Biohydroacoustics

biological hydroacoustics; the study of the sounds produced by aquatic organisms.

Biohydroacoustics arose during World War II in connection with the widespread use of engineering hydroacoustics—submarine detection, echolocation, communications, and so on. A great many aquatic organisms (fish, mammals, and crustaceans) had already been observed to emit sounds. The biological sounds were so varied and loud that they interfered greatly with hydroacoustic apparatus, even causing acoustic mines to explode. Therefore, data on the spectral composition of the sounds and on sound pressure were needed to ensure the normal operation of hydroacoustic devices and develop protective equipment. In some countries the noise of torpedoes and submarines began to be masked by sounds emitted by fishes. Biohydroacoustics is of great importance to the navy. One of the problems of hydroacoustics is to identify and classify the objects detected, especially because of the appearance of relatively quiet nuclear submarines. Biohydroacoustics is useful in determining whether a target is actually a submarine rather than a school of fish or a whale. The intensity of the sounds emitted by fish (in this case the source of the hydroacoustic interference) may be very great. Therefore a knowledge of the physical structure and composition of the sounds, their regionalization in the seas, and the times when they are most intense is important for the proper organization of systems for the detection and recognition of underwater objects.

Hydrobionics has raised some specific practical questions for biohydroacoustics. On the basis of data obtained by biohydroacoustics, engineers are creating devices to protect the acoustic lines of underwater communications. Biohydroacoustics can help to find ways of increasing the resistance of underwater telemetry systems to noise interference.

One of the principal and most widely studied branches of biohydroacoustics is the bioacoustics of fish. Research has shown that fish are capable of emitting acoustic signals in a frequency range from 20–50 Hz to 10–12 kHz (see Table 1). Since many fish make sounds common only to their own species, they can serve as biological criteria for identifying fish by species and age. The sounds change with the biological cycles of the fish at different times of the year (reproduction, fattening, wintering) and with the diurnal changes in light. Fish behavior (their reaction to fishing gear, interrelations of predator and prey, maintenance of contact in a school, acoustic signaling) can be understood and properly acted upon only with knowledge of the characteristics of fish hearing and, accordingly, of the possibility of their perception of different sounds.

Biohydroacoustics is promising for commercial development of certain fish species and identifying the species to which detected concentrations of fish belong. The main equipment used to search for fishes is hydroacoustic fish-searching apparatus, which utilizes methods of echolocation and permits the precise determination of the depth and size of schools of fish discovered, their rate of movement, and density of concentration. However, it is difficult to determine the species of fish making up a given concentration with this apparatus, although it can sometimes be done from the shape of the echo recording provided that the region has been well studied and the persons handling the apparatus have had sufficient experience with it.

The search for certain fish species—for example, tuna—with ordinary fish-searching devices is very arduous owing to the great speed at which they move. Hydrolocation is not sufficiently effective when searching for bottom fishes living in offshore rocky regions because of the complex relief

of the bottom. Some crustaceans of commercial value—for example, shrimp—cannot be readily detected by sounding devices because of low reflecting ability. Another method, location by fish noise, can be used in all these cases.

Biohydroacoustics holds considerable promise for the creation of artificial concentrations of fish and other aquatic organisms and for the control of fish behavior either for fishing purposes or for regulating their movements in installations through which the fish pass.

The data on the acoustic organs of cetaceans (whales and dolphins) are highly interesting. These animals have an ability that very few others have: by sending and receiving hydroacoustic impulses, they can determine under water the presence of various objects that may be a source of danger or food and communicate and give signals within and between species. They can emit acoustic signals in a very wide range of sonic and ultrasonic frequencies. They also have highly efficient organs and systems for receiving, processing, and analyzing hydroacoustic information that sometimes has a very low signal-to-noise ratio.