Estimation of the target strength of juvenile salmonids at any aspect

Abstract

Many tasks, such as assessment of the state of stocks and observation of migrant behavior, elemental for recovery of Columbia River basin salmonid stocks, are currently performed using active acoustic methods. Detectability models are required to configure acquisition systems, design elements of studies, and process and analyze data. Accurate and precise estimates of juvenile salmonid target strength are necessary input to detectability models. The target strength at 420 kHz of 87 juvenile salmonids was measured within the length range of 45 to 300 mm. The fish were rotated through pitch, roll, and yaw planes. Forty target strength measurements were obtained for each degree of rotation in each plane for each fish. Mean target strengths were computed for 1° and 5° increments of rotation. The mean target strength estimates for each 5° increment of rotation for all fish were fit to the equation σ/λ2=a(L/λ)b. Estimates of mean target strength for regions of the sphere encompassing the fish between the planes of rotation were estimated by interpolation to complete a description of the target strength of juvenile salmon at any aspect.