A day in the Fitzroy

[Douglas]

Bit about barramundi hearing. Decided to get "hearing" out of the way first as the light issue has a lot of twists and turns.

We've now done lateral lines and hearing.


What role does hearing play in barramundi fishing


No, its not that a fisherman must be able to hear “barra boofs”. It’s how well can barramundi hear.

Don’t get hearing confused with lateral line operation. Lateral lines deal with low frequency vibrations in the water by objects displacing water as they move around thus sending out waves of energy at low frequencies.

Hearing is to do with acoustic signals being transmitted through the water. These acoustic signals are frequencies higher than those to which lateral lines are tuned. These are within normal human hearing range. Barramundi are classed as hearing specialists in the piscatorial world.

Think about the car enthusiast with the $5,000 audio system in his car. Every vehicle stereo enthusiast has to have the biggest and best “woofer” speakers. A “woofer” is a low frequency speaker which produces base sounds.

A sub-woofer goes even lower in frequency to ranges that we can’t hear (we oldies anyway). But we can feel them in our body well before it becomes evident which ratbag driving past has his volume up (and we don’t even have a lateral line).

What about hearing then? Barramundi don’t have ears and we do.

Let’s go diving into the barramundi’s world where sound travels faster (up to 5 times faster than in air) and there’s less absorption. Put your head underwater and note the difference.

I can recall somebody on a fishing DVD with a black box and underwater microphone listening to underwater sounds one rainy night on the Fitzroy.

You’ll hear an outboard motor underwater long before you hear it with your head out (and louder too). So, can barramundi match our hearing? Yep, sure can. Their hearing (without ears) is just as sensitive as ours and occupies the bottom section of our audible frequency range.

Barramundi have a hearing threshold of 70db of signal pressure while a diver has a threshold of about 67db of pressure wave. Forget the technical figures. It just means they can hear about the same as us.
Barra also have a lateral line, so they’re streets ahead of us already.

But where are their ears? They don’t have ears that we would recognise but they have bony things called otoliths and they work in conjunction with their gas bladder to “hear”.

Now we’re starting to get into the subject of rattles. Rattles don’t work very well on lateral lines but they do on barra hearing if they produce noise between 100 hz and about 1,000hz (1 khz). Above 1 khz barra start going deaf. Steel ball bearings hitting together inside a lure produce sounds towards the upper limit of barra hearing. Lead bearings would produce lower frequency noise being denser but probably need lure design to overcome weight problem. Wooden balls would produce lower frequency noise but of a lower level.

To produce the rattle noise though, there'd need to be stabbing retrieves to get the balls to hit together or against the walls of the rattle chamber in the lure. A straight slow retrieve wouldn't produce any rattle noise.

We’re also into the area of boat, anchor chain, items dropped, walking around in boat etc….Anything that produces a noise. Trebles rattling around on split-rings during stabbing retrieves produce noise.

Remember the warnings in pet shops about not tapping on glass of aquariums? The close-up loud noise produced scares the hell out of the fish.