The following excerpt is from the book The Fisherman’s Ocean by David A. Ross, Ph.D. Reprinted with permission from Stackpole Books, Mechanicsburg, Pennsylvania.
The Inner Ear
[dropcap]L[/dropcap]ooking at fish, it is easy to assume that they are not very sensitive to sound, since they do not have obvious ears or even auditory openings through which sound may carry. They do have ears (actually only one), but it is an inner ear enclosed within their head. A fish does not need external ears because its body tissue is about the same density as water and any sound in the water can easily pass through its body to its inner ear. The inner ear system is mainly used for detecting high-frequency vibrations, though fish also use it to determine their orientation relative to the Earth’s gravitational field—in other words, to maintain their balance.
The inner ear has several fluid-filled canals, some of which contain relatively heavy ear stones, or otoliths. (The term otolith comes from a Greek word meaning “ear stone.”) A fish’s otolith is composed of calcium carbonate and frequently has growth rings similar to those seen on tree rings or fish scales. These rings can be used to determine the age of a fish. Otoliths vary so much in size and shape among species that they often can be used to identify the fish species from which they came.
The inner ear is often linked to an internal organ that can amplify sound, such as the swimbladder. Swimbladders are filled with gas that can be compressed by sound waves, which usually will make them more sensitive to sound than the inner ear. Fish with especially sensitive hearing, such as tarpon, have closely connected inner ears and swimbladders. Fish that do not have a swimbladder, or do not have their swimbladder in close connection with their inner ear, generally are less responsive to high-frequency sound and respond to a smaller range of sound frequencies.
The same as humans, fish are essentially deaf to sounds having a frequency above 20,000 cycles per second (so-called supersonic sounds). Most fish hear relatively low frequency sound of around 1,000 cycles per second, and are relatively insensitive to sound above 3,000 cycles per second. Certainly, some fish are more sensitive to sound than others. Recent research has shown that shad and herring species can detect high-pitched signals considerably beyond what humans can hear.
The Lateral Line
A fish’s lateral line picks up nearby or near-field (within 20 to 30 feet or so) low-frequency sound caused by water movement and turbulence, which can be caused by currents or breaking waves, by movements or activity of other animals, or by movements of the fish itself. The lateral-line system is extremely accurate when the sound is within 5 feet or so. The ability to sense sound through the lateral-line system is sometimes referred to by scientists as a “distant touch” sense. Fish living in relatively quiet waters usually have a simpler lateral-line system than those living in more turbulent water. Deep-sea fish usually have well-developed lateral-line systems.
A fish can use information from its lateral line in several ways, but especially to detect nearby moving prey or predators, and to avoid running into objects at night or in the darkness of turbid or deep water. These abilities are highly efficient because the lateral-line system is very sensitive to low-frequency sound vibrations. In some instances, the fish itself will cause a water vibration, by swimming, and then use the rebounding sound information caused by its own movement and received by its lateral-line system to detect nearby objects (including lures) in the water.
A fish also uses its lateral-line system to determine the direction and distance to the source of a sound, sometimes with startling accuracy, and to orient itself in a current or position itself among waves. Experiments have shown that certain fish can detect and catch prey within 10 to 20 feet by using their lateral-line system.
Schooling fish can use their lateral-line systems to maintain their positions relative to adjacent fish. The sounds from adjacent swimming fish, or just the general noise made by a school, can help individual fish maintain their tight-knit pattern in the school, especially when visibility is poor. To test the importance of the lateral line in schooling, scientists conducted experiments using “blindfolded” schooling fish. These fish had no difficulty maintaining their schooling pattern with adjacent fish by using their lateral-line systems. If, however, the nerve endings of the lateral-line systems of selected fish were cut, they were unable to school with other fish. Scientists also recently found that the lateral-line systems of some fish may be sensitive to certain types of chemicals in the water. Additionally, certain fish use their lateral-line systems to generate and receive electrical signals, apparently for communication and for identification of the opposite sex.
In a few species, the lateral-line organ is also part of an electrical sensory system. Sea water is a good conductor of electricity and some fish, particularly sharks and rays, use their electrical sensory system to detect the electrical field generated by their prey (from the activity of the prey’s muscles). Fish may also use these electrical sensory systems for long-distance navigation, whereby the fish navigates by detecting subtle changes in the voltage gradients induced by ocean currents flowing through Earth’s electrical field. Eels are especially sensitive to electrical fields. One study indicated that, amazingly, they could detect the field generated by a one-volt battery whose poles were separated by more than 3,000 miles.
In next Wednesday’s Fish Facts Ross explains more about fish SENSES and how to apply them to improve your fish-catch stats.