Variations in echosounder -transducer performance with water temperature

Electro-acoustic transducers are central components of multifrequency echosounders used in remote-target identification and acoustic surveys for fish and zooplankton. Appreciable changes in echosounder system gains can result from shifts in transducer frequency responses with water temperature. Because it is standard practice to calibrate echosounder systems for fisheries surveys in one environment and apply the resulting gains to interpret data collected over the range of sea temperatures encountered during a survey, the results may be biased. Such biases may be different for estimates derived from each echosounder frequency. In moving to quantify and mitigate these effects, the performances have been measured for ten commonly used survey transducers in water temperatures ranging from 18 Ct o 188C, using three techniques. Results show that the transducer impedances all change with temperature, potentially changing the signal-to-noise ratio from 5 to .20 dB. The resonance frequencies and quality factors also change with temperature, ranging from 0.2% to 2.8% and 2.5% to .130%, respectively. Corresponding directly to changes in the echosounder gains, the transmitting-current and receiver-voltage responses changed 1 dB or less with temperature, except for the Simrad ES120-7, which showed a 2 dB increase. Generally, the magnitudes of frequency-dependent biases in echosounder-system gains depend primarily on the temperature-dependent performances of the survey transducers, the range of temperatures encountered, and whether the operational frequencies are less or greater than the resonance frequency.