福山港水域の海況の概要

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.