Power budget equations and calibration factors for fish abundance estimation using scientific echo sounders and sonar systems

-Acoustic methods used in fish abundance estimation constitute a key part of the analytic assessment that makes the basis for abundance estimation of marine resources. The methods rely on power-budget equations and calibrated systems. Different formulations of power-budget equations and calibration factors have been proposed for use in scientific echo sounder and sonar systems. There are unresolved questions and apparent inconsistencies in prior literature related to this field. A generic (instrument independent) and unifying theory is presented that attempts to explain the different power-budget and calibration factor formulations proposed and used in prior literature, and how these are mutually related. Deviations and apparent inconsistencies in this literature appear to be explained and corrected. This also includes different (instrument specific) formulations employed in important modern scientific echo sounder systems, and their relationship to the generic theory of abundance estimation. Prior literature is extended to provide more complete power-budget equations for fish abundance estimation and species identification, by accounting for echo integration, electrical termination, and the full range of electrical and acoustical echo sounder parameters. The expressions provide a consistent theoretical basis for improved understanding of conventional methods and instruments used today, also enabling improved sensitivity and error analyses, and correction possibilities

The relative frequency response, as derived from individually separated targets on cod, saithe and Norway pout

The relative frequency response is an important acoustic feature used to characterise acoustic targets. This response has been defined as the sv, volume backscattering coefficient, for a specific frequency relative to that of a reference frequency (38 kHz). The acoustic data commonly used in these calculations are derived from integrated measurements in a region containing multiple targets. In this study the relative frequency responses at 18, 38, 70, 120, and 200 kHz have additionally been measured using filtered target strength data on all frequencies. The spatial comparability of the sv-data is thus avoided, while the single-target detection becomes a new challenge. Target strength was extracted from in situ measurements, using calibrated and digitised data from a Simrad EK60 with split-beam transducers transmitting simultaneously at all five frequencies. Selected series with nearly pure catches of Atlantic cod (Gadus Morhua L.), saithe (Pollachius virens L.) and Norway pout (Trisopterus esmarkii L.) were analysed. The frequency response derived by the new method is compared with standard integration method

Nonlinear crosstalk in broadband multi-channel echosounders

Distortion of acoustic wave caused by nonlinear propagation transfers acoustic energy into higher harmonics of the transmitted signal. When operating several broadband echosounders with non-overlapping frequency bands to cover a wide frequency range, higher harmonics generated by one band may interfere with the fundamental band of others. This interference (i.e., crosstalk) can adversely affect the measured backscattered amplitude frequency response and in some circumstances, appears as spurious targets above and/or below the main target in pulse-compressed echograms. The nonlinear propagation of frequency-modulated acoustic waves in a directional beam was modeled and used to assess methods to reduce the deleterious effects of harmonic components in the signal, and was also compared to field experiments using the seabed echo and a metallic target sphere, with good agreement. Two methods are shown to materially reduce crosstalk: (1) reduction in transmit power, which reduces crosstalk amplitude by a larger amount than the associated reduction in transmit power, and (2) selection of a proper Fourier window length in the processing stage. The effect of crosstalk was small (<0.4 dB or 10%) for area backscattering measurements, but could be several dB for target strength measurements at different frequencies, depending on the transmit signals and processing parameters.acceptedVersio

Evaluation of echosounder data preparation strategies for modern machine learning models

Fish stock assessment and management requires accurate estimates of fish abundance, which are typically derived from echosounder observations using acoustic target classification (ATC). Skilled operators are regularly assisted in classifying acoustic targets by software and there has been an increasing interest toward using machine learning to create improved tools. Recent studies have applied deep learning approaches to acoustic data, however, algorithm data-preparation strategies (influencing model output) are presently poorly understood and standardization is needed to enable collaborative research and management. For example, a common pre-processing technique is to resample backscatter data coming from echosounder measurements from the original resolution to a coarser resolution in the horizontal (time) and vertical (range) directions. Using data values derived from the volume backscattering coefficient obtained during the Norwegian sandeel survey, we investigate which resampling resolutions are suitable for ATC using a convolutional neural network trained to classify single values of backscatter data. This process is known as pixel-level semantic segmentation. Our results indicate that it is possible to downsample the data if important information related to acoustic characteristics is not smoothed out. We also show that the classification performance is improved when providing the network with contextual information relating to range. These findings will provide input to fisheries acoustic data standards and contribute to the on-going development of automated ATC methods.publishedVersio

