Richard O'Driscoll, National Institute for Water and Atmospheric Research, New Zealand, Paul Fernandes, University of Aberdeen, Scotland; John Horne, University of Washington, USA; Verena Trenkel, IFREMER, France; Nils Olav Handegard, Institute of Marine Research, Norway.
Using underwater sound to detect fish
Marine mammals have used echolocation to detect fish for millions of years. The history of fisheries acoustics is much shorter – with the first successful experiments on the acoustic detection of fish schools carried out in the 1920s. The rapid rise in electronics and computing power in the late 20th century has resulted in the use of fish-finders, echosounders, and other sonars on just about every fishing vessel.
Basic principles
The basic tool in fisheries acoustics is the scientific echosounder. This instrument produces an electrical signal which is sent to an underwater transducer, where it is converted to mechanical energy as an acoustic pulse or "ping". When the ping encounters fish, part of the acoustic energy is reflected back to the transducer as an echo and converted back to electrical energy. The distance or range to the fish is obtained by timing the interval between transmission of the ping and reception of the echo. On a moving platform this provides a high resolution 2D representation of objects in the water, an echogram, which is continuously sampled along the entire vessel track.
Echo energy is measured to estimate fish densities and, with an appropriate survey design, abundance. When an echosounder is calibrated, densities can be quantified by adding selected echoes from a single ping (echo integration), averaging similar echoes from a number of adjacent pings, and dividing by the expected echo energy from a representative animal. The selected echoes need to be attributed to species and size, and the acoustic properties of that species (target strength) must be known.
Fisheries and ecosystem applications
Within ICES, there are over 20 fish stocks – mostly midwater species such as capelin, herring, and sprat – for which acoustic surveys are used to provide estimates of abundance and distribution. Acoustic surveys are coordinated by the working groups on international pelagic surveys (WGIPS), acoustic and egg surveys for sardine and anchovy (WGACEGG), the Baltic international fish survey (WGBIFS), and international deep pelagic surveys (WGIDEEPS).
Fisheries acoustics is also used internationally for abundance surveys of demersal and deep-water species using towed transducers to detect fish close to the sea-bed, and to depths in excess of 1000 metres.
In ecosystem-based fisheries management, fisheries acoustics is used for a wide range of species, from seagrass to tuna. A current example is the use of acoustics to estimate abundance of mesopelagic fish, with a 2017 workshop on monitoring technologies for the mesopelagic zone (WKMESO), and a 2019 workshop on the development of practical survey methods for measurements and monitoring in the mesopelagic zone (WKMESOMeth).
Challenges and solutions
Fisheries acoustics has the potential to provide information on marine organisms of all sizes across a large range of spatial and temporal scales. Key challenges when acquiring and interpreting acoustic data include classifying targets, animal acoustic properties and behaviour, and processing very large datasets.
Multi-frequency and broadband (“chirp") systems can allow discrimination of targets because different groups of organisms reflect sound differently at different frequencies. Using echosounders with multiple frequencies is now commonplace. Broad-band (“chirp") systems where sound is transmitted and received simultaneously across a continuous frequency band provide even more information and greater spatial resolution. The Working Group on Target Classification (WGTC) has put together a Cooperative Research Report (CRR) on target classification to be published this year.
Understanding acoustic properties of marine organisms remains a focus. Acoustic measurements and models of acoustic scattering are compared with alternative evidence using nets, optical, or other sampling tools, to verify species composition.
Because fish tilt, contort and move, echo amplitudes are variable. Responses to environmental or human (e.g. survey vessel) stimuli can introduce bias in acoustic abundance or biomass estimates. Multibeam sonars, which increase sampling volume using multiple adjacent beams, provide a tool to observe fish behaviour, and to detect problems such as vessel avoidance.
Collecting ecosystem data across a range of spatial and temporal scales has increased traditional research survey efforts and been expanded to include data collection from vessels of opportunity, autonomous vehicles, and moored ocean observatories. Increased data acquisition and volume has created the need for rapid, automatic processing. The desire for data sharing in international programmes, both within and outside ICES, triggered the development of a metadata convention for processed acoustic data from active acoustic systems, and a standardized, flexible, open-source netCDF-4 data convention for multibeam sonar data. This metadata standard was used for creating the ICES database for acoustic and biotic data collected on acoustic surveys in the Northeast Atlantic and the Baltic Sea.
Looking ahead
Estimating population abundance will remain the cornerstone of fisheries acoustics effort. Improved technology combined with enhanced processing and expanded analytic tools will lead to more accurate estimates. Increased government mandates for ecosystem-based management will increase the demand for acoustic data, and the challenges of analysis, interpretation, and sharing. Use of autonomous vehicles and moored ocean observatories for data acquisition is expected to increase, as is the development of processing tools based on machine learning techniques. Understanding organisms' acoustic properties will be critical to inform automated target classification, and for the conversion of echo energy to fish density. Broadband systems are expected to become mainstream acoustic acquisition tools.
Cross-disciplinary collaboration, training, and investment that integrates physics, engineering, biology, oceanography, ecology, and ecosystem modelling is required to realise the full potential of acoustic technologies. WGFAST will continue to contribute to advances in this field by facilitating this collaboration.
