Understanding fish populations is crucial for assessing fish habitats in river environments. Traditional methods, such as diving and using underwater cameras, have been employed for fish habitat surveys, but these techniques often provide limited coverage. To enable more comprehensive surveys in riverine settings, we have focused on utilizing the Multibeam Echosounder (MBES). The MBES is an instrument designed to survey fish schools and seafloor topography by transmitting and receiving ultrasonic waves and is predominantly used in marine environments. However, its application in rivers, which feature shallower depths and more complex bedforms compared to marine environments, has not been well established. This study aims to evaluate the MBES in river environments, validate its effectiveness, and identify potential challenges and considerations for its application in such conditions.
In this study, an MBES (WMB-1320F, manufactured by FURUNO) was installed on the right bank of the Chitose River in Hokkaido, where large fish species, such as chum salmon (Oncorhynchus keta), have been observed. The survey was conducted on October 12, 2023, from 12:19 to 15:59. The MBES was positioned at a depth of 1.0 meter and transmitted and received ultrasonic waves horizontally towards the opposite bank.
The survey identified 431 objects presumed to be fish, 231 of which confirmed as migratory objects. However, noise levels increased significantly at distances greater than 10 meters due to diffuse reflections, complicating fish identification. When using the MBES in riverine settings, it is crucial to minimize the impact of multipath reflections caused by the water surface and riverbed. Nevertheless, when noise levels are low, the MBES's characteristics suggest it can be effective for monitoring temporal changes, even in deep pools and turbid areas where fish surveys are challenging. Additionally, employing MBES for fish surveys in rivers offers advantages such as reduced costs due to decreased manpower requirements, as fish capture is not necessary, and enhanced safety by eliminating the need for underwater operations. |