Calibration of echosounders for biomass estimation using seafloor backscattering at fixed transects

Backscatter from metallic spheres is the conventional reference for calibrating the amplitude response of echosounders when used for scientific acoustic biomass estimation. However, these can be time consuming to do, requiring specialized equipment and expertise. A simpler method is desirable when the expertise, equipment, conditions, and time are not available. This work considers the use of the seafloor at two sites northwest of the South Orkney Islands as calibration reference targets (#1: 100 m depth, 3.7 km long transect; #2: 300 m water depth, 1.8 km long transect). Easily implemented protocols for data collection, processing, and analysis were defined and data were collected in the period 2012-2023. Site 2 had lower within- and between-year backscatter variability than site 1, likely due to a more spatially homogenous seafloor substrate and the greater ensonified area that occurs at 300 m compared to 100 m. The within-year backscatter variability at site 2 was sufficiently low (0.4, 0.3, and 0.1 dB at 38, 70, and 120 kHz) to use as a replacement for sphere calibrations if near-simultaneous reference measurements were also available from a sphere-calibrated vessel. Additional data on the temporal variability of backscatter over periods of days to months are required to relax the near-simultaneous criteria. This would allow the use of backscatter measurements from a sphere-calibrated vessel to be applied to the uncalibrated vessels that visit site 2 throughout a fishing season.