Measuring fish school swimming speeds with two acoustic beams and determining the angle of the school detection

- School swimming speeds measured with two acoustic beams.
- Cisco fish schools swim at an average speed of 0.3 m/s.
- School angle of detection bias not significant if both acoustics beams are similar.
- Angular Positions within the school regions faithfully describe school positions.

The measurement of school swimming speeds across an acoustic beam from a fixed-platform split-beam transducer is difficult because it is impossible to accurately discern individual fish within a school, or track the displacement of the leading edge of the school over time. However, with two acoustic transducers the swimming speed of schools can be estimated as long as the school swims through both beams, and the detection angle when the school first interacts with the beam is known. Here we present a methodology for (1) measuring school swimming speeds with two 120 kHz elliptical (4° × 10°) split-beam acoustic transducers, and (2) estimating the detection angle of the school using Angular Position data within the school region. We verify the use of Angular Position data by comparing our derived detection angle with Diner's Attack Angle algorithm for a set of mobile vertical surveys on the same lake. Our derived detection angle methodology may also provide a method for fisheries biologists to correct school dimensions under conditions for which Diner's Attack Angle is not appropriate (i.e. when schools are smaller than the beam width), which should be common in lake surveys or generally when using elliptical acoustic beams.