Fisheries acoustics
Encyclopedia
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.
estimation is a method of detecting and quantifying fish
and other marine organisms using sonar
technology. 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 ratio
s.
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.
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including 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...
as sensors in aquatic
Aquatic ecosystem
An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems....
environments. Acoustical techniques can be applied to sensing aquatic animals, zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
, and physical and biological habitat
Marine habitats
This article is about the habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea . A habitat is an ecological or environmental area inhabited by one or more living species.Marine habitats can be divided into coastal and open ocean habitats...
characteristics.
Basic Theory
BiomassBiomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...
estimation is a method of detecting and quantifying fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and other marine organisms using sonar
Sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...
technology. 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
Abundance (ecology)
Abundance is an ecological concept referring to the relative representation of a species in a particular ecosystem. It is usually measured as the large number of individuals found per sample...
. The basic components of the scientific echo sounder hardware
Hardware
Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....
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
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
echo sounder equipment, having high signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
s.
Fish counting
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.Echo integration
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. 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.Echosounders
The primary tool in fisheries acoustics is the scientific echosounder. This instrument operates on the same principles as a recreational or commercial fishfinderFishfinder
A fishfinder 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,...
or echosounder
Echo sounding
Echo sounding is the technique of using sound pulses directed from the surface or from a submarine vertically down to measure the distance to the bottom by means of sound waves. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes...
, 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.
Platforms for fisheries acoustics
Research vessels, AUV's, observatories, fishing vessels, vessels of opportunityTarget strength observations and modelling
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.External links
- ICES Working Group on Fisheries Acoustics, Science and Technology
- Case Studies & Applications for fisheries and fisheries habitat, e.g. zooplankton, fish, submersed aquatic vegetation (SAV), and substrate.
- Census of Marine Life - Acoustic Technology]
- Fisheries acoustics discussion forum Web based discussion forum on fisheries acoustics with topics including theory, hardware, software and data processing/analysis.
- Fisheries Acoustics Research University of Washington.
- Publications for Fisheries Acoustics, e.g. acoustic tag and passive hydroacoustic echo sounder research.
- SIMRAD: Manufacturer of scientific single beam and multibeam echo sounders
- BioSonics, Inc: Manufacturers of single beam and split beam scientific echosounders
- Acoustics Unpacked: A General Guide for Deriving Abundance Estimates from Hydroacoustic Data