Search

The advent of nontraditional underwater acoustic instrumentation (e.g., multibeam and sector‐scanning sonar) and incorporating fish behavior in quantitative fisheries assessment require that backscatter be measured and modeled at a variety of aspect angles. Acoustic backscattering measurements were obtained in a large laboratory tank using live alewife (Alosa pseudoharengus). An individual alewife for each series of measurements was tethered and rotated in two planes of orientation (dorsal/ventral and lateral). The alewife were insonified with a broadband (40–100 kHz) chirp signal and bistatic scattering geometry was used. Backscattering amplitudes for all angles of orientation (3‐D scattering ambit) were modeled using a Kirchhoff ray‐mode model [C. S. Clay and J. K. Horne, J. Acoust. Soc. Am. 96, 1661–1668 (1994)] and digital images of the fish body and swimbladder morphometry. Visualization of the scattering ambit provides a quantitative examination of the effects of fish orientation on echo amplitude. Comparisons of backscattering amplitudes from the model and measurements along the dorsal/ventral and lateral planes are given. The utility of broadband measurements for fish backscattering amplitude measurements and the integration of acoustic models in fisheries assessments are discussed. [Work supported by ONR, NOAA/NMFS, and CICOR.]

Broadband midfrequency acoustic measurements of fish backscattering were made with two complementary sonar systems in September 2008 at the northern slopes of Georges Bank in the Gulf of Maine. One system, towed near the surface, was downward‐looking (short‐range: < 0.2 km) and was used to image fish at high vertical resolution throughout the water column. The other system was horizontal‐looking (relatively long‐range: 1–10 km), consisting of a vertical source array and a horizontal receive array, and was used to image fish at high horizontal resolution. In this talk, the focus is on characterizing the longer‐range data. Spectrogram analysis showed that the echoes had resonance frequencies in the 2–4 kHz band, consistent with scattering by Atlantic herring, while examination of normalized match‐filter output revealed strong, spatiotemporally variable clutter. The associated probability‐density functions (PDFs) exhibited variable non‐Rayleigh behavior. Several PDF models were fit to the data, with the K and Poisson‐Rayleigh PDF models providing not only good fits, but physical insights into the clutter process (such as estimates of the expected number of discrete scatterers per unit area). These results provide an improved understanding of both fish behavior and the characteristics of fish aggregations as long‐range clutter fields. [Work supported by ONR.]

The use of midwater trawls to harvest Atlantic herring (Clupea harengus) in the Gulf of Maine has been a controversial topic for many years. A majority of the concern revolves around the potential physical disruption of Atlantic herring aggregations and the potential impact on ecological processes and other industries (e.g., other fisheries and eco‐tourism) that rely on these aggregations. Preliminary acoustic data collection in fall 2008 using a combination of Simrad ES60 echosounders (38 and 120 kHz) and a Simrad SP90 omni‐directional sonar (20–30 kHz) showed promise for quantifying herring aggregation metrics. In this presentation, we report on a pilot study conducted in the summer of 2009 that uses these acoustic systems to quantify and describe herring aggregations before and after midwater trawling with a pair of fishing vessels. We used a systematic grid design to survey an area continuously for 24 h both before and after pair trawl fishing over multiple fishing trips. We report on the efficacy of using acoustic systems to quantify the potential impacts of fishing on herring aggregations and discuss the next steps to fully evaluate these potential impacts at spatial and temporal scales relevant to the fishery.

Ocean acoustic waveguide remote sensing (OAWRS) is a bistatic multibeam wide area sonar system that enables unaliased monitoring of fish populations over ecosystem scales. This allows us to quantify the formation processes of vast shoals of Atlantic herring (Clupea harengus) containing hundreds of millions of individuals, imaged during the Autumn 2006 spawning season. Areal population density estimation requires calibration of the low‐frequency target strength of an individual shoaling herring. This is estimated from experimental data acquired simultaneously at multiple frequencies in the 300–1200‐Hz range using (1) the OAWRS system, (2) areal population density calibration with several conventional fish finding sonar systems (CFFS), (3) fish length distributions obtained from trawl samples, and (4) local low‐frequency transmission loss measurements. High spatial‐temporal co‐registration was found between shoals imaged by OAWRS and concurrent CFFS line transects, which also provided fish depth distributio...

