Underwater Observation using Sound Waves
Fish in the ocean often migrate in schools of the same species. We need to know where and what species and sizes of fish are swimming. If we know these, we will be able to fish more efficiently and manage resources better. If we can accurately detect fish schools using ultrasonic waves, we can avoid excessive fishing. I would like to contribute to the SDGs by protecting the abundance of ocean and making it possible to supply food sustainably.
The United Nations has designated the decade starting in 2021 as the "Decade of Ocean Science," with the aim of contributing to SDGs. Ocean science, as defined by the UN, includes the field of fisheries.
One of the major differences between underwater and land observations is that the spatial information of depth must be considered in underwater observations. For example, in investigating the distribution of species in the ocean, organisms that inhabit the ocean are entirely different depending on depth, even if they are located horizontally in the same location. In other words, underwater observation is the observation of a huge three-dimensional space in the ocean. Furthermore, the direct observation of a location in the ocean is complicated due to problems such as the limited distance light can reach in the ocean and water pressure. Therefore, efficient underwater observation requires a tool that remotely enables a wide range of observations.
Sound waves are transmitted through liquids and solids better than radio and light waves. Therefore, sound waves are used as a superior tool for wide-range observations. In our laboratory, we are working on sound waves in underwater observation, particularly with the main theme of estimating marine living resources. Our research aims to accurately estimate the number of resources of various organisms, ranging from fish to zooplankton, that sound waves can observe.
Since a sound wave spreads as it goes farther, it can search for a wider range as the distance increases. In contrast, the search range is narrow when the distance is short. Therefore, in the observations with a fish finder, the search range is narrow at a shallow depth (surface layer), and a school of fish cannot be observed at a depth shallower than the bottom of the ship. Sardines, mackerels, and saury, which can be found in supermarkets, are mainly distributed in the surface layer, for which fish finders are suitable. Therefore, sonar is used to find such fish efficiently.
Sonar is a device that detects fish echoes like a fish finder, but it can transmit sound waves in a horizontal direction (Figure 4). Moreover, because the sound waves can be transmitted all around the ship, it can search a wider range than a fish finder. If a school of fish is captured by a sonar continuously, its movement can be visualized three-dimensionally. The figure shows an example of a certain angle of depression (angle looking down), which be changed to correspond to various schools of fish.
Because saury catches have been poor in recent years, the survey of its abundance is important. Since sonars are more suitable for searching for saury than fish finders, the estimation of amount of saury resources using sonars has attracted attention.