It’s True Crappie Can’t Hear Sonar by Brad Wiegmann

Dock talk has been spreading the rumor that crappie can hear marine electronic high frequency sonar waves. These anglers are assuming this because whenever the live imaging sonar is on the crappie get spooky and move away from the boat. However, the truth to the matter is crappie are not capable of hearing high frequencies that are being pinged by sonar units.


Sound waves produced by marine electronic units are ultra sound. Humans are capable of hearing sound waves from 10Hz to 20kHz. Most fish can’t hear frequencies above 500Hz to 1kHz. Marine electronic sonar units for freshwater commonly use 83kHz, 200kHz, 455kHz, 800kHz or 1.2MHz well above the hearing threshold of crappie.



The sounds that fishes hear are confined to low frequencies (often to no more than 800-1,000Hz, but this is very species dependent). Dr. Arthur N. Popper, Ahukin.net University of Maryland, Department of Biology.

A quick overview shows that fish get substantial information from the environment around them and provides information from greater distances than their other sensory stimuli. Fish use particle motion that can be measured in terms of displacement, velocity or acceleration; in addition to sound pressure.


So what are fish hearing? The obvious things they are hearing are lids being opened and slamming shut, striking your lure or boat in to docks, noise baitfish make, wakes striking the side of the boat or shallow water anchors and even landing or throwing a fish back into the lake fish can hear. The list goes on, but you understand an angler in a boat being in stealth mode is even noise. I believe that the spooky behavior angler’s are seeing when the unit is in live imaging sonar is caused mostly by trolling motors props and the particle motion and sound pressure they produce.


Trolling motors props create a disturbance every revolution of the blades through the water. An angler changing direction, reversing to forward to sudden acceleration produce prop wash from a trolling motor. When props are turning they will even force particles or large fragments in to the water column even in gin clear water or around floating vegetation. Plus a prop can also be off-balance creating even more sound fish can hear.


Not all trolling motors are built the same. Lowrance Ghost and Garmin Force trolling motors utilize brushless motors this means less noise being produced while in use. Rumor is that Minn Kota is developing a brushless motor to contend with the other manufactures.


In nature there are numerous other sounds being produced that are louder than the undetectable pinging of your sonar unit. Even in natural environments, the level of ambient noise from natural sources can also affect the ability of fishes to detect sounds (Chapman and Hawkins, 1973).


What were are learning from live imaging sonar is crappie behavior and without this technology anglers were just assuming what the crappie were doing. Now we know what the crappie are doing by actually seeing how they react to lures, trolling motors, boat noise, other outside noises, particle motion and sound pressure.


Fact is ice fisherman have used sonar to identify fish along with watching a fish bite their lure since they invented fish finders. If sonar sound waves spooked fish away ice fishermen using sonar would have never caught a fish.


On a recent guide trip I was on the guide acknowledged that he thought that crappie were hearing the trolling motor whenever he was fishing in shallow water. To eliminate using his trolling motor he used a push-pole like the ones to move boats saltwater flats fishing or deep in thick aquatic vegetation. The guide still used his live imaging sonar attached to his boat with a transducer pole, but never turned on his trolling motor to make the trolling motor prop spin.