A pelagic and/or mid water trawl using canvas kites as net-mouth opening device was developed. The wing kites for horizontal net-mouth opening and the para-kites for vertical opening were designed and the optimum dimension of kites was determined according to the drag of a trawl net of 25.96m in total length. As a result of sea trials using the trawl net, the height of the net-mouth was kept to be 6.03m on average and little change in the height of the net-mouth was observed, within the wide towing speed from 1.55m/s to 2.58m/s. The depth of the cod-end was also maintained at the same level as the center of the net-mouth. The drag of the trawl net was approximately 17.8kN at a towing speed of 2.06m/s (4knot). In addition, working depth of the net was controllable with change in the warp length, but at a given warp length the depth of mid water trawl with canvas kites was almost half of that of the one with conventional otter boards.
A recharging system to lead storage battery using solar cells is installed in small fishing vessels. This system supplements the decrease in electric quantity by the self-discharge of the lead storage battery with the output electric quantity of the solar cells. This research intends to obtain the optimum size in the solar cells, in order to prevent the starting failure of the engine by the self-discharge of the lead storage battery. The tests which evaluate the charging efficiency and the self discharge in lead storage battery, and which evaluate the efficiency in the solar cells were conducted. The results show that the charging efficiency in the lead storage battery was 0.209, the estimated self-discharge was 0.0239kWh/day, the efficiency in the solar cell was 0.087. According to the experimental result, the optimum size in the solar cells was estimated with 0.42m^2 in case of the experimental system.
We launched the research project in order to build a scientific and technological framework for comprehensively assessing and framing policies to restore the environment of a semi-closed coastal water area substantially affected by surrounding river basins. Setting Mikawa Bay and its river basins as the target field, first, we chose N and P as primary materials that play substantial roles in the ecosystem of the water area, and then estimated the latest four-decade transition in composition of the materials and their supply to the bay. Comparing the transition and the time series of direct impacts on the water area with the environmental deterioration of the bay, we identified a scenario composed of four stages to explain the deterioration process with respect to material cycles, and validated it with a water/material movement simulation model. By defining "good material cycle" as the state under which oxygen deficient water is much less likely to expand in the water area, and by using the scale of oxygen deficient water as a common index to measure the environmental soundness, we materialized the aforementioned framework, whereby the best mix between policies to be executed in river basins and a coastal water area can be examined.
It's common knowledge that the artificial creation of shallows including tidal flats has great water purification function. Therefore, creating tidal flats in Mikawa Bay of Japan, we believe it is important to choose areas that will be the most effective for the recruitment and/or supply of pelagic larvae. Firstly, we selected the short-necked clam (Ruditapes philippinarum) as a target species, and used a receptor mode model to predict trajectories of the pelagic larvae flow towards the Isshiki tidal flat areas. The results show that: (1) The main sources of larvae supplied to this area are the inner parts of the bay: (2) The trace routes vary greatly with temporal wind-induced currents and vertical distribution of the larvae. Secondly, we tried to map the important areas that are sources of larvae by predicting trajectories from 5 areas in Mikawa Bay. Based on the results, the supply patterns to each area were placed into 2 major patterns: self-contained pattern and ingression pattern. From these simulations, we found that the receptor mode model used in this study has been useful in answering some of the difficult questions faced in restoring bay environment, when determining the best location for creating artificial shallows.
To create a new artificial shallow area in a hypoxic bay, we propose methods to determine the depth for the area and how to evaluate its effectiveness. The study was conducted in Mikawa Bay, Central Japan. In summer season, serious hypoxia is observed in the bottom layer, which becomes progressively lethal to the macrobenthos. Therefore the aim of developing a new shallow area should be to avoid hypoxia. The circumstances on which we developed the new method are described. Future technical subjects for the environmental restoration are also described.
Huge subaqueous borrow pits are widely distributed in coast of Japan as a result of dredging action for sandmining or other purposes. In this study, we first review the present status of the borrow pits in whole of Japan. The borrow pits can be classified into two categories: one is the flatten-out type that is typical in Seto Inland Sea, and the other is the depression type that are widely distributed in whole country. The latter type are often steep sided and much deeper than the surroundings. Water circulation inside of the pits is strongly inhibited and organic materials are easily accumulated in the bottom of the pits and therefore, water quality deterioration such as development of severe oxygen depletion is often observed. Therefore, some remediation actions are urgent task to restore damaged ecosystems. Recontouring, which is one of the promising methods to restore the damaged ecosystems, has been conducted in the borrow pits in Mikawa Bay by using maintenance dredging sediments. In the latter half of this paper, we report the outline of the restoration action and main results of a research project which aims to develop evaluation methods to estimate beneficial effects of the recontouring on the coastal environment. In the research project, we successfully developed a comprehensive procedure to predict the response of benthic community structure and their purification function after restoration actions conducted in the Mikawa bay.
A comprehensive coastal ecosystem model coupled with 3D hydrodynamic model with non-hydrostatic flow was developed to understand the processes of accumulation of anoxic water masses (and hydrogen sulfide) in borrow pits and their upwelling to shallow region. This model was applied to the borrow pit in Mikawa Bay and compared to the observed data. To examine the effectiveness of the model, the numerical results were compared with those of a hydrostatic pressure model. The comparison proved the non-hydrostatic model to be much more adequate at simulating the stratification structure and the vertical mixing in the pit. Moreover, it turned out that the non-hydrostatic model also reproduces reasonably the upwelling process under strong wind conditions. The model we developed can evaluate environmental situation after borrow pits remediation, and the approach will contribute to effective restoration.
Oxygen-deficient water kills benthic fauna, and degrades ecosystem function. Recently, borrow pits, caused by the sand mining at the bottom of the sea, have been identified as a source of oxygen-deficient water. Projects that restore borrow pit areas have already been begun in some coastal areas of Japan. In this study, we predicted the improvement of ecosystem functions expected after restoring two borrow pit areas of Mikawa Bay, and examined the cost-effectiveness of the restoration project. Firstly, we examined the relationship between dissolved oxygen (DO) and the biomass of benthic fauna in the sea area around two borrow pits, located in different depths of water in the north-eastern part of Mikawa Bay. As a result, the biomass of benthic fauna can be predicted using a new index: the oxygen-deficient sensitivity index (OSI), which is calculated from DO saturation and water temperature measurements. Secondly, we predicted the biomass of the benthic fauna after the two borrow pits were restored and calculated the expected economic value of ecosystem functions, such as water purification, that improved owing to the recovery of the benthic fauna. We then compared these values with the restoration cost of the borrow pit areas. When we compared the cost-effectiveness of the borrow pits, we found that the effect of restoration was high for the borrow pit in the shallower sea area. Therefore, it is necessary to examine the cost-effectiveness of restoration projects prior to undertaking similar projects.