The mean low‐frequency target strength (TS) of spawning Atlantic herring populations in the Gulf of Maine is estimated from the experimental data acquired during September–October 2006 near the northern flank of Georges Bank. A low‐frequency OAWRS system with an instantaneous imaging diameter of 100 km was deployed to provide spatially unaliased imaging of fish populations over wide areas. The OAWRS system’s scattering strength measurements are calibrated with areal fish population density estimates obtained from concurrent localized line‐transect measurements with several conventional fish finding sonars (CFFSs). Trawl sampling at selected locations enables the identification of the imaged species. The mean TS estimates of herring individuals exhibits significant variation over OAWRS operating frequency range, in accordance with the results from a resonant scattering model for swimbladder‐bearing fish. The neutral buoyancy depth of herring and the species composition in the imaged population is inferred ...

An ocean acoustics waveguide remote sensing (OAWRS) system was deployed in the Gulf of Maine, near Georges Bank to image Atlantic herring and other fish population from September–October 2006. OAWRS provides spatially unaliased imaging of herring over wide areas, spannning over 100 km diam. Migration and spawning behavior of Atlantic herring were observed using OAWRS over several diurnal periods, including massive movements on and off the bank to spawn. Measurements made simultaneously with a conventional fish‐finding echosounder (CFFS) and a multibeam sonar provide the depth distribution and local 3‐D morphology, respectively, of the herring schools in the water column. Concurrent trawl surveys provide identification of the fish species. Measurements made by OAWRS and CFFS systems are highly correlated. Examples will be provided of the co‐registration between the two systems over a one‐week period. Calibration of the OAWRS system using CFFS estimates of fish population densities, along with a full‐field scattering model that takes into account both coherent and incoherent scattering from a fish group, is discussed. Resonance scattering behavior of herring is observed in the OAWRS system with significant changes in scattering amplitude over the 300 Hz to 1.5 kHz frequency range of the OAWRS system.

In September 2006, several schools of Atlantic herring were simultaneously imaged with a Reson 7125 SeaBat multibeam sonar (400 kHz) and a Simrad EK60 Scientific echo sounder (38 kHz, 120 kHz, and 200 kHz) on Georges Bank in the Gulf of Maine. One school was imaged five separate times over the span of one hour. These two sonar systems, along with pelagic trawl catch data collected on a separate ship, provided a synoptic view of the fish school as it changed depth and size, fragmented, and eventually dispersed. Of particular interest is the combination of data from the two sonars. The multifrequency, split‐beam EK60 provided estimates of the fish school density directly under the ship, but was not capable of accurately extrapolating these measurements to the entire fish school, which was largely out of the EK60 field of view. The multibeam sonar was generally able to image the entire fish school and provided useful information on school volume, fragmentation, and general school morphology, but was more difficult to use for quantifiable measurements, due to the unknown angle of ensonfication of the fish in the outer beams.

A composite scattering model has been developed to describe the backscattering by swimbladder‐bearing fish. This model includes the influence of elongation of the swim‐bladder not only on the resonance frequency, as do most of the existing scattering models, but also on the quality factor Q and the overall scattering level. In addition, within the region covering the transition from resonance scattering to geometric scattering, the effect of asymmetric cross section of the swimbladder has also been included to represent a more realistic shape of fish swimbladder. The model was compared with other scattering models, such as Kirchhoff ray mode (KRM) and Fourier matching method (FMM). Model predictions were then compared with field data collected with a new broadband towed system. For a single‐species Atlantic herring school (Clupea harengus) with a measured length distribution, the predicted resonance scattering including resonance peak location and overall level matched the broadband acoustic data reasonably well. Some modeling issues will also be discussed.

