Sonar
Today, commercial fishing vessels rely almost completely on acoustic sonar and sounders to detect fish. Fishermen also use active sonar and echo sounder technology to determine water depth, bottom contour, and bottom composition.

Companies such as eSonar, Raymarine, Marport Canada, Wesmar, Furuno, Krupp, and Simrad make a variety of sonar and acoustic instruments for the deep sea commercial fishing industry. For example, net sensors take various underwater measurements and transmit the information back to a receiver on board a vessel. Each sensor is equipped with one or more acoustic transducers depending on its specific function. Data is transmitted from the sensors using wireless acoustic telemetry and is received by a hull mounted hydrophone. The analog signals are decoded and converted by a digital acoustic receiver into data which is transmitted to a bridge computer for graphical display on a high resolution monitor.

Echo sounding: Echo sounding is a process used to determine the depth of water beneath ships and boats. A type of active sonar, echo sounding is the transmission of an acoustic pulse directly downwards to the seabed, measuring the time between transmission and echo return, after having hit the bottom and bouncing back to its ship of origin. The acoustic pulse is emitted by a transducer which receives the return echo as well. The depth measurement is calculated by multiplying the speed of sound in water (averaging 1,500 meters per second) by the time between emission and echo return.
The value of underwater acoustics to the fishing industry has led to the development of other acoustic instruments that operate in a similar fashion to echo-sounders but, because their function is slightly different from the initial model of the echo-sounder, have been given different terms.

Net location: The net sounder is an echo sounder with a transducer mounted on the headline of the net rather than on the bottom of the vessel. Nevertheless, to accommodate the distance from the transducer to the display unit, which is much greater than in a normal echo-sounder, several refinements have to be made. Two main types are available. The first is the cable type in which the signals are sent along a cable. In this case, there has to be the provision of a cable drum on which to haul, shoot and stow the cable during the different phases of the operation. The second type is the cable-less net-sounder – such as Marport's Trawl Explorer – in which the signals are sent acoustically between the net and hull mounted receiver-hydrophone on the vessel. In this case, no cable drum is required but sophisticated electronics are needed at the transducer and receiver.
The display on a net sounder shows the distance of the net from the bottom (or the surface), rather than the depth of water as with the echo-sounder's hull-mounted transducer. Fixed to the headline of the net, the footrope can usually be seen which gives an indication of the net performance. Any fish passing into the net can also be seen, allowing fine adjustments to be made to catch the most fish possible. In other fisheries, where the amount of fish in the net is important, catch sensor transducers are mounted at various positions on the cod-end of the net. As the cod-end fills up these catch sensor transducers are triggered one by one and this information is transmitted acoustically to display monitors on the bridge of the vessel. The skipper can then decide when to haul the net.
Modern versions of the net sounder, using multiple element transducers, function more like a sonar than an echo sounder and show slices of the area in front of the net and not merely the vertical view that the initial net sounders used.
The sonar is an echo-sounder with a directional capability that can show fish or other objects around the vessel.

ROV and UUV: Small sonars have been fitted to remotely operated vehicles (ROVs) and unmanned underwater vehicles (UUVs) to allow their operation in murky conditions.  These sonars are used for looking ahead of the vehicle.  The Long-Term Mine Reconnaissance System is a UUV for MCM purposes.

Vehicle location: Sonars which act as beacons are fitted to aircraft to allow their location in the event of a crash in the sea.  Short and long baseline sonars may be used for caring out the location, such as LBL.

Prosthesis for the visually impaired: In 2013 an inventor in the United States unveiled a "spider-sense" bodysuit, equipped with ultrasonic sensors and haptic feedback systems, which alerts the wearer of incoming threats; allowing them to respond to attackers even when blindfolded.

