The bottom layer of Ise Bay has experienced remarkably large areas of hypoxic water since 2006. In order to identify the mechanisms of the expansion of hypoxic water after 2006, we conducted a long-term (2003–2016) numerical simulation that was based on a hydrodynamic and lower trophic ecosystem model of the area. The model reproduces the density structures and variability of hypoxic water in the bottom layer of Ise Bay. Interestingly, the simulated hypoxic water areas on the date of monthly observation differs remarkably from the simulated monthly mean values. This implies that once-a-month observation cannot capture precise variation of hypoxic water due to a lack of sampling frequency. The simulation results indicate that the intrusion of outer sea water into the bottom layer (BLI: Bottom Layer Intrusion) plays a key role for driving the year-to-year variation of bottom layer hypoxic water. Our results show that the occurrence of BLI is principally driven by the fortnight fluctuation of tide as clarified in previous research. In addition, BLI is also related to (1) the vertical mixing of sea water due to strong wind, (2) the intensified estuarine circulation caused by river discharge and (3) the upwelling of cold and high-density waters from the outer sea. Among these three factors we found that wind speed and river discharge are related to the recent expansion of hypoxic water in Ise Bay. Since 2004 the mean wind speed in August-September has become weaker and the river discharge has decreased. This may have led to the decreased occurrence of BLI after August, resulting in the expansion of bottom layer hypoxic water in recent years.
It is widely known that sardine have population cycles, and that the peak of their populations comes approximately every 60 years for multiple sardine species in distant ocean regions in the world. Until present, although various environment factors, such as oceanographic and climate conditions, were studied to explain the population cycles of sardine resources, Earth’s outer liquid core has not been the subject of the study. The fluctuation of Earth’s outer liquid core is not able to be directly observed. Available data are those obtained indirectly through the ray of geomagnetism from the Earth’s outer liquid core. In this study, we used the geomagnetic data, and found out the followings: (i) a statistically significant correlation was identified between the Index of Geomagnetic Intensity (GEOM) and the Index of Sardine Scale Deposition Rate (SSDR: from 1840 to 2010) (R2=0.394, p=4.029E-20<0.001) and (ii) a statistically significant correlation was also identified between the Index of GEOM and the Sardine Catch Volume in Japan (SCV: from 1910 to 2010) (R2=0.243 (p=1.61E-07<0.001) .
Consequently, it can be argued that the fluctuation of geomagnetic intensity from Earth’s outer liquid core is one of the important factors that explain population cycles of sardine resources. Furthermore, we examined the variation of sardine resources back to 3000 years, 10,000 years and 800,000 years by using fluctuations of the geomagnetic intensity. As a result, in addition to the 60-year population cycle, we have found another population cycle that has population peaks in approximately every 300 years.
Environmental DNA (eDNA) metabarcoding is a useful method for detecting fish fauna in areas where fish sampling is difficult. Limited information on factors such as decay rate that can determine the distribution of eDNA, limits the ability to estimate the actual distribution of fish fauna using eDNA measurements. Therefore, to estimate the fish fauna in Matoya Bay, which is a typical estuary located at the mouth of Ise Bay, we used eDNA metabarcoding. The ecological characteristics of the fish fauna detected (freshwater, brackish water, and saltwater) agreed well with the salinity distribution, indicating that eDNA was degraded and transported from the mouth of the bay near the open sea to brackish water Izounoura. Based on these observations, we calculated the diffusion of eDNA of marine fish based on a flow field obtained by a numerical simulation and attempted to estimate a decay rate that closely resembled the observed distribution of eDNA concentration. The estimated decay rate constants ranged from 0.0 to 0.4 per day, which were smaller than those estimated in previous studies based on rearing experiments (0.603 to 1.653 per day). This study demonstrated the possibility of estimating decay rate constants using field observations and numerical simulations rather than rearing experiments.
This paper summarizes the general situation of marine monitoring conducted after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, the experience of development and operation of USVs, and the possibility of applying unmanned vessels as a tool for nuclear disaster prevention in the future. Even now, more than 10 years after the accident, large-scale marine monitoring is still being conducted in the vicinity of the Fukushima Daiichi Nuclear Power Plant. In addition, it is an important policy issue to develop monitoring tools in preparation for post-accident situations. Operational tests of three USVs have been continuously conducted for use in such environmental radiation monitoring. Development of these USVs is underway with a view to utilizing them for seawater sampling, direct measurement of the seafloor soil surface layer, and seafloor soil sampling, depending on their performance. It is necessary to promote the development of USVs for future nuclear power plant accidents.
