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Amplifier linearity accounts for discrepancies in echo-integration measurements from two widely used echosounders


 The Simrad EK60 echosounder is widely used in acoustic-trawl surveys. It has recently been replaced by the EK80, which can be configured to operate in a manner similar to the EK60. To examine whether EK80s can be substituted for EK60s, the echosounders were configured to alternate transmissions from common transducers at four frequencies during three acoustic-trawl surveys. Significant differences between echo-integration measurements of fish were observed at 18, 38, and 70 kHz. EK80 measurements were 3–12% lower than those from EK60. At 120 kHz EK80/EK60 ratios were less than, but not statistically different from one. The EK80/EK60 discrepancy increased with range. The observed discrepancies were identified to be related to slight over-amplification of low-power signals (<−90 dB re 1 W) by EK60. EK80 amplified signals linearly over a wider range of measured powers. After accounting for over-amplification of weak signals by the EK60, the range dependence was removed and both echosounders produced equivalent results. The impact of over-amplification by the EK60 will be relatively small for surveys of strong scatters (fishes with swimbladders) at short ranges, but has the potential to be greater for surveys of weak scatterers and/or long observation ranges.