Abstract:

Calibration of echosounders for fish stock assessment are commonly performed using the standard sphere method. A complete calibration includes characterization of system sensitivity versus detection angle for measuring target strengths (TS) of individual scatterers and the equivalent beam angle for measuring backscattering strengths of insonified volumes. To explore the accuracy of the method, on-axis TS measurements were made of three standard spheres (23-mm copper and 33.0- and 38.1-mm tungsten carbide), under controlled conditions at a frequency of 120 kHz (bandwidth = 1.2 kHz). The TS measurements, derived by both integrated and peak intensities, were compared to their theoretical counterparts. Also, system sensitivities and split-beam detection angles were characterized versus target bearings. To investigate measurement precision, a commercial echosounder (Simrad EK500) was used to measure the TS of the spheres over 15-h periods with constant pulse duration (0.3 ms) and water temperature (19 (degrees)C). To characterize the temperature dependence of the calibrations, measurements of system gain and transducer admittance were made versus water temperature (0--17 (degrees)C). Finally, bootstrap simulations were used to estimate the combined uncertainty in 120-kHz echosounder calibrations using optimal standard spheres and over ranges of temperatures typically encountered in fisheries surveys.