The seakeeping qualities of fishing vessels in Japan are generally poorer in small boats than in larger vessels. Ship motion frequently exhibits strong nonlinear responses to external forces due to the typical ship forms. This has resulted in many serious marine accidents and fatalities. To improve the safety of small fishing boats with structural vulnerabilities, it is necessary not only to understand their basic stability, but also to estimate accurately the structure of the dynamic response characteristics for external forces. In addition, the regulation of gross tonnage (GT) in the Fishing Vessel Act resulted in the restriction of various improvements of fishing vessels. Many scientists studying fishing vessels pointed out this problem, and in 2018, the law was finally revised, so the structure and safety are expected to improve in near future.
In this paper, I show some typical methodologies for studying the dynamic response structure to improve the safety of fishing vessels currently in use and of future fishing boats.
In order to evaluate the effect of artificial rock solidified dredged soil on the organisms, we observed the appearance of diatoms, seaweeds, sessile/mobile animals, and fishes for about 2 years after setting the reef making a comparison with natural stone. And the nitrogen and carbon stable isotope ratios of the biological communities gathering on artificial reefs were compared with surrounding area.
For seaweeds, their appearances differed depending on the species, Sargassum horneri was detected on artificial stone, and Ecklonia cava tended to habit on natural stone. No clear difference was observed between the artificial rock and natural rock in the whole community structure of the sessile and mobile animals. In addition, difference among the taxonomic groups were also small from the viewpoint of appearance status resulting from the cluster analysis. However, comparing with the prey organisms in the stomach contents of fish, many Serpulidae, polycheats appeared in artificial rock. Some fishes preying on them like Acanthopagrus schlegelii and Oplegnathus fasciatus had a greater populations at prey-rich artificial rock, which suggests the relationship between biomass of prey organisms and population of fish. Although the species number of fish differed between artificial rock and natural rock, specific relationship was found neither in the raw materials, nor in biomass of prey and seaweed. The stable isotope ratios of nitrogen and carbon suggested that the artificial rock has a food linkage structure equivalent to that of the surrounding area.
Recentry, various types of mud samplers have been used for collecting bottom sediment. Usually, Smith-Mclntyre sampler or Ekman grab sampler are used for take sediment sample. However, these conventional mud samplers are very heavy, and sometimes become danger operation in rough sea condition. The other hand, only about same volume as one spoon as, is need for analysis sediment component. Therefore, we tried to develop a new compact mud sampler which can be operated by a rod and an electric reel, and evaluated its usefulness compared with conventional grab type mud sampler. As the result, we can make one prototype compact mud sampler, which size has 1.6 kg weight in the air, L=5 cm, B=4 cm, D=7 cm and sampling volume is 140 cm3. And, we carried out actual sea experiment (Sea depth was ranged 3 m to 76 m). From this experiment, we can say that new type compact mud sampler is usefulness in many cases. However, sometimes different sample was obtained, in case actual sedimentation particle size is more than 150 µm, but nothing to do with the sea depth.