In this experiment, the salt water issued into a basin filled with freshwater at various condition. Results of this experiment are also compared with the free buoyant jet theory.
The following results were obtained :
( 1 ) The trajectory of buoyant jet agreed well with one of the free buoyant jet theory from nozzle to bottom.
( 2 ) Collision with the bottom promoted the entrainment of the buoyant jet issuing into a basin.
It is important to estimate dynamic tension of the mooring line for design of floating artificial reef as the magnitude of the dynamic tension is the same order or so as that of the static tension.
In this paper, we explain a simple calculation method for mooring line tension of floating artificial reef by current and wave, and compare the calculated results with the experimental results of Koterayama's experiment and experiments on two types of mooring lines in order to test sure the precision of the method, Moreover, we make it clear that variations of horizontal component of the dynamic tension at the lower end of mooring line became larger than that at the upper end of mooring line in some conditions.
Diurnal changes of dissolved oxygen content were investigated at the yellowtail and red sea bream fish net cage of Ehime Prefectural Fisheries Experimental Station on September 1989. Patterns of diumal changes in dissoIved oxygen content at the depth of 2m and those of 4m were similar during the daytime, but were different at night. Dissolved oxygen content at the depth of 2m and that of 4m sometimes increased transitionally at night. Dissolved oxygen content at the red sea bream fish net cage was less than that at the yellowtail fish net cage before feeding; and the latter was less than the fomer after feeding. These results suggest that stratification of density, exchange of sea water in the fish net cage and also the amount of food for fish had influence on dissolved oxygen content.
Fish gathering effect of precast・ neｓt blocks (GINRIN type and BOX type) for river revetment work was elucidated using various species of fishes such as carp, duce, eel, thread fin goby, roach and catfish which were stocked in an artificial canal partly installed with the two types of precast nest blocks. Actively swimming fishes such as carp and duce showed strong positive reaction against water current. Whereas bottom tending fishes such as eel, thread fin goby, roach and catfish showed a tendency of being attracted to the precast nest blocks. However, recapture rates in the canal after each experimental period of about three months were less than 45% because some bottom tending fishes hid between the blocks which made recapture impossible, and some fishes such as goby and roach seemed to be victim of camivalism.
All of species utilized the precast nest blocks falrly well but the bottom tending species stayed in the block more frequently than actively swimming fishes. More specifically, actively swimming fishes preferred GINRIN type which has large open-space and higher water exchange rate. Whereas the bottom tending fishes preferred the BOX type which has small dark-space with narrow opening. The environmental conditions such as water temperature and dissolved oxygen did not alter their preference. No clear-cut relationship was observed either between utilization of the precast nest block by the fish and day-length partly because the seasonal change of water temperature in the canal was not large enough throughout the experimental period.
The vorticity field was computed by three-dimentional numerical hydrodynamic model applying to Banzu tidal flat waters, in Tokyo Bay, and the juveniles of Short-necked clam were collected from 1984 to 1986. The simulated vorticity distribution was characterized by the large belt with higher magnitude of vorticity stretching in the northem part of the flat, Whereas small patchy regions with high vorticity were found in the southern part of the flat. This may be attributed to the tidal current condition in off-flat region near the tidal flat. In the northern part, tidal current showed relatively higher magnitude, therefore strong current shear can be formed in the interregion between tidal flat and off-flat region.
The significant correlation could be obtained between the distribution of abundance in the larval settlement of short-necked clam and the vorticity field. In addition, the highest abundance of the juveniles less than 20mm in shell length appeared in the proximity of Nakajima, Where the vorticity also showed higher value. It can be suggested that the high vorticity prevents larvae from dispersing to the off-flat water. Our analysis could reveal that the vorticity in the tidal flat area plays an important role on the larval settlement of short-necked clam and distribution of subsequent juvenile.
The distribution of bottom friction velocity on Banzu tidal flat waters in Tokyo Bay, waa simulated by using a 3D time-dependent hydrodynamic model combined with the wave field model based on mild sIope equations. It was found that the contour of friction velocity, u*=4cm/s, became a good indicator for seeing the tidal flat undulation. In the northem part of the flat, the sand bar were formed on this contour line, and the small undulations were formed distinctly outside of this contour line in the southem part. This undulation is called Se (small shoal) and Utari (depression) by fishemen.
For the number of short-necked clam collected in the Se and Utari, relatively large number of clam was found in the Utari for the case of u*‹4cm/s, and no difference for u*›4cm/s, And as the friction velocity became larger, the more dense patchy distributionis was formed. It became clear that the more patchy clam distribution appeared in the region of u*= 3-4cm/s for shell length less than 20mm, and u*=4cm/s for shell length larger than 30mm, respectively.
From our analysis, it was suggested that the friction velocity of u*=4cm/s might be optimum value for the short-necked clam distribution. If the friction velocity became larger than this value, the clam would be dispersed by wave action and advection inspite of larval settling behavior.
At coastal waters, seaweed beds provide nursery for fishes and food for shellfishes, but they are decreasing their habitat due to increased pollution of sea. Mitsubishi Heavy Industries, Ltd. and Shimizu Corporation have worked jointly to develop the system which cin provide sunlight undersea by tension-leg platform. The field tests are carried out over three years. The purpose of the field tests is to confirm the system working properly at sea and seaweeds grow well under the system. The tests gave satisfactory results and we can expect that the system will be one of the main unit of marine ranching system.
In November 1984, the First Marine Ranch using acoustic sound was set up at Saiki Bay. Subsequently, Motonoma and Hotojima Marine Ranches in May and in November 1987, respectively. Sea Bream fry, which had been trained by conditioned reflex of acoustic sound (300Hz, 50dB) with supply of food, were released at the Marine Ranches. Field surveys were conductted to examine the effectiveness of the acoustic sound in making fish shoal stay around the feeding buoy.
As a result, it was shown that fish shoal released in November stayed for a while at the area around the feedingbuoy.
It was also found that although, it moves to adjacent natural fish reefs to pass the winter when, water temperature falls to 15-16℃, it retums to the area of feeding buoy again when water temperature rises 18-19℃. The mean rate of recapture of stocking sea bream were 11.7 and 28.3% for 0+ and 1+ age groups, respectively. Although, the mean body length of the released sea bream was a little smaller than natural one, tOtalmortority coefficient was lower about 25% than natural one. As a whole, it was thus concluded that the Marine Ranches using the acoustic sound were successful in controlling fish shoal.