Plate-type heat-exchangers are increasingly used in the cooling systems of vessels. Those systems have numerous merits, whiie the disadvantage is decreased performance associated with the formation slime on the cooling surface of the heat-exchanger. A new cleaning system for plate-type heat-exchanger mixing particie of natural material in cooling water has been developed in order to solve the problem. Grains of walnut and zeolite, shell powder of scallop were used for the particle on the experiment. Evaluations of the slime removal efficiency were shown to be affected by kinds of the particle, injection quantity of the particle, flow of cooling water and washing time.

View full abstract

The movement of the trolling depressor is important for catching the target species. The purpose of this study is to clarify the dynamics of the trolling depressor by using the data-logger. A fiume tank experiment was conducted to measure the tension of the line and the attitude and the angular velocities of the trolling depressor. The length of the main line was 2.5m. The flow speed was 1.10m/s. In this experiment, a two times larger model of the trolling depressor was used to install the underwater motion measuring unit. The acceleration, the attitude and angular velocities of the trolling depressor were measured by using this unit at the intervals of 0.01 seconds. As a result of the experiment, it was possible to clarify the change of pitching angle, rolling angle, yawing angle of the trolling depressor, and each periods were 0.7, 1.4, 1.4 seconds.

View full abstract

Recently, thinned trees are used in artificiai reefs in Japan but their effect on fish aggregation, increase of periphyton and their usefulness have not been studied. We researched utilization of the thinned tree reefs by a fieldwork and questionnaire survey. Ten thinned tree reefs and the normal reefs those are not using any tree, were placed on the frontage of Shimonoseki city in the depth of 12m and 34m November, 2002. The change of a shape of thinned trees, fish aggregation and the increase effect of periphyton were observed in 7-9 times a year for 3 years. The utilization of the artificial reefs with thinned trees were examined by questionnaire survey for 39 fishery bureaus of the metropolises and prefectures and for 39 companies producing artificial reefs in 2005. As a result, artificial reefs with thinned trees contributed to early increase of periphyton including marine borers, and superiority was recognized in fish aggregation compared with artificial reefs that do not install thinned trees. In addition, some useful knowledge are also reported for the utilization and the durability of thinned tree reefs.

View full abstract

A Micro Bubble Jetter (MBJ) was used in field experiments on improving bottom sediment quality in order to determine whether this new technology can prevent the mass mortality of shellfish during the summer season due to deteriorating sediment quality in the ark shell fishing ground in inner Ariake Bay. The MBJ is mounted on a small fishing boat and is used to draw up surface water and inject it with microbubles and to improve bottom sediment quality by directly squirting microbubbles into surface mud using a towed nozzle unit. Improvements in bottom sediment quality were measured by investigating surface sediment quality before and after the towed device (including oxidation-reduction potential [Eh] and acid volatile sulfides [AVS-S]), benthic organisms (wet weight, number of species, and numbers of individuals), and ark shell mortality rates. Using this method, there was an increase of 100〜200mV in Eh and a reduction of 0.1〜O.2mg/g DW in AVS-S in the region where the MBJ was towed compared with the control region. Moreover, a reduction in organic C, total-N, and total-P concentrations was observed in the region where the MBJ was towed. Finally, in comparison with the control region, there was a significant decrease in the ark shell mortality rate in the region where the MBJ was towed that was commensurate with amount of MBJ towing performed. Therefore, this study demonstrates that this method is useful in improving bottom sediment quality in inner Ariake Bay.

View full abstract

At Lake Biwa, It is commonly said that prevailing northeasterly or southwesterly wind on the lake brings good catch of common carp Cyprinus carpio in set-net fishery. In order to research this phenomenon, the appearance of common carp, water temperature and winds near a set-net were monitored in the south basin of Lake Biwa from 25 June to 29 September 2003. Four fish were surgically implanted with transmitters, released near the set-net and monitored for 95 days using receivers installed on the set-net. Water temperatures at two depths (2.5 and 4.5m) near the net were also recorded continuously every 15 minutes by temperature loggers. The tagged fish disappeared from the area when the temperature increased, but appeared when the temperature declined following prevailing wind occurred. Our results suggest that the change in water temperature may affect the horizontal movement of the fish in the south basin of Lake Biwa during summer.

View full abstract

Marine afforestation of Ecklonia cava by using a net cage for protection from grazing was examined in the Hainan Isoyake area of Shizuoka Prefecture. Algal reef blocks made of concrete were placed on the bottom of a rocky reef at 12 meters deep in October 1997,onto which hundreds of E. cava plants grown in a water tank were transplanted. The E. cava plants were covered with a net cage for protection from grazing by herbivorous fishes, and the net cage could be removed in the winter, when grazing pressure by the herbivorous fish Siganus fuscescens, decreased. E. cava plants consisting of multiple-year classes survived on the algal reef blocks with a net cage in the summer of 2000, and many young plants spread over the natural rocky reefs around the algal reef blocks. The E. cava that spread around the algal reef blocks were grazed by S.fuscescens in the autumn and disappeared. However, E. cava spread again in the spring of the following year and grew in the summer.

View full abstract

The sea urchin Strongylocentrotus nudus is commercially important but their intensive grazing often leads to the formation of extensive, unproductive barrens along the coast of northern Japan. A simple device that could defend urchin invasion and allow kelp to grow by utilizing waves was developed after repeated field experiments with test devices. The final device for practical use had a swing bed that consisted of a horizontal shaft with frames for kelp settlement and growth on both sides and a plate on the underside. The swing bed was supported with two columns and was swayed by hydrodynamic force on the plate bearing wave-induced oscillatory flows. Invasion by urchins into the bed was prevented primarily in moving from the column to the constantly moving surface of the shaft. To enhance the relative movement of the swing element to the immobile supports, disks were attached to the shaft and to the columns with interstices. The device that was placed in S. nudus-dominated barrens protected from ocean waves successfully prevented urchin invasion into the swing bed leading to the formation of a canopy by the perennial kelp Eisenia bicyclis.

View full abstract

To compare the seasonal distribution of the rabbitfish, Siganus fuscescens off Utsumi, where the Sagarame, Eisenia arborea marine forest still remains, with that off Toyohama, where this type of forest has vanished, the authors investigated the rabbitfish catches of small-scale set-net fishery in 2004. Further, to analyze the seasonal distribution of rabbitfish around the eastern coast of Ise Bay and the coast of Mikawa Bay, the authors investigated the rabbitfish catches of small-scale set-net fishery by obtaining information from fishermen in 2004. This result suggests that rabbitfish were mainly distributed in Mikawa Bay. Toyohama lies closer to Mikawa Bay than Utsumi dose; therefore, the grazing pressure of rabbitfish off Toyohama may be stronger than that off Utsumi. It is believed that the high distribution of rabbitfish is one of the main factors responsible for the disappearance of the Sagarame marine forest around the eastern coast of Ise Bay.

View full abstract

In recent years, the heavy infestation of giant jellyfish (Nemopilema nomurai) has been affecting coastal fishery in Japan. The authors analyzed, as a case study, working process in set-net operation in the Hachinohe coast in northeast Japan. By evaluating work posture during the operation using a "pain index", they quantitatively showed the load of fishing workers when giant jellyfish entered the net. Fishing workers scoop a large amount of jellyfish, which cannot be commercialized, after hoisting the net, and then collect useful fish species such as salmons. In the case reported here, fishing workers used to spend 61 min for scooping giantjellyfish, while spending 54 min for hoisting the net, 14 min for collecting fish and 20 min for releasing the net. During the scooping operation, overburdened work posture was repeatedly forced.

View full abstract