A commercial system originally designed for surveying the seafloor has been adapted for measuring the swimbladder resonance of swimbladder‐bearing fish. The new towed system has three broadband active acoustic echosounders with nearly continuous coverage of the frequency band 2–110 kHz (with some gaps). The lower frequency bands are used for measuring the resonance, while the highest band can detect zooplankton. A pulse‐compression approach (cross correlating the transmit waveform with the echoes) is used to exploit the bandwidth through significantly improving the range resolution to 20, 10, and 2 cm (low‐, med‐, and high‐frequency bands, respectively). The system has been successfully deployed as deeply as 200 m and observed resonances at approximately 3 kHz for herring. Because of the 10–20‐cm range resolution at the low frequencies, fish near the towbody could be resolved from one another and fish swimming within 1 m of the seafloor could be distinguished from the seafloor. This system is compared with previous approaches of measuring swimbladder resonance. The potential of the resonance classification method for significantly reducing ambiguities associated with interpreting acoustic echo data compared with traditional high‐frequency narrow‐band systems (10’s to 100’s of kHz) is discussed.

Routine acoustical surveys for estimating Atlantic herring (Clupea harengus) population abundance have been conducted on Georges Bank during the autumn spawning season from 1998 to present. Acoustical data are collected with a Simrad EK500 scientific echo sounder operating at 12 or 18, 38, and 120 kHz, and split‐beam (the 12‐kHz system is a single beam) transducers. Biological measurements and verification of acoustical scatterers are obtained with a pelagic trawl. Acoustical data are evaluated (scrutinized) manually to remove noise, faulty bottom detections, and to classify acoustical backscattering to species. Species classification is currently subjective, and is based on the experience of the scientists and trawl catches. Objective species classification and automated fish density and abundance estimates are an obvious goal for fisheries surveys using advanced technologies. Classification methods using relationships among frequency‐dependent volume backscattering strengths, such as presence–absence an...

A series of calibration trials have been performed with several multibeam sonars by means of the standard‐target method. These have included multiple units of the Simrad SM2000 Multibeam Echo Sounder operating at 90 or 200 kHz, with external transmitter in each case. The principal measurements have been of the full two‐dimensional directivity characteristics of the main lobes. Issues of sensitivity, linearity, dynamic range, and near‐field effects have also been studied. [Work supported by NSF Grant No. OCE‐0002664.]

Most marine fish maintain swimbladder volumes equivalent to 4%–5% of their body weight in order to maintain neutral buoyancy. In many fish the addition or removal of gas from the swimbladder is accomplished with the gas gland, a blood invested portion of the swimbladder wall. However, several families, including the herring family, Clupeidae, lack a gas gland. Instead, these fish possess a pneumatic duct between the esophagus and the swimbladder by which they are believed to inflate their swimbladders by ‘‘gulping’’ atmospheric air at the sea surface. Acoustic measurements at 1.5–5 kHz on fish in the Gulf of Maine showed a swimbladder resonance peak near 2.3 kHz at 180 m depth. Midwater trawls confirmed that the fish were Atlantic herring (Clupea harengus) of 19–28 cm length. Calculations using a model of swimbladder resonance gives swimbladder volumes of 1.3% at 180 m. Extrapolation using Boyle’s law suggests that at the sea surface, these herring would need to inflate their swimbladders by up to four times the volume required for neutral buoyancy. In general, swimbladders of the Clupeidae may resonate at lower frequencies than previously expected. [Work supported by ONR and NMFS.]

