J. Michael Jech

Critical population density triggers rapid formation of vast oceanic fish shoals.

Similarities in the behavior of diverse animal species that form large groups have motivated attempts to establish general principles governing animal group behavior. It has been difficult, however, to make quantitative measurements of the temporal and spatial behavior of extensive animal groups in the wild, such as bird flocks, fish shoals, and locust swarms. By quantifying the formation processes of vast oceanic fish shoals during spawning, we show that (i) a rapid transition from disordered to highly synchronized behavior occurs as population density reaches a critical value; (ii) organized group migration occurs after this transition; and (iii) small sets of leaders significantly influence the actions of much larger groups. Each of these findings confirms general theoretical predictions believed to apply in nature irrespective of animal species.

Protocols for calibrating multibeam sonar

Development of protocols for calibrating multibeam sonar by means of the standard-target method is documented. Particular systems used in the development work included three that provide the water-column signals, namely the SIMRAD SM2000/90- and 200-kHz sonars and RESON SeaBat 8101 sonar, with operating frequency of 240 kHz. Two facilities were instrumented specifically for the work: a sea well at the Woods Hole Oceanographic Institution and a large, indoor freshwater tank at the University of New Hampshire. Methods for measuring the transfer characteristics of each sonar, with transducers attached, are described and illustrated with measurement results. The principal results, however, are the protocols themselves. These are elaborated for positioning the target, choosing the receiver gain function, quantifying the system stability, mapping the directionality in the plane of the receiving array and in the plane normal to the central axis, measuring the directionality of individual beams, and measuring the nearfield response. General preparations for calibrating multibeam sonars and a method for measuring the receiver response electronically are outlined. Advantages of multibeam sonar calibration and outstanding problems, such as that of validation of the performance of multibeam sonars as configured for use, are mentioned.

Broadband acoustic backscatter and high-resolution morphology of fish: Measurement and modeling

Broadband acoustic backscattering measurements, advanced high-resolution imaging of fish morphology using CT scans and phase-contrast x rays (in addition to traditional x rays), and associated scattering modeling using the images have been conducted involving alewife (Alosa pseudoharengus), a swimbladder-bearing fish. A greater-than-octave bandwidth (40-95 kHz) signal was used to insonify live, individual, adult alewife that were tethered while being rotated in 1-deg increments over all angles in two planes of rotation (lateral and dorsal/ventral). These data, in addition to providing the orientation dependence of the scattering over a continuous band of frequencies, were also used (after pulse compression) to identify dominant scattering features of the fish (including the skull and swimbladder). The x-ray and CT scan images of the swimbladder were digitized and incorporated into two scattering models: (1) Kirchhoff-ray mode (KRM) model [Clay and Horne, J. Acoust. Soc. Am. 96, 1661-1668 (1994)] and (2) conformal-mapping-based Fourier matching method (FMM), which has recently been extended to finite-length bodies [Reeder and Stanton, J. Acoust. Soc. Am. 116. 729-746 (2004)]. Comparisons between the scattering predictions and data demonstrate the utility of the CT scan imagery for use in scattering models, as it provided a means for rapidly and noninvasively measuring the fish morphology in three dimensions and at high resolution. In addition to further validation of the KRM model, the potential of the new FMM formulation was demonstrated, which is a versatile approach, valid over a wide range of shapes, all frequencies and all angles of orientation.

Low-frequency target strength and abundance of shoaling Atlantic herring (Clupea harengus) in the Gulf of Maine during the Ocean Acoustic Waveguide Remote Sensing 2006 Experiment.

The low-frequency target strength of shoaling Atlantic herring (Clupea harengus) in the Gulf of Maine during Autumn 2006 spawning season is estimated from experimental data acquired simultaneously at multiple frequencies in the 300-1200 Hz range using (1) a low-frequency ocean acoustic waveguide remote sensing (OAWRS) system, (2) areal population density calibration with several conventional fish finding sonar (CFFS) systems, and (3) low-frequency transmission loss measurements. The OAWRS systems instantaneous imaging diameter of 100 km and regular updating enabled unaliased monitoring of fish populations over ecosystem scales including shoals of Atlantic herring containing hundreds of millions of individuals, as confirmed by concurrent trawl and CFFS sampling. High spatial-temporal coregistration was found between herring shoals imaged by OAWRS and concurrent CFFS line-transects, which also provided fish depth distributions. The mean scattering cross-section of an individual shoaling herring is found to consistently exhibit a strong, roughly 20 dB/octave roll-off with decreasing frequency in the range of the OAWRS survey over all days of the roughly 2-week experiment, consistent with the steep roll-offs expected for sub-resonance scattering from fish with air-filled swimbladders.

Three-dimensional visualization of fish morphometry and acoustic backscatter

Theoretical acoustic models of fish are used to explain variability in backscatter measurements, improve estimation of target size, and improve target recognition and discrimination among acoustic targets. Acoustic backscatter models that incorporate fish morphology potentially provide more realistic predictions of echo amplitudes than models that approximate morphology using simple geometric shapes. Procedures to obtain digital representations of a fish’s body and swimbladder are presented. These digital images are used in a Kirchhoff ray-mode model to predict backscatter amplitude as a function of fish length, acoustic frequency, and angle of insonification. Backscatter amplitude can be displayed as one-dimensional curves, two-dimensional response surfaces, and a three-dimensional backscattering surface (i.e., ambit).

