In the preceding report, the authors treated of the operational limitations during scouting work, on the basis
of the characteristics of ship motion, by using an actual 135 GT purse seiner working in the fishing ground.
In the present paper, the main objective was shifted to the same problem during a cycle of shooting and
hauling work of the net. In order to examine this problem, the process of a cycle of fishing work was classified
into the six steps, and the ship motions and accelerations were measured under the four sea conditions.
From the results of spectrum analysis and statistical analysis of double amplitudes relating to rolling, the
probability of deck-edge immersion was estimated, and the discussion was concentrated to the change in safety
with progress of fishing work under respective sea conditions. And the discussion was extended to the safety of
the crew on deck during fishing work.
In the previous report, a method of estimating in-water image contrast using its black-and-white photograpf was revealed.
The present study proposes a new method of estimation by using televised image from video tape. The televised image of a test board was captured by video-capture program and saved as file into memory unit of a computer.
Using the Image 1.45 software, gray value of televised image was measured to estimate its contrast value Ci. From the relationship between contrast Ci and Cg. calculated from target and background gray values, a high correlation (r:0.95) was found. A relative equation between the Ci and the Cg was presented by the following equation : Ci=0.134 ln Cg+0.933. Using this formula, the application of the technique for small targets such as net set in a large sized water tank are discussed.
The relationship between distribution of juvenile japanese littleneck Ruditapes phillippinarum and physical
environment, especially bottom condition and water current, was investigated in west Mikawa-bay. Sediment
samples were collected by Smith-McIntyre grab. These samples were sieved with lmm mesh screen and bivalves
were picked up and measured shell length. The oxydised layer and the reduced layer of substratum were observed
throughout the transparent corer, and then upper 10 cm of cored sediments were used for particle size analysis.
The water current-of the bay was analyzed by the three dimensional hydrodynamic model. The juveniles always
distributed more abundantly in the mouth of Yahagi river than in other stations. The thickness of oxidized layer
in bottom sediment was more than 20 cm in the river mouth, whereas most of other station showed less than 10 cm.
The landward water current in bottom and lower strata of the river was observed according to the numerical analysis.
Thus, it was considered that the bottom condition and the water movement contributed to the survival of
juveniles and the entrainment of planktonic larvae to the habitat, respectively.