern boundary of TWC, i.e., polar front. In relatively cold winter, the surface water in the northern Japan Sea becomes dense enough to sink to the bottom due to the surface cooling. This homogeneous cold and dense water is called Bottom Water (BW). In ordinary years, after BW is newly formed, the heat transport takes place in a relatively shallow depth above the BW. It is likely that the downward diffusive heat supply from this shallow heat transport path gradually decreases the thickness of the BW layer.

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Among possible causes of the tide and tidal current decrease in Ariake Bay, the present author has shown from analysis of observed data that the effect of topographic change due to the big Isahaya reclamation project is most important. On the other hand, Tsukamoto and Yanagi performed numerical simulations and concluded that the effect of decrease of the tide in the outer sea is most important and the effect of the above topographic change is minor for decrease of the tide in Ariake Bay. These two different results are compared and examined, and the following conclusion is obtained. For the M₂ constituent on an open boundary in their numerical simulations, Tsukamoto and Yanagi assumed that the amplitude decreased by 2.5 cm after the construction of a sea dike in the reclamation project. Observed data show, however, that this decrease is overestimated, causing the effect of the topographic change to be underestimated. The data analysis by the present author was based on the assumption that the tide at the mouth of Ariake Bay is only slightly influenced by the topographic change in the interior of the bay. This assumption is proved to be valid from comparison with observed data and from a simple theory. Thus, it is concluded that the recent decrease of tidal amplitude in Ariake Bay can be mainly attributed to the topographic change due to the reclamation project and the effect of decrease of tidal amplitude in the outer sea is secondary. An examination of the decrease of tidal current in Ariake Bay supports the same conclusion.

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