UTOPIA scheme was applied to advection term for the numerical calculation of tide and tidal current to reproduce the strong tidal current realistically. Numerical model is constructed by boundary-fitted coordinate method vertically using Arakawa A grid in space. The new method is designed to suppress a numerical oscillation usually induced by Arakawa A grid. UTOPIA scheme was confirmed to be suitable to express a strong current around complicated topography. This model was applied to the tidal calculation for M2 constituent in Osaka Bay with two narrow straits, i.e., Akashi and Tomogashima straits. The tidal currents obtained in this model agree with them observed at monitoring stations, and the four eddies in the bay were also reproduced as the residual currents, i.e., tide induced transient eddy (TITE). The generation, growth and lifetime of the eddies also were investigated.
A preliminary study of the effectiveness of an infrared monitoring system to detect the timing of exit and re-entry to a hole used as a daytime refuge by an individual juvenile P. japonicus was carried out at the Kominato Marine Biosystems Research Center of Chiba University. The monitoring system consisted of the infrared beam source and sensor which were attached to either side of the hole and connected by a 5 m cable to an external exchangeable battery and data logger. For a six day period (25th September to 1st October, 1998) clear activity patterns were observed for an individual lobster (ca. 120 mm total length) showing that the nocturnal foraging pattern of the individual lobster had a high degree of variation in both the duration of foraging (between ca. 1 to 4 hrs) and the period remaining in the hole during the night. The sensor showed high levels of response especially around dusk and dawn when we conclude that the lobster was near to the entrance of the hole and also intermittent peaks during the night when the lobster emerged to forage and then returned. In summary, the lobster remained in its hole during the day and moved to the entrance of the hole at dusk observed before emerging to forage and then returned to the hole at intermittent periods during the night. We conclude that the possession of a suitable hole and its defense from conspecifics may be a key factor, along with prey availability, determining the duration of foraging behavior and that the infrared system is highly effective for monitoring the movement patterns of juvenile lobsters.
Fish farms often cause environmental degradation to the surrounding aquatic areas when they operate for a long time. In this study, a simple numerical model was developed to evaluate the water quality (dissolved inorganic nitrogen, dissolved oxygen) of fish farms. This model consisted of a conventional ecosystem model with an added "cultured fish" component. This model was applied to represent the material cycling in a red sea bream culture at Hazamaura, Gokasho Bay, Japan.The analysis was carried out on the basis of field data collected from 1986-1989. In order to verify the model, the simulated concentrations of dissolved inorganic nitrogen and dissolved oxygen were compared to observed concentrations. The simulated results were in good agreement with the observational data for the whole year. In the simulated nitrogen and dissolved oxygen cycle, we found that the factors causing water pollution (eutrophication, anoxia, etc.) were: excretion by the cultured fishes, river load and benthic regeneration in summer, an increase in organic substances from feed scraps and an increase in dissolved inorganic nitrogen through mineralization in winter.