A day in the Fitzroy

[Douglas]

Some people think they can hear the sounder frequencies working away on their boat. They can't.

What they're actually hearing is the audible sound of the sharp rise time of the pulse hitting the membrane of the sounder face. Put your hand over the transducer while its operating and you'll see what I'm referring to. A sounder sends out so many of these pulses in a second. Sometimes these pulses are called "pings" if you watch submarine movies.

Frequencies of 50khz, 83khz, 200 kHz, 300khz, 455 kHz, 800 kHz or 1200 kHz are the frequencies transmitted by the transducer depending on what type of sounder you've got (fish cannot hear these frequencies).

The rate at which the sounder sends these pulses out depends on the depth of water. If deep water, then it takes longer for the pulse to travel there and back so the longer the sounder needs to "listen" for the echo. That dictates that the rate of pulses being sent out is slower. That rate is called the Pulse Repetition Frequency (PRF). War movie buffs will know it as the ping speed.

It shows up as a low level "clicking" sound which can be heard by fish and humans. It sounds like the ticking of an electric fence which operates on a similar principle except the fence sends its pulse along the wire while a sounder sends its pulse out into the water.

It is a low level and low frequency rate between 5 and 30 times per sec. Some sounders allow the operator to "fiddle" with the PRF (or "ping" for the movie watchers) but most change the rates automatically on selection of depth or frequency selected by the operator. There is usually an adjustment for scroll speed to keep up with increased pulse rates when travelling fast. When travelling fast, more pulses are needed per sec to get some detailed echos onto the screen.

I found out last night that there has been a "underwater duck caller" on the market (and in use) by anglers for some time.

Apparently this "fish/duck caller" is marketed under the name of Hydrowave. It plays audio sounds into the water of a variety of sounds of fish activities designed to attract fish to the vicinity of the boat.

Is it a successful "fish caller"? Anybody got one?