Multiple signals must be identified from different fish simultaneously to observe group behavior using acoustic biotelemetry. We demonstrated simultaneous identification of multiple signals using a new phase modulation-coded telemetry system. We deployed eight transmitters with a 1.28-s signal transmitting interval at the same location, and two receivers that were 18.5 m and 38.0 m away from the transmitters. The detection rates of the two receivers during 15 min of recording were 77.0% and 73.0%. These results show that multiple signals from several transmitters were detected simultaneously by the receivers and were discriminated at high resolution (<2 s). This new telemetry system will be useful in studies on fish schooling behavior, aggregation, and predator-prey interactions.
In order to elucidate characteristics of the water current and halocline in brackish Lake Takahoko, which has been partitioned by a levee with water gates at the center, in Aomori Prefecture, the salinity, current direction/velocity, wind direction/velocity and water level were observed from July to November 2015. The water level in the Lake Takahoko varied with tide within smaller tidal range in comparison to that at costal sea area. Air pressure also had effects to the water level variation. The lake water mainly flew to east or west direction affected by hydrological configuration with main river mouths at west end and a channel to sea at east end, lake shape which is wider in east-west direction, and frequently eastern and western wind. Salinity of lake water was higher in the east area from the levee than in the west area, showing a significant effect of levee to salinity in the lake. A halocline was formed during most of the observation period, though we found its occasional collapse by strong wind. From the Richardson number, which are indicators of halocline structural stability, it was shown that the halocline was very stable at the center.
In this study, we estimated the radioactivity concentrations of radioactive cesium (134Cs and 137Cs) in the body of Japanese whiting (Sillago japonica) in Tokyo Bay and evaluated the safety of the fish for human consumption. We performed the estimation of the radioactivity concentrations using the National Institute of Advanced Industrial Science and Technology’s Chemical Bioaccumulation Model for Windows Ver. 4.0. To increase the computational speed of this model, it was utilized by compiled a database of the calculation results obtained using the three-dimensional flow model, the ecosystem model, and the fate model for chemical substances. We also improved the ease of use of this model for general users, rather than just researchers and engineers, by incorporating a graphical user interface. We evaluated the safety of Japanese whiting for human consumption using the committed effective dose. Assuming a typical daily intake of Japanese whiting of 100 g, the committed effective dose was approximately 0.04 mSv, which was much smaller than the committed effective dose specified by the Japanese government (1 mSv). Therefore, Japanese whiting in Tokyo Bay can be considered safe as human food.