It is well known that the volume scattering strength is very sensitive to the size and orientation of scattering objects such as swim‐bladdered fish. Such behavioral information is in general not always available. Inadequate knowledge of target orientations limits our ability to correctly estimate the size and abundance of fish acoustically. To extract orientation information of individual fish and/or fish schools, a scattering‐model‐based image processing method is presented. For fish aggregations within which individual fish can be resolved from the echogram, the geometric shapes and acoustic intensities of the identifiable highlights can potentially be used to infer the sizes and orientations of the individual targets. For fish schools within which individual fish cannot be resolved, the mean fish size and orientation can still potentially be inferred by examining the statistics of the amplitude distribution of the patches resulting from strong backscattering from fish schools. The scattering model is ...

A series of laboratory scattering measurements and associated analysis have been conducted involving alewife fish (Alosa pseudoharengus). A greater‐than‐octave bandwidth, shaped, linearly swept, frequency modulated signal was used to insonify live, adult alewife that were tethered while being rotated. The acoustic scattering time series were measured in 1‐deg increments of orientation angle over all angles in two planes of rotation. Dominant acoustic scattering mechanisms are identified through both spectral and time‐domain (pulse compression) analyses. Results demonstrate the dependence of scattering strength upon frequency and angle of orientation in the lateral (horizontal) and dorsal/ventral aspects. The pulse compression processing of the echoes from the animals temporally resolves multiple returns from an individual which are correlated with size and orientation. [Work supported by ONR, NOAA and CICOR (Cooperative Institute for Climate and Ocean Research).]

Annual fisheries acoustic surveys have recently been implemented in the Gulf of Maine and Georges Bank regions to estimate the abundance and biomass of Atlantic herring (Clupea harengus). Research was devoted to the verification of backscatter measurements from Atlantic herring using multifrequency echo‐integration, omni‐directional sonar, pelagic trawling, and underwater video operations. Results indicated that individual backscatter measurements during the day were significantly higher compared to the night and twilight periods. The diurnal behavior patterns of Atlantic herring contributed to this variability because of their vertical migration to near bottom during day and to surface waters during night. Observations showed that herring shoaled in stationary schools above offshore banks during the night to feed, and tend to swim more actively during twilight and day periods. An increase in the backscatter from herring during the day may be explained by their more horizontal orientation when actively swimming. Fisheries acoustic surveys conducted during day and night must incorporate systematic adjustments to account for the diurnal bias in acoustical estimates.

To improve chances for discriminating and identifying fish and zooplankton species, more acoustic information is better. Multifrequency, digital echosounders increase information bandwidth by increasing the number discrete frequencies transmitted and received through multiple transducers. Multifrequency data provide a range of aural perspectives just as colored spotlights accent different components of a painting. Lake whitefish (Coregonus clupeaformis) represent an increasing biomass in Lake Michigan and have a disproportionately large, single‐chambered swimbladder relative to the body volume. Frequency‐dependent, echo amplitude predictions from a Kirchhoff‐ray mode model were compared to in situ backscatter measurements of constrained and free ranging lake whitefish at five discrete frequencies (38, 50, 120, 200, and 420 kHz). In addition to geometric scattering, backscattering characteristics of whitefish in the resonance region were also modeled. This combination of theoretical model prediction and multifrequency measures allows the quantification of the importance of intraspecies backscatter variability due to fish sizes and behavior. [Work supported by ONR.]

Most acoustic backscatter models predict echo amplitudes of single or aggregations of fish with single‐chambered swimbladders. Lavnun (Acanthobrama terraesanctae), a planktivorous fish in Lake Kinneret, Israel, possesses a dual‐chambered swimbladder. Target strength, length, frequency, or aspect relationships do not exist for this commercially important species. What is the most appropriate way to model backscatter amplitudes from single and aggregations of fish with multi‐chambered swimbladders? Carrier frequency, organism length, organism aspect, and swimbladder shape all influence amplitudes of returned echoes. Predicted backscatter amplitudes and intra‐species variance predictions from an anatomically‐based Kirchhoff‐ray mode model were compared to in situ measures of lavnun backscatter at 120 and 420 kHz. The lavnun swimbladder was modeled as a single chamber (with equivalent volume of both chambers), as two independent chambers, and as two acoustically interacting chambers. Wider application and comparison of theoretical models to field measurements increases understanding of factors influencing backscatter amplitude and variability for all fish species, and should improve methods for identifying organisms. [Work supported by ONR and US–Israel BSF.]

