Hearing and detection of vibrations are the best-developed senses in most fish, making use of the good propagation of low frequency sounds through water. The main sensory organs involved are the lateral-line system, which detects low-frequency (<100 Hz) particle motion in the water contacting the flanks of the fish, and the inner ear, located within the head of the fish, sensitive to frequencies of up to 1-3 kHz. The lateral line organ is almost certainly involved in acoustic repulsion when the sound source is at close quarters (within a few body lengths of the fish) but the inner ear is thought to be the main sensory organ involved. The inner ear, which lies within the skull of the fish is sensitive to vibration rather than sound pressure. In teleost species (bony fish) possessing a gas-filled swimbladder, this organ acts as transducer that converts sound pressure waves to vibrations, allowing the fish detect sound as well as vibration.
Sensitivity to noise and vibration differs among fish species, especially according to the anatomy of the swimbladder and its proximity to the inner ear. Species with no swimbladder (e.g. elasmobranchs) or a much reduced one (many benthic species including flatfish) tend to be of relatively low auditory sensitivity and may be only weakly repelled by acoustic deterrent systems other than those designed specifically for that purpose. Fish having a fully functional swimbladder tend to be much more sensitive. Best results are obtained with those species in which there is some form of close coupling between the swimbladder and the inner ear. In the clupeids (herring family), this takes the form of a gas duct connecting the swimbladder to the hearing system, whereas e.g. in cyprinid fish, a bony coupling is formed by the Weberian ossicles. This creates a super-league of hearing-sensitive fish. There are therefore three broad groups of low-, medium- and high-sensitivity.
The effect of the different anatomical types has been clearly shown at estuarine power station cooling water intakes fitted with acoustic deterrent systems. A feature of coastal and estuarine habitats is the high diversity of fish species. These are often drawn in large numbers and can be quantified when collected from the plant's band- or drum-screens. Typically, long term monitoring at temperate locations will reveal between 30 and 90 different fish species. Their relative frequency when an acoustic deterrent system is turned on or off provides a good test of the different sensitivities. Broadly, the fish can be grouped into different sensitivity categories according to whether they are pelagic, demersal or benthic, as the following table based on European power station trials shows:
|Herring, Sprat, Smolt, Shad, etc.||Sea Trouting, Cod, Whiting, Gunards,etc.||All Flatfish, gobies, rocklings, etc.|
A fairly general classification of fish according to hearing sensitivity can be made according to the anatomical descriptions given above. Although we do not have efficiency figures for every species of fish, if you tell us the scientific (Latin) name of the species, we can classify it accordingly. Unfortunately, it is not enough to give the common name, as many different species are known by the same name in various parts of the world.
For further information on the likely deflection efficiencies for fish you want to deflect please contact FGS.