Relation between the Kuroshio and the high salinity region of the North Pacific Ocean, and the role of the high salinity region in the variation of sea condition in each ocean

It has been known that the route of the Kuroshio is subject to variation, but neither the cause of the variation nor the character of the Kuroshio itself has been fully understood. Previously the author reported the following three facts: (1) The Kuroshio is a part of the boundary current of the geostrophical current system constituted round the high salinity region in the North Pacific Ocean. (2) According as the spreading of the high salinity region varies yearly (Table 1), the geo strophical current system is subject to deformation, which results in variations in the route of the Kuroshio. (3) Variation in the Kuroshio's route brings about changes in the sea conditions in the neighbouring seas of Japan. As is seen in Text-fig. I, we can recognize in each ocean a high salinity region like the one found in the North Pacific. Then, it may be allowed to suppose that phenomena analogous to (1)-(3) should take place in and around each of these high salinity regions. The author, therefore, is strongly inclined to believe that a high salinity region works as the acting center of the variations of sea conditons in each ocean, and that oceanographers must pay more attention to these high salinity regions and study the relation between the spreading of these regions and the accompanying variations in sea conditions. Furthernore, high salinity regions of different oceans differ in the value of salinity (Text-fig. 1). There should be causes for such differences. One of the suspected causes is the sea-weed floating at the surface of the ocean; evaporation from the sea surface is accelerated by the presence of floating seaweeds, resulting in a higher salinity. The author made some experimental observations in the rate of evaporation from the sea water containing sea-weeds in comparison with that from the free sea water. The results are shown in Table 2

An opinion on the forecast of fishing grounds and on the fundamental sea states from the water temperature distribution in the neighboring sea of Japan

If we can predict the hydrographic condition of a sea area, we may well be said to have a ground for inferring the locations of fishing grounds and also the state of fishes. With our present knowledge of oceanography and fish behavior, however, it is often very difficult to predict the sea conditon of a sea area, much more the locations of fishing grounds therein. This is partly because a sea area can be called a fishing ground only when a certain amount of fish catch is expected from it by a commercial fishing method, and partly because distribution and movement of fish schools depend upon both the environmental condition and the fish's habit. In this connection, the writer stresses the importance of studying hydrographic and ecological factors intensively in fishing grounds from the viewpoint that fishing grounds are the environment for the life of fish. The writer describes a few conspicuous hydrographic features found in the Japanese coastal waters and in the neighboring Oyashio and Kuroshio systems. In Table I are summarized the typical seasonal north- and southward movements of the monthly isotherms for the surface tmeperatures of 5°, 10° and 20°C. The 5°C isotherm can be regarded as representing the boundary of the Oyashio water mass, and the 20"C isotherm, that of the Kuroshio water mass. By examining the year-to-year variations in the position and movement of these isotherms, one can deduce a fundamental hydrographic pattern of this sea region. In Figs. 2, 3 and 4 are shown some typical distributions of the current and water mass systems in the offshore waters around Japan. The writer is of opinion that the following are prerequisite to making reliable forecast concerning fishing grounds. 1. Various environmental factors in fishing grounds are measured concurrently with fishing operations, and the data of such meaurement are accumulated. 2. The biology, particularly the habit and the migratory range, of each economically important fish species is studied thoroughly. 3. In forecasting a fishing ground of a migratory fish, attention is paid to the movement of that isotherm which represents the lowest temperature which the species normally inhabits. 4. In infering the distribution of the egg and larva of a fish, attention is given to the current sytsem which washes the spawning area, and to those current rips which are formed along the border of that current system. 5. The efficiency of various fishing gears is studied from the viewpoint that it varies according to the sea condition and the behavior of fish school. 6. The possibility that fish, through learning, may acquire the ability to avoid the fishing gears of usual types is takne into account

