An overview of underwater acoustics applied to observe fish behaviour at the Institute of Marine Research

Underwater acoustics have been used for decades to observe and enumerate fish and other marine taxa. As early as 1935 the technique was used to observe fish vertical distribution, and techniques to quantify fish abundance followed. Today, these techniques are widely used to inform fisheries management. However, fisheries acoustics are well suited to observe other properties than distribution and abundance. In this contribution I give an overview on recent advances at the Institute of Marine Research that enable us to quantify fish behavior using acoustics. The introduction of the split beam echo sounder was motivated by the need to locate single individual fish within the echo beam. By connecting the detections, tracks of fish could be established and tilt angles and swimming behavior could be inferred. This was important for resolving the tilt angle distribution for correct enumeration of the fish populations, but it is also an important tool for tracking individual fish in the open ocean. The technique has been used to track changes in individual fish behavior exposed to vessels in situ. Another tool to observe fish behavior is fisheries and multi beam sonars. Several sonar configurations exist in terms of frequency and beam configurations. Fisheries sonars operating at relatively low frequencies can be used to track the position and velocity to entire schools. Multi beam sonar can also be used to track the internal behavior in fish schools, and applications to collective internal dynamics of fish schools is presented.