Sound, Vibration, and Muskies

In an earlier article I wrote about the underwater environment of muskies, I briefly mentioned their vibrational environment. These ideas are closely related to the properties of sound since sound is produced by vibrations transmitted through a material. For humans, the vibrating material is air whereas for muskies the vibrating material is water. When materials vibrate, their vibration can be thought of in two different ways that boil down to the same thing: 1) the particles of that material are moving back and forth, or 2) the pressure within the material varies above and below the ambient background pressure at a specific rate. How the human ear detects the vibration associated with a sound wave traveling through air is that the pressure at the person’s ear drum membrane varies up and down. When the pressure is at its highest value, it pushes the ear drum membrane into your head slightly, whereas when the pressure is at its lowest value below the ambient pressure, it sucks the ear drum membrane slightly outward. This continually alternating oscillation in the pressure causes your ear drum to vibrate at the same rate as the air. Your inner ear detects this and translates it into a signal that your brain interprets as sound. The rate at which the oscillations happen is called the frequency of the sound. Frequency is measured as the numbers of full wiggles per unit time (eg. oscillations per second, the unit that frequency is expressed in is the Hertz, abbreviated Hz). Our brains interpret different frequencies as different pitches. A very high frequency corresponds to a high note like a piercing whistle, while a low frequency would correspond to a low bass rumble. Human ears have limits to the frequencies we can detect. Sounds below about 20Hz and above about 20000Hz are inaudible to us.

Why the detour into human hearing when we are more interested in muskie hearing? Because there are a lot of similarities between the way muskies hear and how we humans hear. First, muskies don’t really have ear drums or external ear flaps like we humans do. Fish like muskies, instead, have a swim bladder on the interior of their body that serves the same purpose as our ear drums (in addition to regulating the muskie’s buoyancy!). The vibration of the swim bladder due to external vibration in the water is detected by structures in the muskie’s head that have a similar function to the middle ear of humans. The main difference is in the frequency response of these structures compared to humans. Research on fishing hearing appears to show that their ears are capable of detecting sounds from about 100Hz up to 1000Hz or so. This means that we humans are capable of detecting much higher pitches of sound than muskies. I would be remiss, however, if I didn’t say that this research on fish hearing is still being conducted. After all, it is difficult to make fish respond in a definitive way to hearing tests, and it is also very challenging to put fish in their natural environment and conduct such tests in a repeatable and definitive way. If new research comes to light that gives better information on how fish respond to sound, I will change my ideas and I will let you know with a new article!

Now let’s consider the range at which sound waves are detectable by fish. To understand this, let’s consider what happens to a sound wave as it gets further away from its source. As sound moves outward from its source, the size of the vibration gets smaller and smaller with distance. This is due to the damping effect of having to push the material back and forth. This means that the sound is perceived as quieter at greater distances. Eventually, the volume of the sound will be small enough that it is imperceptible. This happens when the strength of the sound signal is smaller than the background noise. For sound propagating in water, this distance of “fading into the background” is probably at most about 100m for sounds that start out loud, and much less than that (maybe 10m or so) for sounds that start out quiet.

The other major structure that fish use to detect vibrations is their lateral line system. The lateral line system in fish has no counterpart in humans, and it functions quite differently than the fish’s “ears”. The lateral line consists of a series of small hairs in pores that run the length of the fish. Rather than detecting pressure variations in the water (like the swim bladder does), these hairs move in response to the motion of water past the side of the fish. If a disturbance is created in the water, that disturbance can be detected by the motion of water past the hairs with the combined signal of all the hairs simultaneously being processed by the fish’s brain. Like the muskie’s ears, the lateral line system has only a limited range of frequencies that it can detect. In fact, the lateral line is sensitive to even lower frequencies of vibration than the muskie’s “ear” system: from a few Hertz up to maybe about 100Hz. The region of highest sensitivity appears to be about the 10-20Hz range.

Next, let’s discuss the range at which vibrations can be detected using the lateral line. Recall that detection by the lateral line requires actual movement of water in the vicinity of the muskie that is trying to detect the vibration. This water movement tends to weaken very rapidly with distance from the source of the vibration. This rapid dampening of the vibrational size explains why researchers have found that the lateral line has a very short range of detection. In studies of the effectiveness of the lateral line in largemouth bass, researchers found that the predators responded to lateral line stimulation when the source of vibration was within only a few feet. These results were supported by research with juvenile muskies. Blind muskies would only respond to swimming minnows when the minnows were within about a body-length of the muskie. The acuity of the lateral line, however, was excellent, meaning that both types of predators were able to get extremely precise positional information for their prey based only upon input from the lateral line. So while the lateral line appears to only have a very short range it is crucial for muskie predation, allowing muskies to combine visual information AND vibrational information when striking their prey.

Now that we know a little something about how muskies detect vibrations, we can speculate a little but about how muskies can perceive these vibrations. A forage fish swimming in the water will create low frequency vibrations via its paddling motion. Also, just by pushing water out of the way as it swims, the forage fish is creating low frequency disturbances in the water. What sort of frequencies are we talking about? Think about the rate at which the minnow swishes its fins back and forth and you have a good sense of the frequencies involved. It’s probably on the order of 3-10 wiggles per second (3-10Hz). Of course, there is some high frequency “noise” put out as the fish slaps the water back and forth, but they will generally be quieter than the main “swish” of the water being moved by the fish’s tail. These low frequency vibrations are not really in the range of frequencies that a muskie would detect as sound using its ears. Instead, these vibrations would be detected via the muskie’s lateral line. Since the lateral line has a short range, the vibrational signals are unlikely to “call” a muskie in to investigate from long range unless it was accompanied by high frequency noise (in the 100Hz-1000Hz range) that could be detected by the muskie’s ears from much further away. Note, that if a low frequency of oscillation has a very big amplitude (amplitude is the size of the oscillation), then they could be perceived at slightly greater distances, of course.

