Acoustic Measurement of Fish Schools Using Array Phase Information

The usefulness of side-looking sonar in fisheries assessment work can be greatly increased by employing array phase information to estimate the echo arrival direction. Vertical and horizontal coordinates (perpendicular to the beam direction) can be measured, permitting beam pattern corrections, tracking from ping to ping, and depth determination. Acoustic data have been gathered during cruises in the Northeast Pacific, Caribbean, and Northeast Atlantic using both side- and down-looking transducers. The down-looking transducer was mounted in a towed body and interfaced with a simrad EK 120 scientific sounder. The side-looking transducer was part of a towed measurement system that provided phase-coherent recording of signals received by transducer quadrants.[1] This system contained sensors for speed, depth, pitch, and roll. Depth distributions have been computed using both side and down-looking data. Estimates of nekton biomass versus vertical and horizontal distance were made by echo integration of down-looking data. Target strength distributions have been computed from the side-looking data, with and without beam loss corrections, which produce shifts in the distributions on the order of 3 dB. Echo-to-Echo fluctuations in side-looking depth measurements correspond to a standard deviation of 10 -20 m for the Northeast Atlantic schools, which are assumed to be Blue Whiting. Information is also presented on Northeast Pacific rockfish schools and on unknown species in the Western Caribbean Sea.