Evaluation of potential bias in observing fish with a DIDSON acoustic camera

The DIDSON multi-beam sonar has become a popular tool for monitoring fish. The aim of this study was to determine how well the DIDSON sonar can detect and measure fish during stationary applications in lakes. Cyprinid fish of known sizes (10–60 cm long) and with horizontal body orientations were deployed in known positions at two ranges (6.3 and 9.5 m) within a DIDSON high-frequency array of beams. A new method for estimating fish length from multi-beam sonars was thus developed and applied.At both ranges, all the deployed fish were invariably detected when they were aligned with their sides perpendicular to the beam. Increasing fish body angle, however, reduced the ability of the system to detect fish. Only the largest fish were detectable when the fish were parallel to the beam (head or tail aspect). The estimated fish length agreed well with the actual length when the fish were positioned perpendicular to the center of the multi-beam array. The lengths were underestimated for fish that were at the edges of the array, further away from the transducer or that had an increased body aspect, especially small-sized fish. Additionally, we observed that the wide girth of large fish can shadow the rest of the body, resulting in a decreased estimated length.We showed that determining the actual length of fish is challenging and is not a trivial task, and we raise the question of where to define fish length along the echo intensity of fish. We conclude that including the error functions for length estimates allows the DIDSON to obtain more reliable and accurate biological information.