Accuracy and Precision of Salmon Length Estimates Taken from DIDSON Sonar Images

Experiments were conducted with a multibeam dual-frequency identification sonar (DIDSON) to evaluate the accuracy and precision of estimating lengths from images of tethered fish insonified at side aspect in an Alaskan river. Live tethered Chinook salmon Oncorhynchus tshawytscha and sockeye salmon O. nerka were suspended in front of a long-range DIDSON (1.2 MHz, 48 beams) equipped with an ultra-high-resolution lens. Lengths measured manually from DIDSON images were highly correlated with the actual lengths (R(2) = 0.90, RMSE = 5.76 cm). No range dependency in the accuracy of the range estimates was documented. We conclude that relatively accurate and precise estimates of fish length are now possible with certain DIDSON system configurations at up to 21 m.