Hydroacoustics may be one of the most misunderstood techniques for fisheries research. In essence, hydroacoustics monitoring is based on a few relatively simple principles. An acoustic echo sounder transmits a pulse of acoustic energy into the water. The pulse of energy travels through the water at a speed of approximately 1500 m/sec. When the acoustic pulse encounters an object, such as a fish or the bottom,
some of the energy (i.e. an echo) is reflected back to the transducer. The echo sounder amplifies the received signal and then sends it to an output device (such as a chart recorder or
video display) and digital echo processor.
If the signal level exceeds a mark threshold level, a mark appears on the output display device. The distance from the top of the display to the mark is proportional to the travel time for the pulse to travel from the transducer to the target and back. Since the velocity in water is known, range (distance from the transducer) can be calculated from this travel time.
By displaying the output resulting from many consecutive transmissions, the time “change of range” and direction of objects in the acoustic beam can be monitored.
The basic system includes a high frequency echo sounder, one or more transducers with cables, a chart recorder, an oscilloscope, and a computer-based echo processing system. If more than one transducer is used, a multiplexer is also required. To learn about the individual components, mouse over the “Typical hydroacoustic system” (left).
Only research-quality equipment that does not drift in its parameters and can be accurately calibrated will allow month-to-month, or even hour-to-hour, comparisons of results.