Pelagic fish are known to school and shoal. Such behaviors cause fish to be non-uniformly distributed within the aquatic environment. Non-uniform or patchy fish spatial distributions create many logistical and statistical problems for net or trap surveys of fish abundance. With the aid of acoustic data processing software, the return echoes are used to measure fish densities, map fish spatial distributions, and measure the size of individual fish targets.
Acoustic beams are most sensitive directly below the transducer and less sensitive on the edges.
A small fish will appear larger on axis than a big fish on the edge, so it is important to know where a fish is in the acoustic beam. Locating Fish within the Acoustic Beam Split beam transducers measure acoustic signals on four independent receivers that correspond to the quadrants of a circular transducer face. Differences in the time of arrival of an individual echo to each quadrant may be used to locate the off-axis angle of a fish target and its position within the acoustic beam. The air bladder is the primary sound-scattering component of fish. Changes in the shape and orientation of the air bladder strongly influence the target strengths of individual fish.