Chemical composition of pond waters on the ilands located in the Seto Inland Sea

Freshwater ponds are found on a number of islands located in the Seto Inland Sea. The author made observation of the chemical properties of pond water at some of the ponds located on the islands in the western part of the Seto Inland Sea. The primary purpose of this survey was to know the effect of the "sea breeze" from the chlorine contents of pond water. Some of the surveyed ponds were utilized for irrigation. Almost all the surveyed ponds were relatively shallow, and dissolved oxygen was poor in the bottom layer. Chlorine contents were considerably lower than expected. Other chemical compositions and their contents in each pond were not extraordinary; they were of the same order as the average values reported by Dr. S. YOSHIMURA for the lake waters of Japan. The results of the observation and water analysis are shown in Table I. It seemed that all the surveyed ponds could be utilized for the culture of freshwater fishes

On the tldes of the Seto Inland

We studied on the following elements of the tides in the Seto Inland Sea. (I) Meteorological tides observed at four tidal stations (Uno, Takamatsu, Matsuyama and Kure) during the recent five years, 1952-1956. (Appendix Table 1.) (2) The profiles of the tidal level along the east-westward section of the Sea in different seasons. Osaka, Kobe, Akashi, Takamatsu, Imabari, Hashihama, Mitsuhama and Dannoura were selected as the standard stations for the calculation of the sea level. (3) Harmonic and non-harmonic constants for the tidal stations located in the Sea. (4) The time difference and the ratio of the range of tide for the tidal stations in the Sea. I. Meteorological tides at the four stations were computed numerically with the following formula: Δh = Ht - (Ht-25 + Ht+25) / 2 Δh : Height of the meteorological tide. Ht : Observed height of tide at the time when the meteorological tide Δh occurred. Ht-25, Ht+25 : Observed heights of tide 25 hours before and after H, was observed, respectively. II. Simultaneous heights of tide at the 15 base stations (which are distributed between Osaka and Dannoura over a distance of about 260 nautical miles (Text-fig. I)) are shown in Text-figs. 2-1 to 2-8 for different seasons of the year. From these figures we can see the sectional configuration of the sea level in various seasons, and are enabled to predict the districts where local slope currents will occur. III. By taking advantage of the tidal currents, one can greatly reduce the time of navigation in traversing the Seto Inland Sea from the east to the west (or in the reverse direction). For example, as is shown in Text-figs. 3-1 to 3-3, a ship with a self-propelling speed of 6 knots and cruising from Osaka to Moji (about 260 nautical miles) can save about 7 hours by making the best use of tidal currents, as compared with the same ship navigating against tidal currents. IV. The latest values of harmonic constants for 142 stations are summarized in Appendix Table 2. Non-harmonic constants for these stations were calculated from the latest data of tidal observations and are summarized in the following charts: 1. Mean high water interval (Text-fig. 4-1) 2. Diurnal inequality (Text-fig. 4-2) 3. Mean tidal range (Text-fig. 4-3) 4. Spring range (Text-fig. 4-4) 5. Neap range (Text-fig. 4-5) 6. Spring rise (Text-fig. 4-6) 7. Neap rise (Text-fig. 4-7) V. In Appendix Table 3 we have shown the new time differences and ratios of the range of tide to be applied to the results of tidal observations at the 138 stations located in the Seto Inland Sea and Tosa Bay. In computing these values, we selected as standard stations those 20 stations whose tidal elements had been observed and published in the tide tables by the Meteorological Agency or by the Maritime Safety Agency

