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Discovery of Sound in the Sea
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How is sound used to identify fish?

Experienced fishermen can use fish finders to identify particular species of fishes. As a fish moves through the sonar beam of a fish finder, a mark specific to that fish species appears on the chart display. To determine what species of fish produces what kinds of marks, a fishermen must be familiar with the area they are fishing, the fish that swim there, and the swimming patterns of different schools of fish. For instance, slow swimming carp produce short fat marks. Stripers swim much faster and move around a lot more, so they produce dotted lines. Baitfish may swim in circles as other fish herd them or as they swim to the surface, producing a third type of mark. These are just examples, and each fish finder will have different markings for a particular fish. Therefore, experience and close watching of patterns will help to perfect a fisherman's skill in using fish finding sonars to identify fish.

A fish finder display showing the seafloor and individual fishes.
Fish finder display shows the seafloor gently sloping down to 33.9 feet (black horizontal line with gray beneath). Individual fishes appear as arches on the display. (Courtesy of Lowrance Electronics, Inc.)

Scientists are developing new and improved methods to differentiate between the marks, also called echo signatures. Each species of fish has an unique size and shape of its swim bladder. The differences in swim bladders cause differences in the return echo of a sonar signal. Echo signatures for specific species can then be determined and used to identify fish.

Various fish species and their acoustic signatures
The unique echo signatures of 3 different species of Hawaiian snapper. Echo signatures on the left were taken from an anesthetized fish under controlled conditions at the surface. Echoes on the right were taken from free-swimming fish at 250m deep. Differences in echo signature structure are observed between species, but differences between control and free-swimming measurements are minimal for each species. Echoes measured at the surface, under controlled conditions, can therefore be applied to identify different fish species at depth. This type of acoustic data is important for fisheries surveys. (Courtesy of Kelly Benoit-Bird)

This method is especially useful when studying deep-sea bottom fish. Many fish are found below diving depth, and therefore, can only be studied using submersible vehicles or fishing gear. Acoustics provide an additional means for scientists to identify bottom fish and monitor them in their natural environment.

It is important to determine the factors that affect echo signatures. As water gets deeper, the pressure increases. The increased pressure can compress the air in the swim bladder as the fish dives. Using acoustics in conjunction with video and low light cameras, scientists have found that many fish have the ability to regulate the size and shape of their swim bladders, even under high pressure. The ultimate goal of the research is to identify the echo signatures of different species and make sure that these signatures do not change with movement. This will allow new fish finders to differentiate one species from another very accurately and quantitatively.