Observation of aquatic biota in eutrophied pond using stationary acoustic monitoring system

Active acoustic monitoring system using a dual frequency identification sonar (DIDSON) is applied for the follow-up observation of aquatic biota after Hydrilla verticillata planting. The field experiment was performed in a small pond adjacent to Lake Izunuma (northern latitude of 38.43 degrees and east longitude of 141.04 degrees; surface area, 1,584 m2), and 24 Hydrilla verticillata were prepared and planted at the pond bottom. The DIDSON unit was mounted on an original frame with a 3 degree concentrator lens. The data were collected at 1.8 MHz (high-frequency mode) and at a maximum range of 3.0 m from the imaging sonar. The frame rate was 2 fps and the recording time was about 20 hours. Successfully, we found out the cause of plants disappearance by the acoustic imaging data. All Hydrilla verticillata were disappeared in a day and strongly affected by the predation pressure of crayfish, and whole scene of feeding was recorded. Other aquatic creatures, such as snail, frog, and several species of fish, were also appeared in the acoustic images. Then, we have originally developed an image processing program and quantified the biological features of the aquatic creatures for the better understanding of aquatic biota in freshwater environment.