Passive acoustic monitoring is an effective method to observe feeding behavior of dugongs in their natural environment. In this paper, we developed a new method to automatically detect the feeding sounds of dugongs from acoustic recording data. In January 2018, we deployed an autonomous underwater sound recorder on the seafloor in an intertidal seagrass bed off Talibong Island, Trang, Thailand. We developed custom-made software to detect the feeding sounds of dugongs automatically. The feeding sound consists of a set of pulses with a constant interval. A morphological opening filter was applied to identify pulses and autocorrelation coefficient threshold was used to extract the feeding sounds. Typical ambient noise that caused false detections included the sounds of snapping shrimps, other crustaceans, rainfall, waves, and boat traffic. Applying a threshold on water depth effectively reduced the false detection rate from 16.4 to 4.4%, and the correct detection rate increased from 72.2 to 76.8%. The proposed method will reduce time and effort for manual detections, and thus contribute to extension of temporal and spatial scale of passive acoustic monitoring of dugong feeding behavior.
Rokujo tidal flat called Rokujogata in the inner part of Mikawa Bay, is an estuary tidal flat developed at the mouth of the Toyo River. Juvenile Manila clams Ruditapes philippinarum larvae were caught there and then released to various places in Ise and Mikawa Bay, supporting the clam fishery. The authors focused on the feeding environment of juvenile clams as one of the causes of the mass outbreak. The appearance of phytoplankton in the coastal area around Rokujogata, which is used by these clams as food, was considered to be strongly affected by Toyo River, so we performed observations under various flood conditions. As a result, the relatively small size of chlorophyll a of 2–20 µm tended to increase under low salinity conditions around Rokujogata, and Cryptophyceae was considered to be important in its size. Cryptophyceae were suitable for feeding on juvenile clams because of their size, ecology, and food value, and their proliferation was thought to promote the mass outbreak of juvenile clams in Rokujogata.
In order to ascertain the size and phytoplankton group of the food that the settled juvenile and the early juvenile of the Manila clam Ruditapes philippinarum ingest in the sea area, Manila clams of each growth stage were bred in natural seawater collected in Mikawa Bay through laboratory experiments. Changes in size-fractionated chlorophyll a content and phytoplankton composition were investigated. The experimental results show that the settled juvenile ingested cyanobacteria and nanoflagellates such as Cryptophyceae less than 10 µm, the early juvenile ingested diatoms less than 20 µm in addition to nanoflagellates, and that the juvenile clam further ingested the diatoms and dinoflagellates size to 20 µm or more. The food environment is very important for the growth and survival of juvenile Manila clams. It is necessary to understand the size composition of the phytoplankton to evaluate the quality of food environment to these clams.
A method of correction for misalignment errors in ship-mounted Acoustic Doppler Current Profiler (ADCP) measurement is proposed by comparing ADCP current data with geostrophic currents based on eXpendable Conductivity, Temperature and Depth (XCTD) profiler observations. Oceanographic observations using ADCP and XCTD were carried out in Toyama Bay and adjacent area on board the training vessel ‘Wakashio-maru’ of the National Institute of Technology, Toyama College in September 2016 and October 2017. Cross-track components of ADCP currents are averaged between two neighboring XCTD stations to compare them with the geostrophic currents calculated from respective XCTD data. For the first cruise in 2016, an offset velocity is commonly observed between the ADCP and geostrophic currents, while those show a coincident current profile in terms of vertical shear. The offset seems to be caused by a misalignment in ADCP observation and the misalignment angle estimated from the offset value is 1.7 degree, which is comparable with 1.5 degree estimated by the software ‘ADCP Tracker.’ For the second cruise in 2017, there is no offset in the same comparison and the vertical profiles of geostrophic currents well trace those of ADCP currents. The present study proposes a practical method of misalignment correction by using geostrophic calculation based on a small number of XCTD measurements, which is different from that proposed by Michida & Ishii (2000). The present one has an advantage that it will be applicable to non-research vessels such as training vessels, which will not always be able to share enough ship-time for scientific observations for operational and/or logistic reasons. It is also suggested that the proposed methodology allows the training vessel ‘Wakashio-maru’ to be more effectively utilized to obtain higher quality observation data toward better understanding of the oceanographic conditions in Toyama Bay and adjacent areas.