Feasibility study on the use of scientific multibeam sonar to characterize the Atlantic Bluefin Tuna spawning stock

The Institute of Marine Research (IMR) and the Polytechnic University of Valencia (UPV) were invited by ICCAT Atlantic-Wide Research Program for Bluefin Tuna (GBYP) to carry out a feasibility study on the use of scientific multibeam sonar, Simrad MS70, to characterize the Atlantic Bluefin Tuna (BFT) spawning stock in the Balearic Sea. The study was carried out on board the research vessel B/O Ángeles Alvariño owned by the Spanish Institute of Oceanography. The aim was to map the distribution of BFT along a pre-defined track (part of an ictyoplankton survey) in the first part of the cruise and in the second part of the cruise do detailed measurements of BFT schools detected and measured by an aircraft. In the first part of the cruise 12 schools were detected with the MS70, but none of these were likely to be BFT schools. In the second part of the survey no BFT schools were detected by the aircraft in the coordinated search. There may be several reasons for the lack of BFT schools; too little time dedicated to school search, wrong area was covered or BFT avoided the research vessel. There was also a problem with electrical noise in the sonar, which reduced the effective range and made data processing more challenging. However, BFT were successfully measured with the MS70 sonar inside and near a transport cage. These data indicate that the MS70 is suitable for detailed measurements of BFT schools. However there is a need to validate estimates made with the sonar and there is a need for lateral target strength estimates of tuna at the frequency range of the MS70 sonar. We recommend that the project is continued, the experience from the current study used to improved survey design and focus is aimed at basic studies on the acoustic properties of tuna.publishedVersio

Postprocessing system for echo sounder data

Author Posting. © Acoustical Society of America, 1991. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 90 (1991): 37-47, doi:10.1121/1.401261.Echo sounding is a powerful and widely used technique for remote sensing of the marine environment. In order to enhance the power of the echo sounder, a postprocessing system has been designed and realized in standard software that is essentially machine independent. This has been done by adhering to the following international standards: UNIX operating system, C programming language, X Window Systems, Structured‐Query Language (SQL) for communication with a relational database, and Transport Control Protocol/Internet Protocol (TCP/IP). Preprocessed data are transferred from the echo sounder to the postprocessing system by means of a local‐area network (LAN), namely Ethernet. Development of the postprocessing system, for analysis of such diverse scatterers as plankton, pelagic, and bottom fish, and the bottom itself, is documented in the following way. The history of echo integration is summarized. User requirements for the new system are listed. Reasons are given for the choice of the particular computing environment, including both hardware, software, and external communications. The system design, consisting of data flow and graphical user interfaces, is described. Implementation of the system is defined through integration techniques and a discussion of performance issues. Operating procedures and the first field trials of the system are described. Several features characteristic of and perhaps unique to the postprocessing system are, for example: (1) user definition of arbitrarily shaped integration regions, including non‐constant‐depth intervals, by means of interactive graphics; (2) preprocessor error correction, e.g., adjustment of the noise threshold or redefinition of the detected bottom; (3) use of several color map techniques in order to extract such information as signal strength and shape; and (4) the scheme of interconnections of graphical user interfaces, database, and data files. This work does not introduce a set of computer instructions. It does describe a design philosophy and method of realization that may have broader applications in acoustics than that ostensibly concerned only with the quantitative estimation of fish abundance

Mesopelagic fish gas bladder elongation, as estimated from wideband acoustic backscattering measurements

