The authors attempted to examine the use of the Habitat Suitability Index (HSI) Model for quantitative estimation of Sargassum spp. seaweed bed in the case of low biological and physical disturbances on the coast of Kunda Cove in the western part of Wakasa Bay. A strong correlation was found between the prediction of the HSI model analysis and the field observations (r=0.72〜0.80), while model predictions for the artificial construction were low depending on the substrate configuration. Possible causes preventing seaweed beds formation are feeding damage by herbivores inhabiting in the substrate crevasses. By using a net over the artificial constructions where Sargassum spp. had not previously grown, young Sargassum patens sporophytes were able to survive. To form a submarine forest effectively in this area, it was suggested that it is necessary to prevent herbivores from using substrate crevasses as habitats. The feeding experiments verified the feeding activity of a decapod crustacean Heptacarpus futilirostris on young Sargassum patens sporophytes. Although the grazing amount of each crustacean species would be slight, young Sargassum spp. survival might be affected by the grazing of crustaceans including H. futilirostris especially in the area where the seaweed beds have deteriorated and seaweed bed formation has not been successful.
In this paper, ecological relationship between seaweed / seagrass bed and fisheries production is discussed on the results of correlation analysis between fisheries catch and the area of seaweed / seagrass beds of the Seto Inland Sea. In the analysis, the area of coastal ecological components (seaweed / seagrass beds, tidal flats and shallow (<10m) sea areas) and fisheries catch data was prepared from the past governmental statistics for the nine sub-sea areas (Suo-Nada, Iyo-Nada, Aki-Nada, Bingo-Geiyo-Seto, Hiuchi-Nada, Bisan-Seto, Harima-Nada, Osaka-Bay and Kii-Channel) in the Seto Inland Sea. The distribution patterns of these ecological components were quite different among the sub-sea areas, which show the Seto Inland Sea comprises diverse environmental and ecological characteristics. A number of positive correlations were detected between the area of these ecological components and fisheries catches, e.g., catches of Japanese flounder, fine spotted flounder, red sea bream, black sea bream, rock fishes (Sebastes spp.), sea urchins and turban shell (Turbo cornutus) with Zostera or Sargassum bed areas. The ecological roles of seaweed / seagrass beds to support coastal fisheries production was discussed, with referring to the extinction of these beds which the Seto Inland Sea has experienced in the past several decades.
A sonar responding acoustic tag (SRAT) was developed. The SRAT responds to the signal transmitted from a sonar and transmits the pulse signal to the sonar. The SRATs response signals are appeared on the sonar display. The prototype of SRAT responded to the sonar signal between 20 to 90 kHz. The frequency of the transmitting signal was 31.25kHz or 62.50kHz. The size of SRAT was φ25×150mm, 100g in air and 70g in water. The multilayer piezoelectric element was used as the transducer. The battery life was about one month. Experiments to examine the response range were performed using 68 kHz sonar whose source level was 210dB re 1μPa at 1m. The SRAT signals could be observed around 200m distance on the sonar display. Experiments were performed using a tuna purse seine. The sonar frequency of the fishing vessel was 28kHz and the source level was 220dB re 1μPa at 1m. The SRAT was attached to the purse seiner for measuring shape of net. As the result of this experiment, the maximum response distance of the SRAT was confirmed as 400m. The SRAT is useful for actual fishing such as the observation of purse seine shapes.
We carried out a demonstration experiment using the Intertidal Flat Experimental Facility (IFEF), which is a semi-closed and experimental intertidal flat ecosystem, to investigate the effects of hypoxia and hydrogen sulfide (H_2S) water on organisms and water quality. We created three tide pools on the experiment tidal flat and divided them into three experiment areas : one area where sodium sulfide (Na_2S) was added to examine the effect of hypoxic water alone; another area where hydrogen chloride (HCl) and sodium sulfide (Na_2S) were added to examine the effect of both hypoxic and hydrogen sulfide (H_2S); and a control area where nothing was added. Then we observed changes in the water quality and the survival situation of the benthic organisms in each area. The results indicated a possibility that the mortality of organisms increases due to a combination of factors, i.e. hypoxic water and high hydrogen sulfide and high water temperature (>20℃). The results also suggested a possibility that in a tidal flat, where the sea level changes frequently, the length of time that organisms are exposed to two factors, i.e. hypoxic water and high hydrogen sulfide, is short, and therefore the mortality can be avoided.
In recent years, various types of coastal structures have been increasingly constructed using rubble mound covered with concrete armor blocks. The large amounts of resources used for these structures and greenhouse gas emitted from their construction have a strong impact on the environment, requiring consideration. On the other hand, it has been pointed out that seaweed tends to grow on concrete blocks placed under the sea and seaweed beds can absorb and fix CO_2. With this as a background, the authors calculated CO_2 emissions from the processes of the production and transportation of materials and during construction from the production to placement of concrete blocks, with the objective of contributing to the establishment of a method of designing environment-friendly coastal structures. Longterm CO_2 fixation by seaweed beds formed after the placement of concrete blocks was estimated as well. These calculations revealed the relationship between the amount of CO_2 emitted from the construction process of a coastal structure and the quantity of CO_2 fixed by seaweed beds on the concrete blocks. The authors also proposed a new type of armor block to reduce the environmental loads of coastal structures.
To identify the effects of horizontal spreading of macro-algal culture (Nori Porphyra yessoensis) nets on promoting colonization of juveniles of Asari clam Ruditapes philippinarum at intertidal sandy bottom, seasonal changes in density of clams and sea water flow were monitored at the field plots which established in the tidal flat in Ise Bay, Japan, during 2007. There was no significant difference of densities of juvenile clams between plots during the period of spat-recruitment in summer (nets were not spread). However, in the experimental plot (nets were spread at +50cm higher than the bottom surface), spatial densities of new settlers (shell length<0.3mm) and small bivalves (shell length 0.3-1.0mm) were observed denser than in the control plot (nets were not spread), during the period of spat-recruitment in autumn. The current velocity, which was measured at +10cm above the bottom surface, decreased after nets were spread in the experimental plot. Both in the experimental plots and in the control plot ones, whereas the shear stress of currents was lower than the calculated critical tractive force for the sediment grains and the new settlers throughout the experiment, shear stress of waves sporadically exceeded the critical tractive force. Consequently, movement of the new settlers may be influenced by the sea water current when the shear stress of waves exceeded the critical tractive force. We concluded that the reduction of the current velocity by the horizontally spread Nori-culture nets may have facilitated to trap the settled juveniles of Asari clam under the net at the experimental plot in Ise Bay.
It is necessary to investigate actual fisheries' work conditions to secure labor for fisheries in Japan; it may also be necessary to redesign tasks so that the work is more acceptable. This study quantitatively analyzed fisheries work on a small trawler that belonged to the Toyohama Fishermen's Cooperative Association in Aichi Prefecture, and that worked out of Ise Bay. Almost all crew movements on the bow deck and quarterdeck were recorded using CCD cameras and hard disk recorders during a fishing trip. The recordings were sorted, and times required for each task were measured. Physical loads on the crew were estimated using the Ovako Working-posture Analyzing System (OWAS), a popular approach that is based on subject work posture. The trip was about 12 hours long and the crew worked about 69% of the time. Approximately 22%, 24%, 95%, and 58% of the net casting, net hauling, fish sorting, and fish storing work, respectively, was judged as harmful and as needing to be improved.