Scientific applications: 

Biomass estimation: Detection of fish, and other marine and aquatic life, and estimation their individual sizes or total biomass using active sonar techniques. Sound pulses reflect off any object that has a different density than the surrounding medium. This includes fish, or more specifically, the air-filled swim bladder on fish. These echoes provide information on fish size, location, abundance and behavior. This is especially effective for fish swim bladders (e.g. herring, cod, and pollock), and less useful for fish without them (e.g. sharks, mackerel, and flounder). Data from the watercolumn is usually processed differently than seafloor or object detection data, this data type can be processed with specialized software.

Wave measurement: An upward looking echo sounder mounted on the bottom or on a platform may be used to make measurements of wave height and period.  From this statistics of the surface conditions at a location can be derived.

Water velocity measurement: Special short range sonars have been developed to allow measurements of water velocity.

Bottom type assessment: Sonars have been developed that can be used to characterise the sea bottom into, for example, mud, sand, and gravel. Relatively simple sonars such as echo sounders can be promoted to seafloor classification systems via add-on modules, converting echo parameters into sediment type. Different algorithms exist, but they are all based on changes in the energy or shape of the reflected sounder pings.  Advanced substrate classification analysis can be achieved using calibrated (scientific) echosounders and parametric or fuzzy-logic analysis of the acoustic data.

Bathymetric mapping: Side-scan sonars can be used to derive maps of seafloor topography (bathymetry) by moving the sonar across it just above the bottom. Low frequency sonars such as GLORIA have been used for continental shelf wide surveys while high frequency sonars are used for more detailed surveys of smaller areas.
Hull-mounted multibeam echosounders on large surface vessels produce swathes of bathymetric data in near real time. One example, the General Instrument "Seabeam" system, uses a projector array along the keel to ensonify the bottom with a fan beam. Signals from a hydrophone array mounted athwartships are processed to synthesize multiple virtual fan beams crossing the projector beam at right angles.

Sonar imaging: Creating two and three-dimensional images using sonar data.

Sub-bottom profiling: Powerful low frequency echo-sounders have been developed for providing profiles of the upper layers of the ocean bottom. One of the most recent devices is Innomar's SES-2000 quattro multi-transducer parametric SBP, used for example in the Puck Bay for underwater archaeological purposes

Gas leak detection from the seabed: Gas bubbles can leak from the seabed, or close to it, from multiple sources. These can be detected by both passive and active sonar (shown in schematic figure by yellow and red systems respectively).

 Natural seeps of methane and carbon dioxide occur. Gas pipelines can leak, and it is important to be able to detect whether leakage occurs from Carbon Capture and Storage Facilities (CCSFs; e.g. depleted oil wells into which extracted atmospheric carbon is stored). Quantification of the amount of gas leaking is difficult, and although estimates can be made use active and passive sonar, it is important to question their accuracy because of the assumptions inherent in making such estimations from sonar data.

Synthetic aperture sonar: Various synthetic aperture sonars have been built in the laboratory and some have entered use in mine-hunting and search systems.  An explanation of their operation is given in synthetic aperture sonar.

Parametric sonar: Parametric sources use the non-linearity of water to generate the difference frequency between two high frequencies.  A virtual end-fire array is formed. Such a projector has advantages of broad bandwidth, narrow beamwidth, and when fully developed and carefully measured it has no obvious sidelobes: see Parametric array. Its major disadvantage is very low efficiency of only a few percent.  P.J. Westervelt summarizes the trends involved.

Sonar in extraterrestrial contexts: The use of both active and passive sonar has been proposed for various extraterrestrial environments. One example is Titan, where active sonar could be used to determine the depth of its hydrocarbon seas, and passive sonar could be used to detect methanefalls.
Proposals that do not take proper account of the difference between terrestrial and extraterrestrial environments could lead to erroneous measurements.

Ecological impact: 

Effect on marine mammals: Research has shown that use of active sonar can lead to mass strandings of marine mammals. Beaked whales, the most common casualty of the strandings, have been shown to be highly sensitive to mid-frequency active sonar. Other marine mammals such as the blue whale also flee from the source of the sonar, while naval activity was suggested to be the most probable cause of a mass stranding of dolphins.