East of the Sanriku coast, there are three water masses, namely Tsugaru Warm current (TuWE), Kurisio extension, and Oyashio cold current (OyCWM). A current rip is generated in the region by which the warm water mass and the cold water mass are bounded. Sound propagation is influenced by the current rip region. In this study, bi-directional propagation along 41°N off the Sanriku coast is investigated. As a result, when the receiver was at a shallow depth with the sound source approaching from OyCWM to TuWE, the transmission loss (TL) is large until it approached the receiver near the shallow source. The combination of a shallow source and a deep receiver has a similar result. When the receiver comes close to the source, the TL decreases greatly because of the correspondence between the receiver position and the position of the bottom reflection. When the receiver is approached to the sound source in OyCWM from TuWE, the TL decreases as the receiver depth increases. Because the sound channel (SC) propagation predominates in OyCWM, the receiver depth can reduce the TL near the SC axis.

Biological production is thriving in the coastal areas of Southeast Asia. However, resource evaluation has not progressed. In addition, the valid acoustic research needed for resource assessment is not being conducted or disseminated. Therefore, we developed a simple measurement system to perform acoustic investigations and provide basic education. We then conducted a field test in an effort to understand the distribution of fish schools in Tateyama bay, Chiba, Japan. Depth of the target research area was about 20 m. The acoustic device was composed of a GPS plotter and echo sounder (FURUNO GP1670F)+ Interface Box and PC. The concept has been used previously. However, the equipment is simple and easy to use in collecting acoustic data. First, we assumed the amount of resources for recreational boats to measure the distribution of fish using the fish finder. Results confirmed that the system was sufficiently effective.