Fisheries acoustics

Fisheries acoustics includes a Range of research and practical application of topics using acoustical devices have sensors in aquatic environments. Acoustical techniques can be applied to aquatic animals , zooplankton , and physical and biological habitat characteristics.

Basic Theory

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 are so different from the surrounding medium, such as fish, they reflect some sound back to the source. These provide information on fish size, location, and abundance . The basic components of the scientific echo sounder hardwareto record the sound, receive, filter and amplify, record, and analyze the echoes. While there are many manufacturers of commercially available “fish-finders,” quantitative analysis requires that measurements be made with calibratedecho sounder equipment, having high signal-to-noise ratios .

History

An extremely wide variety of fish taxa produce sound. Sound production behavior provides an opportunity to study various aspects of fish biology, such as spawning behavior and habitat selection, in a noninvasive manner. These methods are noninvasive, can be conducted at low cost, and can cover a large study area at high spatial and temporal resolution. [2]

Following the First World War, when sonar was first used for the detection of submarines, echo sounders. The French explorer Rallier du Baty reported unexpectedly midwater echoes, which he attributed to fish schools, in 1927. In 1929, the Japanese scientist Kimura reported disruptions in a continuous acoustic beam by sea ​​bream pond aquaculture. [3]

In the early 1930s, two commercial fishermen, Ronald Balls, an Englishman, and Reinert Bokn, a Norwegian, began independently experimenting with echosounders as a means to locate fish. Acoustic traces of sprat schools recorded by Bokn in Frafjord, Norway. [4] In 1935, Norwegian scientist Oscar Sund reported observations of cod schools from the research vessel Johan Hjort, [5] tagging the first use of echosounding for fisheries research.

Sonar technologies developed rapidly during the Second World War, and was later adopted by commercial fishers and scientists soon after the end of hostilities. This period saw the first development of instruments designed specifically to detect fish. Large uncertainties persisted in the interpretation of acoustic surveys, however: calibration of instruments was irregular and imprecise, and the sound-scattering properties of fish and other organisms were poorly understood. Beginning in the 1970s and 80s, a series of practical and theoretical investigations. Echosounders, digital signal processing, and electronic displays also appeared in this period.

At present, acoustic surveys are used in the assessment and management of many fisheries worldwide. Calibrated, split-beam echo sounders are the standard equipment. Several acoustic frequencies are often used, allowing some discrimination of different types of animals. Technological development continues, including research into multibeam, broadband, and parametric sonars.

Techniques

Fish counting

Where they are divided up, they are distinguished by one from the other, it is easy to estimate the number of targets. This type of analysis is called echo counting , and was historically used for biomass estimation.

Echo integration

If more than one target is located in the acoustic beam at the same depth, it is not possible to resolve them separately. This is often the case with schooling fish or aggregations of zooplankton. In these cases, echo integration is used to estimate biomass. Echo integration assumes that the total acoustic energy scattered by a group of targets is the sum of the energy scattered by each individual target. This assumption holds well in most cases. [6] The total acoustic energy backscattered by the school or aggregation is integrated together, and this total is divided by the (previously determined) backscattering coefficient of a single animal, giving an estimate of the total number.

Instruments

Echosounders

Main article: Echo sounding

The primary tool in fisheries acoustics is the scientific echosounder. This instrument operates on the same principles as a commercial fishfinder or echosounder , but is engineered for greater accuracy and precision, allowing quantitative biomass estimates to be made. In an echosounder, a transceiver generales a short pulse which is sent to the water by the transducer, an array of piezoelectric elements arranged to produce a focused beam of sound. In order to be used for quantitative work, it must be calibrated in the same configuration and in which it will be used; This is typically done by examining a metal sphere with known acoustic properties.

Early echosounders only transmitted to a single beam of sound. Because of the acoustic beam pattern , different targets are different azimuth angles will return different echo levels. If the beam pattern is known, this directivity can be compensated for. The two -beam echo sounder , which forms two acoustic beams, one inside the other. By comparing the phase difference of the same in the inner and outer beams, the angle-off-axis can be estimated. In a further refinement of this concept, a split-beam echosounderdivides the transducer face four quadrants, allowing the location of targets in three dimensions. Single-frequency, split-beam echosounders are now the standard instrument of fisheries acoustics.

Multibeam echosounders

Main article: Multibeam echosounder

Multibeam sonars project a fan-shaped set of sound beams in the water and record echoes in each beam. These have been widely used in bathymetric surveys, but Their major advantage is the addition of a second dimension to the narrow water column given by an echosounder. Multiple pings can thus be combined to give a three-dimensional picture of animal distributions.

Acoustic cameras

Acoustic cameras [7] are instruments that have a three-dimensional volume of water instantaneously. These typically use higher-frequency sound than traditional echosounders. This is their resolution so that individual objects can be seen in detail, but their range is limited to tens of meters. They can be very useful for studying the behavior of children in the water, for instance monitoring the passage of anadromous fish at dams

Platforms for fisheries acoustics

Fisheries acoustic research is conducted from a variety of platforms. The most common is a traditional research vessel, with the echosounders mounted on the ship’s hull or in a drop keel. They may be deployed on the ship’s side or they may be deployed on the ship’s side, or on a towed body or “towfish” pulled behind or alongside the vessel. These are particularly useful for studies of deep-living fish, such as the orange roughy , which typically live at the surface of the surface.

In addition to research vessels, acoustic data can be collected from a variety of “ships of opportunity” such as fishing vessels, ferries, and cargo ships. Ships of opportunity can offer low-cost data collection over large areas, though the lack of a true survey can make analysis of these data difficult. In recent years, acoustic instruments have also been deployed and operated under the supervision of the United States.

Target strength observations and modeling

Target strength (TS) is a measurement of how well a fish, zooplankter, or other target scatters back to the transducer. In general, larger animals have larger target strengths, though other factors, such as the presence or absence of a gas-filled swimbladder in fishes, may have a much larger effect. Target strength is of critical importance in fisheries acoustics, since it provides a link between acoustic backscatter and animal biomass. TS can be derived theoretically for simple targets such as spheres and cylinders, but in practice, it is usually measured empirically or calculated with numerical models.

Applications

Surveys, stock assessment, management Ecology Behavior

See also

  • Deep scattering layer

References

  1. Jump up^ Simmonds J. & MacLennan D. (2005). Fisheries Acoustics: Theory and Practice, second edition. Blackwell
  2. Jump up^ Gannon DP (2008)” Transactions in the American Fisheries Society,137(2): 638-656. doi:10.1577 / T04-142.1
  3. Jump up^ Kimura, K, 1929. On the detection of fish-groups by an acoustic method. Journal of the Imperial Fisheries Institute, Tokyo.
  4. Jump up^ Anon, 1934. Forsøkene med ekkolodd ved Brislingfisket (Trials with an echosounder during the sprat fishery). Tidsskrift for hermetikindustri (Bulletin of the Canning Industry), July 1934, pp. 222-223.
  5. Jump up^ Sund, O. (1935). “Echo sounding in fishery research” . Nature . 135 : 953-953. doi : 10.1038 / 135953a0 .
  6. Jump up^ Linearity of fisheries acoustics, with additional theorems. Kenneth G. Foote, 1983. Journal of the Acoustical Society of America 73, pp. 1932-1940.
  7. Jump up^ Martignac F., A. Daroux, Bagliniere JL, Ombredanne D. Guilalrd J., 2015. The use of acoustic cameras in shallow waters: new tools for hydroacoustic monitoring migratory fish population. A review of DIDSON technology. Fish & Fisheries, 16 (3), 486-510. DOI: 10.1111 / fa.12071

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