Inferring fish orientation from broadband-acoustic echoes

A new method has been developed for inferring the orientation of fish through the use of broadband-acoustic signals. The method takes advantage of the high range resolution of these signals, once temporally compressed through cross-correlation. The temporal resolution of these compressed signals is inversely proportional to the bandwidth, thus the greater the bandwidth the higher the resolution. This process has been applied to broadband-chirp signals spanning the frequency range 40–95 kHz to obtain a range resolution of approximately 2 cm from the original, unprocessed resolution of about 50 cm. With such high resolution, individual scattering features along the fish have been resolved, especially for angles well off normal incidence. The overall duration of the compressed echo from live, individual alewife, as measured in a laboratory tank, is shown to increase monotonically with orientation angle relative to normal incidence. The increase is due to the greater range separation relative to the transducer between the echoes from the head and tail of the fish. The results of this study show that with a priori knowledge of the length of the fish, the orientation could be estimated from the duration of a single, compressed broadband echo. This method applies to individual, acoustically resolved fish. It has advantages over previous approaches because it derives the orientation from a single ping and it does not use a formal, mathematical scattering model. Design parameters for applications in the ocean are given for a range of conditions and fish size. 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights

Visualizing fish movement, behavior, and acoustic backscatter

Acoustic surveys of aquatic organisms are notorious for large data sets. Density distribution results from these surveys are traditionally graphed as two-dimensional plots. Increasing information content through wider acoustic frequency ranges or multiple angular perspectives has increased the amount and complexity of acoustic data. As humans are visually oriented, our ability to assimilate and understand information is limited until it is displayed. Computer visualization has extended acoustic data presentation beyond two dimensions but an ongoing challenge is to coherently summarize complex data. Our goal is to develop visualizations that portray frequency- and behavior-dependent backscatter of individual fish within aggregations. Incorporating individual fish behavior illustrates group dynamics and provides insight on the resulting acoustic backscatter. Object-oriented applications are used to visualize fish bodies and swimbladders, predicted Kirchhoff-ray mode (KRM) backscatter amplitudes, and fish swimming trajectories in three spatial dimensions over time. Through the visualization of empirical and simulated data, our goal is to understand how fish anatomy and behavior influence acoustic backscatter and to incorporate this information in acoustic data analyses.

Vast assembly of vocal marine mammals from diverse species on fish spawning ground

Observing marine mammal (MM) populations continuously in time and space over the immense ocean areas they inhabit is challenging but essential for gathering an unambiguous record of their distribution, as well as understanding their behaviour and interaction with prey species. Here we use passive ocean acoustic waveguide remote sensing (POAWRS) in an important North Atlantic feeding ground to instantaneously detect, localize and classify MM vocalizations from diverse species over an approximately 100,000 km2 region. More than eight species of vocal MMs are found to spatially converge on fish spawning areas containing massive densely populated herring shoals at night-time and diffuse herring distributions during daytime. We find the vocal MMs divide the enormous fish prey field into species-specific foraging areas with varying degrees of spatial overlap, maintained for at least two weeks of the herring spawning period. The recorded vocalization rates are diel (24 h)-dependent for all MM species, with some significantly more vocal at night and others more vocal during the day. The four key baleen whale species of the region: fin, humpback, blue and minke have vocalization rate trends that are highly correlated to trends in fish shoaling density and to each other over the diel cycle. These results reveal the temporospatial dynamics of combined multi-species MM foraging activities in the vicinity of an extensive fish prey field that forms a massive ecological hotspot, and would be unattainable with conventional methodologies. Understanding MM behaviour and distributions is essential for management of marine ecosystems and for accessing anthropogenic impacts on these protected marine species.

Interpretation of multi-frequency acoustic data: Effects of fish orientation

One goal of fisheries acoustics is to develop objective classification or identification methods to automate allocation of acoustic backscatter to species. Classification schemes rely on consistent relationships for successful apportionment of acoustic backscatter to species. A method is developed that compares frequency-dependent volume backscatter from an acoustical survey of Atlantic herring (Clupea harengus) to investigate the potential for classifying herring. Predicted backscattering patterns by a Kirchhoff-ray approximation are used to explain the observed relationships and evaluate the potential for classification of multi-frequency data. Combining predicted backscatter with observations of the frequency-dependent volume backscatter gave approximately 40% classification success, which is not sufficient for survey purposes. However, this method highlighted potential consequences that fish orientation may have on classification schemes and density and abundance estimates. This method of comparing multi-frequency volume backscatter appears to be beneficial for detecting behavioral changes by groups of fish, which may be used to select target strength values for density or abundance estimates. Utilizing predicted target strengths from numerical or analytical solutions or approximations, appropriate target strengths could be selected and would provide more accurate estimates of fish density and abundance.

Comparisons among ten models of acoustic backscattering used in aquatic ecosystem research

Analytical and numerical scattering models with accompanying digital representations are used increasingly to predict acoustic backscatter by fish and zooplankton in research and ecosystem monitoring applications. Ten such models were applied to targets with simple geometric shapes and parameterized (e.g., size and material properties) to represent biological organisms such as zooplankton and fish, and their predictions of acoustic backscatter were compared to those from exact or approximate analytical models, i.e., benchmarks. These comparisons were made for a sphere, spherical shell, prolate spheroid, and finite cylinder, each with homogeneous composition. For each shape, four target boundary conditions were considered: rigid-fixed, pressure-release, gas-filled, and weakly scattering. Target strength (dB re 1 m(2)) was calculated as a function of insonifying frequency (f = 12 to 400 kHz) and angle of incidence (θ = 0° to 90°). In general, the numerical models (i.e., boundary- and finite-element) matched the benchmarks over the full range of simulation parameters. While inherent errors associated with the approximate analytical models were illustrated, so were the advantages as they are computationally efficient and in certain cases, outperformed the numerical models under conditions where the numerical models did not converge.