Unmanned observation technologies such as Unmanned Surface Vehicle (USV) have been studied as a low-cost substitute for oceanographic observations previously conducted by manned ship. Many USVs have a long cruising time and a slow cruising speed, making them difficult to reach quickly to observation points. For this reason, we developed a flying boat type Unmanned Aerial Vehicle (UAV) that has both the functions of the USV and the UAV. Two aircrafts with different specifications were developed to correspond to various observation methods and purposes. The HAMADORI 3000 has a wing span of approximately 3 m, weighs approximately 19 kg, and is able to fly for approximately two hours. The HAMADORI 6000 has a wing span of approximately 6 m, weighs approximately 100 kg, and is designed for approximately 8 hours of flight. Using the developed aircrafts, we operated them as USVs on the sea surface and conducted a flying experiment by take-off from sea. As a result of the experiment, neither aircrafts function without serious errors in regard to the set route. In addition, no divergence or other problems occurred in the wake when turning or straightening. Therefore, both aircrafts showed practicality to operate as USVs at sea. It was also shown that the aircrafts can be safely operated as UAVs when taking off from the sea by operating a function that immediately aborts the takeoff if the attitude of the aircrafts is disturbed. Based on these results, the developed fling boat type UAVs has can be practically used for cost-saving and high-frequency observations.
In 1987, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), the former Japan Marine Science and Technology Center, conducted ocean observations in the equatorial region of the Pacific Ocean as part of the El Niño emergency study, and from 1988 to 1993 as a part of JAPACS project. After that, time-series observations in the western tropical region using sub-surface mooring systems were added by the in-house TOCS project of JAMSTEC, and these observation activities were further developed within an international framework. This marked the beginning of the buoy network observations that continued for about 20 years and followed by TRITON buoys project. In 2000, the buoy array by the TRITON buoys in the western Pacific officially became to play a part of the TAO buoy network (west side) operated by the United States and conducted long-term observations until 2021 as the TAO/TRITON buoy array. After that, in response to the critical situation of maintaining the Pacific buoy network (TAO/TRITON buoy array) in both Japan and the United States, the TPOS2020 project was launched in 2015 with the support of international organizations (WMO and IOC/UNESCO) to design a new observation network and governance to operate, and finally in 2022 the new Tropical Pacific Observing System (TPOS) project was launched for its implementation. In this paper, based on the history of long-term time-series observations mainly in the tropical Pacific Ocean at JAMSTEC, and based on these experiences and the recent scientific and technical discussions at TPOS2020, future perspectives of ocean observations in the Indo-Pacific are discussed.
Our company’s main service is construction consultancy, and we undertake surveys, investigation, and inspection work using ROVs and drones as a part of our infrastructure inspection and surveying services. In recent years, demands for underwater surveys work using ROVs have been increasing. The equipment used is a BlueRobotics BR2, to which a mechanical sonar (BlueRobotics Ping360) and on imaging sonar (BlueprintSubsea OcullusM750d) are added and outfitted as necessary.
First, a low visibility scour survey of the piers of a dam lake bridge is introduced. The transparency was about 1 m at underwater, and almost nothing could be seen on the camera image. Observations were made using the mechanical sonar. It was attached to the bottom of the ROV so that it could observe the area near the lake bottom.
Next, the breakwater mound damaged by earthquake was surveyed. The sonar was installed horizontally so that the deformation of the cross section could be observed.
Third, port structures damaged by earthquake were surveyed. We installed GPS on the structures on a trial basis in order to obtain location information, even if only while they were above the water surface. It was confirmed that the addition of location information was useful in understanding the situation under the water surface. Currently, we install USBLs for underwater location monitoring.
The fourth survey was conducted after an agricultural canal tunnel was blocked due to a heavy rain disaster. The collapsed section was 650 m long. The collapse was confirmed at 337 m through the tunnel.
Finally, we expected that a screening test for scour could be conducted using an inexpensive imaging sonar. The results of the test in the pool showed that the target geometry could be determined with very high accuracy, and we believe that this would be applicable for the moat scour surveys.