Measurements of sound scattering as a means of examining ecological interactions between marine organisms and their environment

Abstract

Abshacf -Significant advances in hydroacoustic systems have occurred in recent years and the availability and application of such systems has greatly improved the quality of quantitative sound scattering measurements from towed and moored sensors. There remain questions of relating the quantity of sound scatter observed (a highly precise measurement) to the quantity of biota or other material causing reflections or volume reverberation (a less well understood process, where behavior of the ensonified organisms causes variation in their target strength). Measurements of sound scattering coupled to directed sampling of target organisms (using nets or video) frequently reveals many aspects of interactions between organisms and their environment. This paper presents methods and examples of measurements and comparisons between sound scatter, causative organisms, and environmental data. INTKODUCTIONThe specifics of echo integration of hydroacoustic data have been well described [ 13. The author has developed and used the system described here for a number of one week to o n e month surveys in a diversity of environments (the Antarctic, Georges Bank off Cape Cod, Hood Canal in Puget Sound, Jervis Inlet in British Columbia). In each of these cases, large quantities of data were collected and analyzed in real time and results placed in the database system to permit further analyses and production of data products. Results of these surveys are reported elsewhere [2-41 and they incorporate many of the data products described in this paper. DATA PROCESSINGDuring field operations, some or all of the available acoustic data may be recorded in digital format (e.g. using a digital audio tape system or similar format) for detailed post-survey re-analysis or to provide for collection and analysis of a second simultaneously collected frequency. The data collection system provides visual displays of the incoming data in several formats, these include paper chart records, oscilloscope signals and color scaled video display of integrated echo intensity for both ping by ping data and the integration interval. Periodic transfer of data files to a second computer, permits summarization and visualization of data collection by means of graphic and printed output, including color echograms.Other analyses may be performed in the field or as a postsurvey activity. This method of data analysis provides timely production of data items of assistance in field work (e.g. sampling depths for nets) and makes a complete analysis of results available for examination immediately following a survey.Many of the details of this process are described in [5]. The use of color coded displays of the data as echograms or other graphical plots, provides a means of rapidly assessing regions of interest and sections of data which may of special interest. The large volume of hydroacoustic data precludes dfective assessment of features within a data set by examination of numerical data alone. For example, a screen of color coded pixels 640 by 480 can be used to rep...