Current efforst to model fish backscatter (e.g., Kirchoff ray‐mode model [C. S. Clay and J. K. Horne, J. Acoust. Soc. Am. 96, 1661–1668 (1994)] use digitized images of fish anatomy for a more realistic representation of swimbladder shape, volume, and aspect. However, the degree of image resolution necessary for acoustic characterization and improved predictions, and the variability associated with resolution has not been quantified. Does a more accurate and higher resolution image result in improved correlation between theory and empirical measurements? Backscatter amplitudes display strong relationships with carrier frequency, swimbladder shape, and swimbladder aspect at rather high frequencies. Are predictions of these relationships influenced by image resolution? X‐ray images of Atlantic cod (Gadus morhua) and brook trout (Salvelinus fontinalis) were digitized at fine resolution, and sensitivity analyses were performed to quantify the effect of varying image resolution on predicted backscatter as a function of carrier frequency and swimbladder shape and aspect. Quantifying image resolution is a step towards determining morphometric effects on backscatter variability and estimating fish abundance using multi‐frequency data and the inverse approach. [Work supported by ONR.]

Fisheries researches are increasing the use of frequency‐, length‐, and aspect‐dependent backscatter models in population abundance estimates of fish and zooplankton. Model predictions are often based on measurements from a single or a limited number of organisms. The resulting model parameters are used to predict backscatter for organisms of any length in the population. At rather high frequencies (i.e., length/wavelength ratios of 1 to 20), choice of carrier frequency, swimbladder shape, and swimbladder aspect influence amplitudes of returned echoes. Morphological differences among individuals will therefore determine the variance observed in amplitudes of echo ensembles. To quantify variance in predicted backscatter within species as a function of carrier frequency, fish length, and swimbladder aspect, we combined data from digitized x rays with Kirchhoff ray‐mode backscattering models of brook trout (Salvelinus fontinalis) and Atlantic code (Gadus morhua). Quantifying backscatter variances within and ...

Fisheries sonar systems typically operate at discrete frequencies between 38 and 420 kHz. Can length‐frequency distributions of aggregated fish be accurately estimated using available frequencies and the inverse problem? The inverse problem requires measured fish lengths and realistic scattering models. The size distribution of threadfin shad (Dorosoma petenense) in Lake Nornman was estimated using 120‐, 200‐, and 420‐kHz data, and a recently developed scattering model [Jech et al., J. Acoust. Soc. Am. (in press 1995)]. Size distribution estimates were compared to length frequency measurements from purse seine catches. Fits of probability density functions (PDF’s) using the inversion technique to those using length frequency measures were sensitive to the choice of fish size classes. Preliminary results indicate that estimation of length frequencies using multi‐ frequency data and the inverse problem appears dependent on the shape of measured length‐frequency distributions. [Work supported by NOAA Coastal...

A quantitative approach, Wiener optimal least‐squares filtering, was used to determine the linear correlation between physical and biological structure along the western edge of the Gulf Stream. Temperature, vertical temperature gradient, and diurnal effects on the vertical distribution of biological sound scatterers in the top 200 m of Gulf Stream frontal regions were studied. Coordinated temperature and 70‐ and 120‐kHz acoustic backscatter data were obtained 105 km northeast of Cape Hatteras along a transect perpendicular to the Gulf Stream front. The data show that temperature is a better predictor of 70‐ and 120‐kHz scattering than is temperature gradient at the fine scale. Diurnal differences suggest biological influences can override temperature preferences by nekton and zooplankton. A universal filter is developed to determine a consistent temporal relationship between acoustic scatterers and temperature structure. Application of this technique is a unique means of quantitatively determining relati...