Annual life cycle of rabbitfish Siganus fuscescens were examined in Tateyama Bay, Chiba Prefecture. A total of 1775 specimens (35-339mm BL) were collected by set net and dip net from 2005 to 2007. Rabbitfish were caught by the set net from May to December when the water temperature was over 17.5℃. From the seasonal changes in gonad somatic index and ovarian maturity, the spawning season was estimated to occur from July to August. The minimum size for maturation was 155mm BL in males and 168mm BL in females. Young rabbitfish (35-43mm BL) were caught by the dip net in October after the spawning season. This result suggested that the rabbitfish were reproduced in Tateyama Bay. Frequency of occurrence in gut contents of adult rabbitfish were 48.1% in brown algae, 22.9% in red algae, 17.5% in green algae, 12.5% in hydroid, and 7.4% in caprellid amphipod. Gonad somatic index inversely correlated with gut contents somatic index.

View full abstract

The marine forest Sagarame, Eisenia arborea, which had grown on the mouth of Ise bay, decreased becaused of grazing by rabbitfish, Siganus fuscescens, from 1998 and almost disappeared in 2005, remaining confined to only a limited area. It is said that the marine forests of the members of Phaeophyta are multifunctional. Massive restoration of these marine forests requires consideration of the expense involved. However, the authors did not have the required data to calculate the expense. In this study, the authors fed 3 brown algae, namely, E. arborea, Ecklonia cava, and Sargassum micracanthum, to disk abalone, Nordotis discus discus, and calculate the it's catching the fish from the growth formula of for these. Further, the authors calculated the cost of the protecting the marine forests from grazing by rabbitfish and selected beneficial one from three algae. The survival rate of abalones fed on S. micracanthum was low at approximately 40%; this algal species was not suitable for the creation of marine forests. The survival rate of abalone that fed on E. cava was the same as that of abalone that fed on E. arborea; the growth rate of abalone fed on E. cava was approximately 70% of that of abalones that fed on E. arborea. Based on these results, the authors conclude that developing E. cava forests is a suitable low-cost method for restoring and maintaining food resources for disk abalone; further, for the maintenance of these forests, protection from grazing by rabbitfish is not required.

View full abstract

The Sagarame (Eisenia arborea) marine forest has depleted because of grazing by rabbitfish (Siganus fuscescens) on the eastern coast of Ise Bay. If the meristem of Sagarame are protected against grazing by rabbitfish, renewal of the Sagarame lateral blade is possible. The authors selected the 8,000D PCL (Poly-ε-caprolactone) biodegradable fiber stretched five-fold in indoor. The authors could renew the lateral blade of Sagarame by attaching the six 1.5m PCL fibers with a Sagarame on the sea area. The biodegradable fibers completely disappear after 3 months during which the rabbitfish graze, i.e., from September to November. Compared with the method of protection involving nets, this method is easy and cheap and dose not require the setting, checking, repairing, cleaning, and removing of nets.

View full abstract

The seaweed and seagrass beds have the ecological functions by providing nursery grounds, spawning grounds and food for aquatic animals, purifying the waters through absorption of nitrogen and phosphorus in seawater and the bottom, and reducing greenhouse gases in the atmosphere by fixing carbon through photosynthesis. Among these functions, the calculation technique of economic values about the increase of fishery production and water purification in the fishery infrastructure improvement project is proposed. However the economic value of the air quality conservation effect by carbon fixation has not been calculated. To improve the cost-effectiveness of the seaweed and seagrass beds development in the fishery infrastructure improvement project, the trial calculation of the long-term carbon fixation by main types of seaweeds and seagrass beds was made by analyzing carbon cycle processes based on previous knowledge. The trial calculation results showed that the quantity of long-term carbon fixation, which varied with the type of seaweed and seagrass beds, was 0.10kg-C/m^2/year in Zostera marina beds, 1.67kg-C/m^2/year in Eisenia and Ecklonia beds, 0.13kg-C/m^2/year in Laminaria beds and 0.60kg-C/m^2/year in Sargassum beds respectively. It was suggested that the main process of carbon fixation was the sedimentation of withered grass body and underground stem without degradation as for Zostera marina beds, and transfer of the drifting seaweeds to the offshore mesopelagic zone as for the Eisenia and Ecklonia beds and the Sargassum beds. The benefit of the long-term carbon fixation by the seaweed and seagrass beds development was estimated from 10 thousands of yen to 190 thousand of yen in 30years per 1ha of the seaweed and seagrass beds based on the market price of the carbon emissions trading.

View full abstract

The formation of oxygen depleted water mass during summer is a main environmental issue of the Mikawa Bay. Sinking fluxes of plankton and detritus greatly influence on the generation process of oxygen-depleted water mass because they are decomposed by bacteria using oxygen. The terrestrial loading of nutrients and organic substance had increased in Mikawa Bay until about 1980, and also intensive reclamations were carried out in shallow sea area of Mikawa Bay in 1970's. Therefore we have thought that the formation of the oxygen depleted water mass in Mikawa Bay could be related to an increase of the loading and the land reclamation. In this study, we attempt to determine which is the main process to form the oxygen depleted water mass, by using a numerical ecosystem model. The model run covered from 1961 to 1990. The model results show increased sinking flux of particulate organic matter that related to the oxygen consumption in the sediment in the first half of 1970's compared to early 1960's. It exceeded 500mgC/m^2/day in the latter half of 70's, and this level was maintained afterwards, even though the nutrient loading from terrestrial origin decreases in the latter half of 80's. The reclamation of Mikawa ports started in 70's, and then the amount of the shellfish resource decreased. The model result shows the negative correlation between the shellfish resource and total area of oxygen depleted water mass exists. Therefore, we can conclude that a decrease of the shellfish due to the reclamation greatly influenced on formation of the oxygen depleted water mass.