Backscattered acoustic energy from a target varies with frequency and carries information about its material properties, size, shape, and orientation. Gas-bearing organisms are strong reflectors of acoustic energy at the commonly used frequencies (∼18–450 kHz) in fishery surveys, but lack of knowledge of their acoustic properties creates large uncertainties in mesopelagic biomass estimates. Improved knowledge about the volume and elongation (i.e., longest to shortest dimension) of swimbladders of mesopelagic fishes has been identified as an important factor to reduce the overall uncertainties in acoustic survey estimates of mesopelagic biomass. In this paper, a finite element approach was used to model gas-filled objects, revealing the structure of the backscattering, also at frequencies well above the main resonance frequency. Similar scattering features were observed in measured broadband backscattering of several individual mesopelagic organisms. A method is suggested for estimating the elongation of a gas-bubble using these features. The method is applied to the in situ measured wideband (33–380 kHz) target strength (TS) of single mesopelagic gas-bearing organisms from two stations in the North Atlantic (NA) and Norwegian Sea (NS). For the selected targets, the method suggested that the average elongation of gas-bladder at the NA and NS stations are 1.49 ± 0.52 and 2.86 ± 0.50, respectively.publishedVersio

Quantitative processing of broadband data as implemented in a scientific split-beam echosounder

The use of quantitative broadband echosounders for biological studies and surveys can offer considerable advantages over narrowband echosounders. These include improved spectral-based target identification and significantly increased ability to resolve individual targets. An understanding of current processing steps is required to fully utilise and further develop broadband acoustic methods in marine ecology. We describe the steps involved in processing broadband acoustic data from raw data to frequency dependent target strength ( ) and volume backscattering strength ( ) using data from the EK80 broadband scientific echosounder as examples. Although the overall processing steps are described and build on established methods from the literature, multiple choices need to be made during implementation. To highlight and discuss some of these choices and facilitate a common understanding within the community, we have also developed a Python code which will be made publicly available and open source. The code follows the steps using raw data from two single pings, showing the step-by-step processing from raw data to and . This code can serve as a reference for developing custom code or implementation in existing processing pipelines, as an educational tool and as a starting point for further development of broadband acoustic methods in fisheries acoustics.publishedVersio

Micronekton biomass distribution, improved estimates across four north Atlantic basins

Distribution of micronekton was investigated during early summer of 2013, using data from a cruise covering the central parts of four north Atlantic basins, the Norwegian Sea (NS), Iceland Sea (ICS), Irminger Sea (IRS), and Labrador Sea (LS). Continuous underway acoustics mapped vertical and horizontal distributions, and trawl sampling provided data on biomass and taxonomic composition. The hull mounted acoustics and trawl catches suggested that, among the four basins, biomass of epipelagic, larger nektonic species (>20 cm length) during the cruise was highest in the NS and ICS basins, while mesopelagic non-gelatinous micronekton biomass peaked in the IRS and LS basins. Biomass of Scyphozoa was also about 1 order of magnitude higher in IRS and LS compared to ICS and NS. In ICS and NS, crustaceans made up about 50% of total non-gelatinous micronekton biomass, with fish making up less than 20% of total biomass. In contrast, fish constituted more than 60% of non-gelatinous biomass of catches in IRS and LS. In catches from ICS and NS the myctophid Benthosema glaciale dominated the catches, whereas bathylagids, gonostomatids, barracudinas and stomiids contributed to the high biomass densities of fish in IRS and LS. In addition to the differences in biomass between the basins, the acoustic measurements suggested gradients within the north-eastern basins, and large differences in vertical distribution of biomass between the basins during the cruise.publishedVersio

Survey report for CRIMAC SFI 2023

This cruise report describes the objectives, methods, and preliminary results from the tasks carried out at the CRIMAC SFI survey. The survey was conducted on board RV G.O. Sars between November 15th (Tromsø) and November 21st (Tromsø); in the fjords around Tromsø, from Malangen to Kvænangen. The main objective was to test the Kongsberg Sounder capabilities, including noise testing, weather window testing and safe operations. Other tasks were broad banded calibration procedures, testing two new Kongsberg discovery transducers (18kHz and 333kHz), automated predictions from the deep vision system, and an experiment testing whether herring are affected from a whale deterring device or not (FHF project).Survey report for CRIMAC SFI 2023publishedVersio