Acoustic survey

An acoustic survey estimates aquatic animal abundance by measuring sound reflected from organisms underwater. It uses calibrated sonar measurements to infer the distribution and density of animals in the surveyed water.

Echolocation: Biological sonar in which animals emit sounds and interpret returning echoes. Its echo principle parallels active sonar, though surveys use instruments rather than animal calls.
Fish swim bladder: A gas-filled organ that helps many fish regulate buoyancy. Its gas boundary often makes fish strong acoustic targets.
Fisheries acoustics: The use of underwater sound to study fish distribution, abundance, and behavior. Acoustic surveys are a central method within this broader field.
Trawl survey: A survey that samples aquatic animals by towing a net through the water or along the seabed. Trawls capture organisms directly, while acoustic surveys estimate them from echoes.
Sonar: A technique that uses sound propagation to detect or locate underwater objects. Acoustic surveys use sonar to transmit sound and record echoes from organisms.
Acoustic impedance: A material property equal to density multiplied by sound speed, governing sound transmission and reflection. Impedance contrasts between an animal and water shape how much sound returns.
Hydroacoustic assessment of fish stocks: An estimate of fish-stock abundance and distribution based on underwater acoustic measurements. This is a direct application of survey data to fisheries assessment.
Underwater visual census: A method for estimating aquatic animal abundance by observing and counting individuals underwater. Visual counts offer direct observations but cover less space and depend on visibility.
Target strength: A measure of how strongly an object reflects sound toward a sonar receiver. Converting echo intensity into animal numbers depends on the target strength assigned to organisms.
Sound speed profile: The variation of sound speed with depth in a body of water. Sound-speed changes affect sonar travel paths and the accuracy of depth assignments.

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