Let’s put these observations in a muskie fishing context. A bucktail spinner produces a low frequency vibrational profile because of the rotation of the blades. Consider how fast the blades spin: maybe 10 rotations per second at maximum? Definitely not 100 times per second. So these vibration do not fall into the realm of perception of the muskie’s ear; the frequency is just too low. However, those vibration would light up the muskie’s lateral line like crazy, but that only happens at short range. So what would be necessary to make these lures call in muskies from greater distances? Sound and flash. By “sound”, I mean frequencies that are high enough to be audible to a muskie’s ear, something above 100Hz. This is why bucktail spinners that have a grind or a squeal to them tend to be very effective at attracting muskies at greater ranges. High frequency sound vibrations are perceived at longer distances compared to the very low frequencies that excite the muskie’s lateral line. Flash is also important, because it appeals to a second sense that muskies use to locate prey: their sight. So when a big, double-bladed spinner with different-sizes blades is spinning and grinding, it can have significant calling power. The different-sized blades make the whole lure wobble at the same rate as the blades rotate. This causes a very large amplitude of low frequency oscillation while it is creating high frequency oscillations, both of which can be detected at range.

There has been some discussion lately about whether muskies and other fish can detect sonar pulses from our electronics units and whether some fish are becoming negatively conditioned to this phenomenon. To weigh in on this subject, I need to discuss a little bit about the sound that is being produced by our sonar units. Sonar units put out sound in the ultrasonic range of frequencies. The LOWEST frequency of sound put out by sonar units is at 83kHz (kilohertz are thousands of Hertz), that is 83000Hz. The frequencies used by side imaging and live imaging sonar units are in the 400kHz to 1MHz (megahertz is a 1000kHz i.e. a million Hertz). As I mentioned before, the highest frequency that fish have been scientifically demonstrated to perceive is about 1000Hz (maybe 2000Hz if the sound is outstanding loud). What this means is that if the sonar transducers were putting out a continuous vibration at its normal ultrasonic frequency, muskies couldn’t perceive it. Even with our superior high frequency sensitivity, we humans couldn’t detect a vibration with that high of a frequency. It is inaudible, and it really isn’t a matter of the transducer putting out a very loud version of that high frequency. These frequencies are about 1000 times too high to be perceivable by fish using their ears.

There is a feature of these sonar signals, however, which may make them perceivable to humans and fish. Sonar transducers don’t put out continuous sound vibrations. What they do is send out short pulses of sound with measurable pauses between pulses. To use a familiar analogy with music, it is like they are playing a repeated note on a guitar. When you pluck the string of a guitar, the string vibrates with a certain frequency to make the note (eg. the note on a guitar’s low “E” string is about 82Hz). If you pluck that guitar string repeatedly and regularly, three times per second, you are producing an 82Hz sound three times per second. That is a pulsed sound: 82Hz on then off then back on then off, etc… The same thing is happening with sonar. The transducer plays a very short 500kHz “note” a few times per second with a measurable pause between “notes”. The thing is you can’t hear a 500kHz note. But it MAY be possible that the housing of the transducer (or the piezoelectric crystals within the transducer) will change shape a bit during the time they are emitting sound and then contract again when the ultrasound is “off” during the pulse. This would give rise to a relatively quiet clicking sound from the pulsing of the transducer (not the overall carrying signal). This is the only effect that I can think of that would give rise to fish being able to perceive a sonar signal: they can hear the “tap-tap-tap” of the ultrasound being pulsed out of the transducer.

That said, I have not noticed this conditioning effect myself. Fish like bluegills, crappie, etc… do not avoid a live sonar beam in my experience. They certainly respond to the motion of lures, but they exhibit no response to sonar in my experience. As for muskies’ response, I have used live sonar to observe muskies shying away from the boat while following a lure, often at a range of about 30-40 feet from the boat. However, this is consistent with these muskies having seen the boat rather than being spooked by the presence of sonar pulses in the water. As they say in the world of automobile reviews, your mileage may vary: your experience may differ from mine.

For those that wonder whether the muskie’s lateral line is what may be detecting sonar pulses, the frequency response of the lateral line does not bear that out. The top end of frequencies that can be detected by the lateral line is about 100Hz, far less than even the lowest sonar frequency of 83000Hz. The million hertz frequencies associated with live sonar are even less likely to be detected by the lateral line than standard sonar. And that’s not even considering the very limited range of the lateral line system. Remember, the lateral line system detects the motion of water past the fish. The size of the back-and-forth motion associated with a vibrating transducer crystal is extremely small and gets smaller very rapidly with distance.

My observation that it is unlikely that sonar is detectable by fish could be incorrect. But the reason it may be incorrect would be due to errors made in measurements of the frequency response in fish hearing and lateral line sensitivity within the scientific community. I guess we’ll have to wait to see if we get better information about hearing in fish.

Best of luck on the water!

Dr. Bob