Hydrographic conditions of Fukuyama Habor

A series of hydrographic observations were carried out from September, 1958 through July, 1960 in order to clarify the physical and chemical properties of sea water and bottom muds of Fukuyama Harbor, Hiroshima Pref. (Text-figs. 1,2, Table 4). The results are presented in Tables 6, A-F and Text-figs. 4-12 and discussed in comparison with previously published data on the hydrography, ecology and fishery of this water area. Fukuyama Harbor is the former estuary of Ashida River. It occupies the western part of Kasaoka Bay and measures 6.1km2 in area. Being trapezoidal in shape, it is bordered by a reclaimed land in the west, by hilly peninsula in the south and north, and opens wide in the east. The fiat muddy bottom slopes down gently toward the east. Water depth does not exceed 7m below the mean sea level at any part of the harbor. Tidal flats are exposed along the shore at low waters. Water temperature differs very little from air temperature all the year, the monthly mean varying between 7o (January and February) and 28oC (August) at the surface (Text-fig. 3). Water is turbid with the Secchi disc depth seldom exceeding 4 m. These two features can be ascribed partly to such local conditions as the small water depth and the muddy bottom, but are principally due to the fact that the water can not freely pass from the high sea (i. e., the Pacific Ocean off Bungo Strait) to this locality owing to the channels and shallow seas lying on the course. Monthly mean of chlorinity varies within the range of 16.6-17.9‰ at the surface (Text-fig. 3). Seasonal variation of chlorinity is not very great and reaches minima in summer when local precipitation and the drainage of Ashida River reach maxima (Tables 1, 2). Tidal range is comparatively great owing to the interference of the two tidal waves traversing the Seto Inland Sea in opposite directions, one wave from Bungo strait toward the east and the other from Kii Strait toward the west. In an average tide, tidal range measures 2.2m (Table 3) and 50% of the water that is present in the harbor at the high water is drained off during the ebb. Since water is mixed and replaced by the tide, vertical stratification seldom develops and the water is rich in dissolved oxygen from surface to bottom throughout the seasons. Tidal current, however, is not very fast. At low tides C.O.D. increases and dissolved oxygen decreases in the water in the northwestern part of the harbor, where polluted water is discharged from Fukuyama Inlet. In rainy months fresh water is discharged from flood-gates at low tides, temporarily lowering the chlorinity of the surface layer nearby. In the area where the effect of the polluted water or fresh water is not appreciable, various measurements on water are generally within the following range: water color in Forel's scale, 5 or more; pH, 8.2-8.3; C.O.D. by Saeki's alkaline permanganate method, 0.9-1.9ppm; acid-soluble total iron by aa' –dipyridyl method, less than 0.05ppm. The water near the sea bottom often gives greater values of C.O.D. and acid-soluble total iron than those mentioned above. The bottom mud is principally composed of the silt of particle diameters between 2 and 20f-b. The bottom is harder on the inshore side of the 2m depth contour than on the offshore side: the penetration value obtained with the Furukawa's penetrometer averages 30 and 50cm respectively. Other measurements on the bottom mud are within the following range: ignition loss, 3.2-13.9%; organic carbon by Tiurin's rapid titration method, 4.3-18.2mg/g (chlorine error not corrected); total nitrogen by Kjeldahl method, 0.34-1.64mg/g (Table 6). Carbon-nitrogen ratio (chlorine error corrected) is usually close to I 0. Ignition loss, Kjeldahl nitrogen and the oxygen consumption of mud (measured at room temperatures) respectively hold linear relation with organic carbon content (Text-fig. 13). Bottom mud is rich in organic matter and gives small carbon-nitrogen ratio in the area affected by the drainage from Fukuyama Inlet, flood-gates or Ashida River. Major fisheries in Fukuyama Harbor are the "masu-ami" fishery and the culture of the ark shell (Anadara subcrenata) and the laver (Porphyra tenera). The "masuami" is a pound net consisting of a pound 25m wide and 13m long and a leader net about 40m long. It is usually set between the 0 and 2m depth contours, being held in place by bamboo poles driven into the bottom (Text-fig. 14). Fishes, crustaceans and cephalopods are trapped in it as they move with the tide. In 1959 about 70 nets were operated in Fukuyama Harbor with the total catch of 79 metric tons. In the same year, 333 tons of ark shell and 1.8 tons of dried laver were produced by culture and 390 tons of littleneck clam (Venerupis semidecussata) were harvested from the natural beds on tidal flats (Table 7). All the species represented in the commercial catch are typical inhabitants of such inshore waters where water is relatively turbid and seasonal variation of water temperature is great.主として昭和33 ，34年度農林省農林漁業試験研究費によっ