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Output of a computer model of underwater acoustic propagation in a simplified ocean environment. A seafloor map produced by multibeam sonar. Underwater acoustics (also known as hydroacoustics) is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water, its contents and its boundaries.
Biomass estimation is a method of detecting and quantifying fish and other marine organisms using sonar technology. [1] An acoustic transducer emits a brief, focused pulse of sound into the water. If the sound encounters objects that are of different density than the surrounding medium, such as fish, they reflect some sound back toward the source.
Electric fish can detect electrical signals using tuberous electroreceptors which are sensitive to high-frequency stimuli. Electroreceptors exist in different forms and can be found in various parts of the body. Sharks, for example, have electroreceptors called ampullae of Lorenzini in the pores on their snouts and other zones of the head ...
The lateral line in fish and aquatic forms of amphibians is a detection system of water currents, consisting mostly of vortices. The lateral line is also sensitive to low-frequency vibrations. It is used primarily for navigation, hunting, and schooling. The mechanoreceptors are hair cells, the same mechanoreceptors for vestibular sense and hearing.
Sonar systems are generally used underwater for range finding and detection. Active sonar emits an acoustic signal, or pulse of sound, into the water. The sound bounces off the target object and returns an echo to the sonar transducer. Unlike active sonar, passive sonar does not emit its own signal, which is an advantage for military vessels.
Sonar image of the wreck of USS O-9.. The target strength or acoustic size is a measure of the area of a sonar target. This is usually quantified as a number of decibels.For fish such as salmon, the target size varies with the length of the fish and a 5 cm fish could have a target strength of about -50 dB.
Aggressive sounds tend to be middle-frequency (about one kHz). Distressed sounds can be a bit higher frequency (about 6 kHz). [6] Longspine squirrelfish make two kinds of sounds, a single "grunt" and a short burst of sounds called a "staccato." The grunt sounds are used mostly to defend their territory. The staccato sound indicates alarm.
Fish are least sensitive to low frequencies, with a minimum threshold of 10 Hz and have peak sensitivity at 75–150 Hz; the highest frequency they can detect is 250 Hz. [5] These waves have small amplitudes (on a micrometer-scale), and fish can detect waves with minimum peak-to-peak displacement of 1 μm at 10 Hz and 0.0007μm in their most ...