Fisheries acoustics includes a range of research and practical application topics using acoustical devices as sensors in aquatic environments. Acoustical techniques can be applied to sensing aquatic animals, zooplankton, and physical and biological habitat characteristics.
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. These echoes provide information on fish size, location, and abundance. The basic components of the scientific echo sounder hardware function is to transmit 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 calibrated echo sounder equipment, having high signal-to-noise ratios.
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. Passive acoustic methods can be an attractive alternative or supplement to traditional fisheries assessment techniques because they 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 began to find uses outside the military. The French explorer Rallier du Baty reported unexpected 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 swimming in an aquaculture pond. [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 was the first echogram of fish to be published. [4] In 1935, Norwegian scientist Oscar Sund reported observations of cod schools from the research vessel Johan Hjort, [5] marking the first use of echosounding for fisheries research.
Sonar technologies developed rapidly during the Second World War, and military surplus equipment was 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 was poorly understood. Beginning in the 1970s and 80s, a series of practical and theoretical investigations began to overcome these limitations. Technological advances such as split-beam 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 echosounders are the standard equipment. Several acoustic frequencies are often used simultaneously, allowing some discrimination of different types of animals. Technological development continues, including research into multibeam, broadband, and parametric sonars.
When individual targets are spaced far enough apart that they can be distinguished from one another, it is straightforward to estimate the number of fish by counting the number of targets. This type of analysis is called echo counting, and was historically the first to be used for biomass estimation.
If more than one target is located in the acoustic beam at the same depth, it is not usually 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.
The primary tool in fisheries acoustics is the scientific echosounder. This instrument operates on the same principles as a recreational or commercial fishfinder or echosounder, but is engineered for greater accuracy and precision, allowing quantitative biomass estimates to be made. In an echosounder, a transceiver generates a short pulse which is sent into 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, the echosounder must be calibrated in the same configuration and environment in which it will be used; this is typically done by examining echoes from a metal sphere with known acoustic properties.
Early echosounders only transmitted a single beam of sound. Because of the acoustic beam pattern, identical targets at different azimuth angles will return different echo levels. If the beam pattern and angle to the target are known, this directivity can be compensated for. The need to determine the angle to a target led to the development of the twin-beam echosounder, which forms two acoustic beams, one inside the other. By comparing the phase difference of the same echo in the inner and outer beams, the angle off-axis can be estimated. In a further refinement of this concept, a split-beam echosounder divides the transducer face into four quadrants, allowing the location of targets in three dimensions. Single-frequency, split-beam echosounders are now the standard instrument of fisheries acoustics.
Multibeam sonars project a fan-shaped set of sound beams outward into the water and record echoes in each beam. These have been widely used in bathymetric surveys, but have recently begun to find use in fisheries acoustics as well. Their major advantage is the addition of a second dimension to the narrow water column profile given by an echosounder. Multiple pings can thus be combined to give a three-dimensional picture of animal distributions.
Acoustic cameras [7] are instruments that image a three-dimensional volume of water instantaneously. These typically use higher-frequency sound than traditional echosounders. This increases their resolution so that individual objects can be seen in detail, but means that their range is limited to tens of meters. They can be very useful for studying fish behavior in enclosed and/or murky bodies of water, for instance monitoring the passage of anadromous fish at dams
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. If the vessel does not have permanently installed echosounders, they may be deployed on a pole mount attached to the ship's side, or on a towed body or "towfish" pulled behind or alongside the vessel. Towed bodies are particularly useful for studies of deep-living fish, such as the orange roughy, which typically live below the range of an echosounder at the surface.
In addition to research vessels, acoustic data may 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 design may make analysis of these data difficult. In recent years, acoustic instruments have also been deployed on remotely operated vehicles and autonomous underwater vehicles, as well as at ocean observatories.
Target strength (TS) is a measurement of how well a fish, zooplankter, or other target scatters sound back towards 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.
In the use of scientific echosounders for biomass estimation, calibration of the echosounder is necessary, as the target strength provides an estimate of animal biomass, which in turn may be used for stock estimation and regulation of fisheries. This is normally done using objects of known frequency response, known as calibration spheres. They may be made of copper or tungsten carbide, and are of a specific size according to desired backscatter and frequency response. They are attached with a thin monofilament fishing line, strong enough to hold the sphere but thin enough to reduce unwanted backscatter. By moving the sphere inside of the acoustic beam of a given transducer, the echosounder software will correct the observed target strength over the area of the transducer. This is repeated for all transducers and pulse shapes as required. [8] [9]
Surveys, stock assessment, management Ecology Behavior
Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical engineer. The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio and noise control industries.
Sonar is a technique that uses sound propagation to navigate, measure distances (ranging), communicate with or detect objects on or under the surface of the water, such as other vessels.
Side-scan sonar is a category of sonar system that is used to efficiently create an image of large areas of the sea floor.
A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potential when subjected to a pressure change, such as a sound wave.
Echo sounding or depth sounding is the use of sonar for ranging, normally to determine the depth of water (bathymetry). It involves transmitting acoustic waves into water and recording the time interval between emission and return of a pulse; the resulting time of flight, along with knowledge of the speed of sound in water, allows determining the distance between sonar and target. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes.
Hydrographic survey is the science of measurement and description of features which affect maritime navigation, marine construction, dredging, offshore wind farms, offshore oil exploration and drilling and related activities. Surveys may also be conducted to determine the route of subsea cables such as telecommunications cables, cables associated with wind farms, and HVDC power cables. Strong emphasis is placed on soundings, shorelines, tides, currents, seabed and submerged obstructions that relate to the previously mentioned activities. The term hydrography is used synonymously to describe maritime cartography, which in the final stages of the hydrographic process uses the raw data collected through hydrographic survey into information usable by the end user.
