Understanding a fish's hearing

Fisheries scientist Dr Mark Burdass has a warning for you – fish might not have ears that you can see but they can hear almost every noise you make. So tread lightly...

MOST anglers are aware that fish can pick up vibrations because they have both ears and a lateral line which can pick up any disturbance.

The problem for most anglers is that water is an excellent conductor of sound and vibration. A clumsy angler can rapidly denude his chosen swim of fish by driving up in the car, slamming the door then dumping his gear on the bank.

Then, when all but the most tolerant fish have been spooked off, he bangs in some rod rests and bank sticks for good measure. He then wonders why he hasn’t had any bites for a few hours!

Our freshwater fish can be divided into two general groups – the hearing specialists and the non-specialist.

The specialists are the cyprinids such as carp, roach and tench. All these fish have their ears hard-wired to their swim bladders by a series of bones from the spine.

This connection means that they increase their hearing range considerably, because sound is actually created by rapid changes in pressure.

This means that the air in the swim bladder will be affected more than the surrounding body of the fish. The connection to the ears passes on these vibrations so the fish can sense the sound.

A human can generally hear between 20 and 20,000 Hertz, (Hertz is a measure of sound frequency, or pitch). The average cyprinid can hear between 5 and 2,000 Hertz so they can tune into lower, deeper sounds than us but not up to the higher frequencies.

This means fish can probably detect someone walking much better than we can but would struggle to hear a bite alarm.

The non-specialists include fish like perch and pike. These species don’t have a connection between the swim bladder and their ears so they cannot hear sounds much above 500 Hertz.

These fish are often sight specialists relying on their hearing as a secondary sense. A recent scientific study showed that hearing specialists are particularly badly affected by noise from boat traffic. It seems excess levels of noise interferes with their ability to feed, while the non-specialists were unaffected.

All fish have a pair of ears – just behind the skull – and, as in people, they are responsible for hearing and balance.

But unlike our ears those on a fish don’t have any obvious outlet to the outside world. Floppy ears on a fish would be no use at all because they pick up sound from water so well without them.

This could also cause the fish a problem. As the fish itself is mainly constructed of water, a sound vibration may pass straight through it without the fish being able to sense it.

To allow the fish to hear, the noise or vibration must affect the relevant bits of the fish differently.

Hence the connection of the ears to the swim bladder, because noise travels differently through air than water.

In all fish ears there are also three bones called otoliths which are much denser than water.

These will also pick up the sound vibration differently to the rest of a fishes body.

The lateral line picks up low frequency sound and water movement close to the fish. It works by having a series of blobs of jelly in the lateral line under the skin which are disturbed by water movement or sound. Sensors in the blobs of jelly note the movement and pass this information to the brain.

Fish can also determine the direction of the sound as they have two ears and a lateral line on both sides of their body. The slight time delay between the two sides is enough for the fish to work out which direction the sound came from.

So, it’s best to keep quiet even though fish can become accustomed to loud noises if they occur regularly. I fish a lake close to a busy railway line and they are not upset by the regular rumbling of trains – but will spook easily by the sound of me setting up my fishing chair.