View full abstract

A newly developed ecosystem model-the first model describing the ecological connectivity consisting of both benthic-pelagic and central bay-tidal flat ecosystem coupling while simultaneously describing the vertical micro-scale in the benthic ecosystem-was developed and applied to Tokyo Bay. The model outputs demonstrated the significant ecosystem responses as follows. First, the benthic oxygen consumption during summer was quite low due to the low level of dissolved oxygen (hypoxia), although reduced substances, Mn^<2+>, Fe^<2+>, and S^<2->, were highly produced and accumulated in the pore water. Second, both the tidal flat creation and nutrient load reduction decreased the anoxic water volume and mass of detritus in Tokyo Bay. However, the creation of tidal flats led to the higher biomass of benthic fauna, while the nutrient load reduction led to the lower biomass of benthic fauna compared to the existing situation. Lastly, in the simulation, Tokyo Bay reproducing reclaimed tidal flats (earlier Tokyo Bay system) prevented the increase of oxygen consumption potential (hypoxia potential) and the decrease of higher trophic production to red tide, compared to the existing Tokyo Bay system with reclamation of tidal flats.

View full abstract

In Mikawa Bay, 6.2×10^6m^2 artificial tidal flats had been created for improvement of damaged bay environment from 1998 to 2004. However, recently materials of the artificial tidal flat were difficulty supplying. A facility experimental tidal flats can be created in is in Aichi Fisheries Reserch Institute. Effects of the sand accumulated at Yahagi dam in case of using as a material of artificial tidal flats was examined by the experimental tidal flats managed over 2 years. The planktonic larvae of Japanese littleneck clam (Ruditapes philippinarum) landed at the sand accumulated at Yahagidam almost as much as at natural coastal sand.

View full abstract

With respect to the sediment problems in the Yahagi dam, we examine both short-term and long-term measures. First, as an urgent, short-term measure, we are excavating the sediment accumulated on the upper parts of the reservoir, so that the dam can retrieve the flood control function up to the beginning level. The excavated sand is used effectively for park maintenance in Toyota city and for the tide-land development project in corporation with Aichi prefecture. Second, as a long-term resolution, we consider developing a sand-exhausting system that excretes only the amount of sand that goes beyond dam capacity by combining Hydraulic Sedimentation Removal System and Sand-Exhausting Bypass. However, it is supposed, according to a result of a trial calculation, that most of the sand exhausted from the Yahagi dam will flow downward and accumulate on the lower dams for power generation. We thus are required to consider such sediment problems from the viewpoints of comprehensive sediment management. Specifically, what should be taken into consideration is to maintain the functions of the Yahagi dam as well as of the lower power dams, to run the sand exhausted from the Yahagi dam along through the lower rivers without making further sand accumulation, and to suppress the total cost.

View full abstract

The Mikawa Port Office, Chubu Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism, is promoting three environmental projects for rehabilitation and creation with Aichi Prefectural Government. (a) Sea blue project; new tidal flats and shallow waters are created through the utilization of high quality dredged sand arose from the development of Nakayama waterway. After completion of artificial tidal flats, monitoring survey and sharing the information with relevant institutions are continued. According to the survey, implementation sites indicate that these areas provide habitats for fry, bivalves and wild birds. (b) Filling depressions with dredged material into the sub aqueous borrow pits in Mikawa bay to prevent blue tides. (c) Next phase Sea blue project; after finishing Nakayama waterway's dredging, it is started to research new materials for creating mud flats, such as compound of slug and silt.

View full abstract

After the 1960's, especially, a lot of fishing port facilities were constructed at the high economic growth period. Facilities maintained at this time have already exceeded 30 years, the future, it is expected that maintenance and renewal of facilities increase by decrepitude. The fishing port is an important infrastructure to supply fishery products, appropriate maintenance and renewal of the fishing port facilities are important policy because it supplies fishery products with stability. Then, we totaled the construction year and the amount of construction of outlying facilities and berthing facilities in the fishing port in Japan. Next, the cost of the renewal in the future was calculated by the amount of construction and the construction year. Finally, we clarified the point of problem of maintenance and renewal in the future. This result is as follows; the renewal of outlying facilities and berthing facilities increases from about 2000 and becomes a peak in around 2030. The renewal cost that exceeds 50 billion yen is needed in the seventeen prefectures. The renewal cost of twenty-four prefecture is higher than the budget in fiscal year 2009. Therefore, the renewal cannot be constructed by the budget in fiscal year 2009. In addition, the renewal cost of twelve prefectures becomes twice or more the budget in fiscal year 2009.

View full abstract