Assessing a dual-frequency identification sonars' fish-counting accuracy, precision, and turbid river range capability

Accurately assessing migrating salmon populations in turbid rivers with hydroacoustics is challenging. Using single, dual, or split-beam sonars, difficulties occur fitting acoustic beams between the river's narrow boundaries, distinguishing fish from nonfish echoes, and resolving individual fish at high densities. To address these issues, the fish-counting capability of a dual-frequency identification sonar (DIDSON), which produces high resolution, video-like images, was assessed. In a clear river, fish counts generated from a DIDSON, an echo counter, split-beam sonar, and video were compared to visual counts from a tower, a method frequently used to ground-truth sonars. The DIDSON and tower counts were very similar and showed the strongest agreement and least variability compared to the other methods. In a highly turbid river, the DIDSON's maximum detection range for a 10.16 cm spherical target was 17 in, less than absorption and wave spreading losses predict, and 26 in in clear water. Unlike tower and video methods, the DIDSON was not limited by surface disturbances or turbidity. DIDSON advantages over other sonars include: better target resolution; wider viewing angle; better coverage of the water column; accurate direction of travel; and simpler to aim and operate.