Re: Why do certain sound frequencies attract or disperse fish?

This is a wonderful question! In a short answer, we don't know why fish respond in different ways to different types of sound. You and I also respond differently to different sounds, and that process is also not well understood. There are many people currently studying the ways in which animals hear and how that information is processed in the central nervous system.

Fish have two main organs that they use to "hear", the otolith organs and the swim bladder. The otolith organs are similar to our ears in location and function. They contain the otoliths, which are dense, gelatinous structures consisting of calcium carbonate crystals . When sound waves reach the otoliths, they cause specialized hair cells to send signals to the brain. The otolith organs are responsible for detecting a variety of frequencies and providing directional information. The other "hearing" organ is the swim bladder. It's main function is to allow the fish to change its depth in the water. However, sound causes the volume of the swim bladder to change briefly, and the fish can use this to detect very sensitive frequencies of sound. However, the swim bladder provides no directional information. Therefore, fish detect the type of sound with both otolith organs and the swim bladder, and determine where the sound is coming from through the otolith organs.

Once these signals reach the brain, the fish interprets them and decides how to react. The way the brain translates this kind of information is a very important question and there are no good answers yet. It is safe to say though, that fish are probably attracted to sound which they associate with food or other friendly fish, while they probably swim away from something that sounds dangerous. How the fish knows which sounds are bad and which are good is another interesting question with no good answers yet. There are a number of people studying the neural pathways responsible for hearing, but how hearing relates to behavior is unknown. Such behavior could be innate or learned over time. But a fish that does not know to swim away from what sounds like a predator is likely to soon be lunch, so only the fish that make these appropriate distinctions will live see another day! Therefore, only the "smart" fish will be selected to create the next generation of fish.

For a picture showing the otolith organs and the swim bladder, go to http://zfish.uoregon.edu:80/zf_info/zfbook/stages/figs/fig39b.jpg. Frame G shows the front of an embryonic zebrafish. The eye is the top dark circle and the swim bladder is the bubble-like organ lower in the picture. In between the eye and the swim bladder is a clear circle of similar size which is the otolith organ.