Visualizing Fish Movement, Behavior, and Acoustic Backscatter
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Aquatic Living Resources 16 (2003) 277–282 www.elsevier.com/locate/aquliv Visualizing fish movement, behavior, and acoustic backscatter Richard H. Towler a,*, J. Michael Jech b, John K. Horne a a University of Washington, School of Aquatic and Fishery Sciences, Box 355020, Seattle, WA 98195, USA b NOAA Northeast Fisheries Science Center, Woods Hole, MA 02543, USA Accepted 27 January 2003 Abstract 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. © 2003 Éditions scientifiques et médicales Elsevier SAS and Ifremer/IRD/Inra/Cemagref. All rights reserved. Keywords: Acoustics; Backscatter; Kirchhoff-ray mode; Target strength; Visualization 1. Introduction Simultaneously measuring the influence of all biological factors on backscatter amplitudes is not yet possible. As an Fisheries acoustic surveys collect large data sets. A com- alternative, numeric and analytic models estimate backscat- mon challenge when presenting any large data set is to ter as a function of biological or physical factors of interest. parsimoniously represent data characteristics (e.g. ampli- Backscatter models augment experimental measures by pre- tudes, ranges, variances), while maximizing the amount of dicting echo amplitudes from individuals under known con- information portrayed. Acoustic data contain quantitative ditions. To illustrate by example, Kirchhoff-ray mode information on density and size distributions of backscatter- (KRM) backscatter models (Clay and Horne, 1994) have ing organisms. An important analytic task is to partition the been used to characterize frequency- and behavior- backscattered energy into categories representing size dependent backscatter of individual and aggregations of fish classes, species, or species groups. Classification of acoustic (Horne and Jech, 1999; Jech and Horne, 2001). Visualization backscatter utilizes echo amplitude or echo envelope metrics of results include backscatter response surfaces over a desig- when targets can be resolved, or patterns of backscatter from nated range of aspect angles, lengths, and carrier frequencies aggregations when targets are not resolvable. Fluctuations in (Fig. 6, Horne and Jech 1999; Fig. 5, Horne et al., 2000) and backscatter amplitudes from an individual impede consistent in interactive representations of fish bodies, swimbladders, and accurate classification of single targets. Variability in the and the corresponding acoustic backscatter ambits (Jech and amount of sound reflected by an aquatic organism is caused Horne, 2001, Fish3d www.acoustics.washington.edu). by the transmission of sound through water and by a suite of One approach used to examine how biological factors biological factors including anatomy, physiology, and behav- influence echo amplitudes integrates modeling of organism ior. behavior with acoustic measurements of fish distributions in computer visualizations. These visualizations should present * Corresponding author. large data sets in a coherent and comprehensive manner; E-mail address: [email protected] (R.H. Towler). reveal several levels of detail within data sets; avoid distor- © 2003 Éditions scientifiques et médicales Elsevier SAS and Ifremer/IRD/Inra/Cemagref. All rights reserved. doi:10.1016/S0990-7440(03)00011-1 278 R.H. Towler et al. / Aquatic Living Resources 16 (2003) 277–282 tion of measurements; prompt the viewer to think about mechanisms that cause observed patterns; and encourage comparisons among data sets (Tufte, 1983). This paper was motivated by an interest in visualizing three-dimensional swimming behavior and the resulting effects on acoustic backscatter. Simulations of adult walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus) dynamics are integrated with backscatter models to visualize the influ- ence of fish anatomy, orientation, and kinematics on acoustic measurements. We believe that visualization enhances the understanding of backscatter variability and can be used to improve conversions of acoustic data to biological meaning- ful numbers such as fish length, density, and abundance. 2. Materials and methods Our goal is to integrate echosounder properties with fish anatomy, backscatter model predictions, and fish trajectories to visualize factors that influence patterns in backscatter data. Three autonomous visualizations are integrated into a single computer application. First, digitized walleye pollock and capelin anatomy are visualized with the corresponding frequency-dependent acoustic backscatter. Second, the movements of fish within schools are visualized from the Fig. 1. Schematic diagram showing the relationship between the visualiza- perspective of a transducer or a member of the school. Third, tion classes, objects, and data. these two components are integrated to visualize the effect of fish orientation and movement on echo amplitude. Interac- The echofish class combines individual fish anatomical tion among the three components is facilitated using object- and acoustic data with behavioral and physical properties. oriented programming. The echofish class defines the location in space, velocity, and Object-oriented programming is a ubiquitous feature of the orientation as physical properties of echofish objects. The computer applications from games to scientific simulations. anatomical data stored in an echofish object are used to create An object-oriented approach improves efficiency and flex- a three-dimensional model of the fish used in the display. ibility of computer programming by coupling data and a set Acoustic backscatter data, either modeled or empirical, can of actions within an object class. An object class defines the be queried at specified tilt and roll orientations of an echofish data that are stored in an object and the actions that can be object. Because the echofish was designed to interact with performed on the object. Objects are derived from classes. any number of behavior simulators, specific behavioral pa- Each object is an instance of a class with its’ own unique set rameters are not defined in this class. The echofish class can of data that can be queried, manipulated, and visualized. The read and write behavioral parameters so that behavior simu- three classes used in this application correspond to the echo- lators can define and modify parameters at any time. This sounder, fish anatomy and associated backscatter, and fish permits simulators with varying parameter requirements to behavior. The echosounder class includes transducer proper- interact with echofish through a common interface and per- ties; the echofish class includes acoustical, anatomical, and mits each echofish to be parameterized independently if behavioral characteristics of individual fish; the trajectory needed. class defines fish movements in a shoaling and schooling Fish behavior is simulated by the trajectory class. Fish simulator (Fig. 1). velocities are calculated using a three-dimensional shoaling The echosounder class contains all properties of the echo- and schooling model. The formation and maintenance of fish sounder and transducers used in the application: location, aggregations is based on opposing forces of attraction and orientation, beam width, and frequency. The echosounder repulsion among individuals (Parr, 1927; Breder, 1954; tabulates backscatter by determining which targets are in the Horne, 1995). Magnitudes of attraction and repulsion forces beam, calculating the incident angle relative to the target’s are a function of the distance between individuals, canceling local axes, and then querying each target for a backscatter at the mean distance between individuals. Parameter values value. Backscatter amplitudes incorporate target orientations of the behavioral model are based on experimental or empiri- (i.e. tilt and roll) within the beam but are not corrected for cal observations adapted to approximate walleye pollock and transducer beam patterns. Echo amplitudes are equivalent to capelin kinematics. In our simulations, all fish are acousti- on axis targets at a variety of tilt and roll angles. cally resolved at any frequency. One walleye pollock is R.H. Towler et al. / Aquatic Living Resources 16 (2003) 277–282 279 chosen as the “lead”fish among the 20 echofish objects in the each point in the backscatter matrix is represented in the simulation. This pollock follows a specified trajectory ambit by color (purple