title: Assessing a dual-frequency identification sonars' fish-counting accuracy, precision, and turbid river range capability.
abstract: 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 m, less than absorption and wave spreading losses predict, and 26 m 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.
authors.affiliation: Alaska Department of Fish and Game, 43961 Kalifornsky Beach Rd, Suite B, Soldotna, Alaska 99669, USA. suzanne.maxwell@alaska.gov
authors.forename: Suzanne L
authors.initials: SL
authors.lastname: Maxwell
authors.forename: Nancy E
authors.initials: NE
authors.lastname: Gove
mesh_terms: D000162:Acoustics / Q000295:instrumentation; D000413:Alaska; D000465:Algorithms; D025041:Animal Migration; D000818:Animals; D003198:Computer Simulation; D004867:Equipment Design; D005618:Fresh Water; D019015:Geologic Sediments; D008954:Models, Biological; D009391:Nephelometry and Turbidimetry; D011156:Population Density; D015203:Reproducibility of Results; D045483:Rivers; D012474:Salmon / Q000502:physiology*; D013550:Swimming; D014465:Ultrasonics; D014741:Video Recording / Q000295:instrumentation