Bioacoustics is a cross-disciplinary science that combines biology and acoustics. Usually it refers to the investigation of sound production, dispersion and reception in animals. This involves neurophysiological and anatomical basis of sound production and detection, and relation of acoustic signals to the medium they disperse through. The findings provide clues about the evolution of acoustic mechanisms, and from that, the evolution of animals that employ them.
Acoustic homing is the process in which a system uses the sound or acoustic signals of a target or destination to guide a moving object. There are two types of acoustic homing: passive acoustic homing and active acoustic homing. Objects using passive acoustic homing rely on detecting acoustic emissions produced by the target. Conversely, objects using active acoustic homing make use of sonar to emit a signal and detect its reflection off the target. The signal detected is then processed by the system to determine the proper response for the object. Acoustic homing is useful for applications where other forms of navigation and tracking can be ineffective. It is commonly used in environments where radio or GPS signals can not be detected, such as underwater.
A fishfinder or sounder (Australia) is an instrument used to locate fish underwater by detecting reflected pulses of sound energy, as in sonar. A modern fishfinder displays measurements of reflected sound on a graphical display, allowing an operator to interpret information to locate schools of fish, underwater debris, and the bottom of a body of water. Fishfinder instruments are used both by sport and commercial fishermen. Modern electronics allow a high degree of integration between the fishfinder system, marine radar, compass and GPS navigation systems.
An acoustic doppler current profiler (ADCP) is a hydroacoustic current meter similar to a sonar, used to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water column. The term ADCP is a generic term for all acoustic current profilers, although the abbreviation originates from an instrument series introduced by RD Instruments in the 1980s. The working frequencies range of ADCPs range from 38 kHz to several megahertz.
A scientific echosounder is a device which uses sonar technology for the calibrated backscatter measurement of underwater physical and biological components—this device is also known as scientific sonar. Applications include bathymetry, substrate classification, studies of aquatic vegetation, fish, and plankton, and differentation of water masses.
Sodar, an acronym of sonic detection and ranging, is a meteorological instrument used as a wind profiler based on the scattering of sound waves by atmospheric turbulence. Sodar equipment is used to measure wind speed at various heights above the ground, and the thermodynamic structure of the lower layer of the atmosphere.
Acoustic tags are small sound-emitting devices that allow the detection and/or remote tracking of organisms in aquatic ecosystems. Acoustic tags are commonly used to monitor the behavior of fish. Studies can be conducted in lakes, rivers, tributaries, estuaries or at sea. Acoustic tag technology allows researchers to obtain locational data of tagged fish: depending on tag and receiver array configurations, researchers can receive simple presence/absence data, 2D positional data, or even 3D fish tracks in real-time with sub-meter resolutions.
A multibeam echosounder (MBES) is a type of sonar that is used to map the seabed. It emits acoustic waves in a fan shape beneath its transceiver. The time it takes for the sound waves to reflect off the seabed and return to the receiver is used to calculate the water depth. Unlike other sonars and echo sounders, MBES uses beamforming to extract directional information from the returning soundwaves, producing a swathe of depth soundings from a single ping.
Underwater acoustics 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. The water may be in the ocean, a lake, a river or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and 1 MHz. The propagation of sound in the ocean at frequencies lower than 10 Hz is usually not possible without penetrating deep into the seabed, whereas frequencies above 1 MHz are rarely used because they are absorbed very quickly.
Acoustic seabed classification is the partitioning of a seabed acoustic image into discrete physical entities or classes. This is a particularly active area of development in the field of seabed mapping, marine geophysics, underwater acoustics and benthic habitat mapping. Seabed classification is one route to characterizing the seabed and its habitats. Seabed characterization makes the link between the classified regions and the seabed physical, geological, chemical or biological properties. Acoustic seabed classification is possible using a wide range of acoustic imaging systems including multibeam echosounders, sidescan sonar, single-beam echosounders, interferometric systems and sub-bottom profilers. Seabed classification based on acoustic properties can be divided into two main categories; surficial seabed classification and sub-surface seabed classification. Sub-surface imaging technologies use lower frequency sound to provide higher penetration, whereas surficial imaging technologies provide higher resolution imagery by utilizing higher frequencies.
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.
Bistatic sonar is a sonar configuration in which transmitter and receiver are separated by a distance large enough to be comparable to the distance to the target. Most sonar systems are monostatic, in that the transmitter and receiver are located in the same place. A configuration with multiple receivers is called multistatic.
Acoustic survey in fishing is one of the research methods that can detect the abundance of target species using acoustic detectors. For example, many pelagic fisheries are generally very scattered over a broad ocean and difficult to detect. Hence survey vessel with acoustic detector emits sound waves to estimate the density of plankton and fish shoal. Generally, the transducer is put under water, which is linked to an echo sounder in the vessel which records the shoals of fish as "marks" on a screen or paper trace. Then the density and number of marks are converted into biomass.
An underwater survey is a survey performed in an underwater environment or conducted remotely on an underwater object or region. Survey can have several meanings. The word originates in Medieval Latin with meanings of looking over and detailed study of a subject. One meaning is the accurate measurement of a geographical region, usually with the intention of plotting the positions of features as a scale map of the region. This meaning is often used in scientific contexts, and also in civil engineering and mineral extraction. Another meaning, often used in a civil, structural, or marine engineering context, is the inspection of a structure or vessel to compare actual condition with the specified nominal condition, usually with the purpose of reporting on the actual condition and compliance with, or deviations from, the nominal condition, for quality control, damage assessment, valuation, insurance, maintenance, and similar purposes. In other contexts it can mean inspection of a region to establish presence and distribution of specified content, such as living organisms, either to establish a baseline, or to compare with a baseline.
