Practical calibration of ship-mounted omni-directional fisheries sonars

Conventional ship-mounted vertically-oriented echosounders are poor at detecting organisms that are close to the sea surface. In contrast, omni-directional sonars can ensonify these near-surface waters unavailable to hull-mounted echosounders. If calibrated, sonars can provide quantitative biomass estimates of pelagic aggregations. However, for sonars that have not been designed as scientific and research instruments, the quantification and verification of the system performance is of heightened importance, and should include how parameters such as the shape and gain of the beams vary with system and operational configurations. We present a practical methodology for absolute calibration of omni-directional sonars when conventionally mounted on a vessel, illustrate the achievable calibration accuracies and precision, and document their variability over time and for a range of operating parameters. This work forms an essential prerequisite to the use of such sonars for quantitative measurement of backscatter, such as for echo-integration surveys and individual school density and biomass estimation.