Shut up and fish!
  |  First Published: February 2014

Over the past couple of issues we’ve looked closely at the critically important senses of sight and smell in fish, and examined how these senses impact upon our success (or otherwise) as anglers. Before leaving this fascinating area, it’s worth considering one more fishy sense: hearing.

Hearing is definitely a significant sense in the fish world although, like sight and smell, it’s importance varies between species and habitats. A quick look at the way fish detect and translate sound waves travelling through the water (in other words, how they ‘hear’) is helpful in understanding the importance of this sense.

Fish’s bodies have roughly the same density as water, so any sound waves moving through the water also tend to travel right through their bodies. It’s reasonable to assume that fish can therefore ‘feel’ at least some sounds, much as we actually ‘feel’ the thump coming from a big bank of amplifiers at a rock concert. Fish with large swim bladders (gas sacs located near the gut cavity) are likely to be able to ‘feel’ sound much more readily than those without, as these hollow, low density organs will be momentarily compressed by the passing sound waves.

Even more importantly, fish also have an inner ear mechanism specifically designed to detect sound waves. There are small bones called ‘otoliths’ suspended in jelly within the inner ear of most fish, and these are considerably denser than either water or the rest of the fish’s body. As a result, these ear bones move more slowly in response to sound waves than the rest of the fish. The difference between the motion of the fish’s body and the otoliths bends little hairs or ‘cilia’ on the wall of the fish’s inner ear. That movement is interpreted by the animal’s brain as sound.

Most fish also have lateral lines, used to detect vibration and water flow. One source of vibration is obviously sound waves. It’s thought that fish’s lateral lines are mostly used to detect and analyse low frequency sound waves generated fairly close to the fish, typically within a few metres. This sense is particularly useful for maintaining a tight school formation, but probably also aids in detecting both prey and potential predators.

It’s also worth thinking about the way sound travels in water. For starters, did you know that sound travels more than four times faster in water than it does in air? Under specific conditions, sound waves can also travel a lot further underwater than they can through the air.

But what about the old story that sound can’t travel from air into water, so you can scream your lungs out up above and the fish won’t hear a thing down below? Well, remember when you were a kid swimming at the busy local pool, diving down to the bottom of the deep end and still being able to hear your mates shouting, yelling and skylarking above? The sound was garbled and subdued, for sure, but it still reached your brain, even via ears that had evolved purely for above water applications.

Certainly, when any wave reaches an interface between two media with different properties, it splits. Much of the wave is reflected back into the original medium by the interface, while the remainder is transmitted across the interface into the second medium. This is exactly what happens with sound waves at the air/water interface. Interestingly, the waves that make it into the water begin to travel much more readily, because of their increased speed.

So, what’s the bottom line? For me, the answer seems obvious: fish can definitely hear and react to sounds, whether they’re generated beneath or above the surface. Furthermore, fish with larger otoliths (ear bones), big gas bladders, linkages between the two and well-defined lateral lines obviously have extremely keen hearing. One such species that springs to mind as ticking all those boxes is the mulloway. And, as if to confirm the importance of sound in the mulloway’s world, these are also highly ‘vocal’ fish, capable of producing a distinctive drumming or croaking sound, no doubt as a form of school communication. It seems to be a lot more than coincidental that most of my more memorable mulloway luring sessions have occurred when there was very little boat traffic in the area and I approached as quietly as possible, using a fortuitous wind drift or an electric motor operating at low revs. In fact, I’ve even been known to switch my depth sounders off for ‘silent running’ when hunting these fish.

I’m a great believer in stealth when fishing for any species in any environment. I heartily dislike (and avoid) rattling anchor chains, booming doof-doof music, heavy-footed companions and even shouted conversations from boat to boat. While there may be plenty of exceptions to such rules of thumb, I believe that you’ll generally catch more and better fish by turning the volume down.

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