Sonar systems and aquatic organisms: matching equipment and model parameters

Abstract

Acoustic technology is an accepted and important component of aquatic research and resource management. Despite the widespread use of echosounders, few guidelines aid in the choice of appropriate sonar system parameters for acoustic surveys. Choice of acoustic carrier frequency is analogous to the choice of spotlight colour used to illuminate a painting. Three primary biological factors influence the scattering of sound by aquatic organisms: swimbladder presence, organism length, and organism behaviour. We illustrate the influence of these factors on the amplitude of backscattered echoes using a Kirchhoff-ray mode scattering model to quantify fish and zooplankton backscatter as a function of carrier frequency, fish length, and swimbladder aspect. Model results illustrate that echo amplitudes from aquatic organisms are largely dependent on the presence or absence of a swimbladder. Target strengths generally increase with increasing carrier frequency and organism length. Swimbladder angle relative to the incident sound wave affects scattering amplitudes at all frequencies. Measurements of backscatter from swimbladdered fish are relatively robust when the ratio of fish length to acoustic frequency wavelength ranges between 2 and 10. As fish length to frequency wavelength ratios increase, echo amplitudes become more dependent on aspect and peak when the swimbladder is perpendicular to the acoustic wavefront.

Acoustic approaches to remote species identification: a review

“…Investigating the in¯uence of an organism's morphology and behaviour on echo amplitude and shape is just beginning. Effects of an organism's aspect (Foote, 1980b;Midttun, 1984;Horne and Clay, 1998) and boat avoidance (Olsen et al, 1983;Soria et al, 1996) are fairly well documented, but other factors such as abdominal cavity contents (Ona, 1990), degree of aggregation (Stanton, 1985b;Misund, 1993), animal orientation (Clay and Horne, 1995;Medwin and Clay, 1997), and the material properties of scattering structures (Chu and Wiebe, 2000) require further examination.…”

“…A measure of the amount of sound reflected by a specific target at a specific frequency is the backscattering cross section. At any distance R from the sound source, the backscattering cross section ( σ bs ; units: m 2 ) is defined as: where I r is the sound intensity reflected or backscattered from the target, and I i is the intensity of the incident pulse measured at an arbitrary distance, usually 1 m. A common convention in underwater acoustics is to express the echo amplitude as a target strength ( TS ), which is the logarithmic transformation (units: dB) of the backscattering cross section: Backscattering cross sections or target strengths can be measured using caged or tethered animals, measured in situ using dual or split‐beam echosounders, or modelled using theoretical backscatter models (review: Horne and Clay, 1998). Experimental measures of individual fish lengths and acoustic backscatter can be used to derive empirical relationships between target strength and fish length (Love, 1971; Nakken and Olsen, 1977; Foote, 1987).…”

“…Backscattering cross sections or target strengths can be measured using caged or tethered animals, measured in situ using dual or split-beam echosounders, or modelled using theoretical backscatter models (review: Horne and Clay, 1998). Experimental measures of individual ®sh lengths and acoustic backscatter can be used to derive empirical relationships between target strength and ®sh length (Love, 1971;Nakken and Olsen, 1977;Foote, 1987).…”

Using Multiple Gears to Assess Acoustic Detectability and Biomass of Fish Species in Lake Superior

“…To overcome the problem of distinguishing species, synoptic sampling with midwater trawls is necessary to determine the proportional numerical abundance of species producing the acoustic backscatter (McClatchie et al 2000). The effect of variable fish orientation on average target strength, as used to scale backscatter to estimate fish abundance, is more problematic (Horne and Clay 1997;Eckmann 1991;McClatchie et al 2000;Rudstam et al 2003;Hjellvik et al 2004). Moreover, when average target strength is overestimated (e.g., when fish school or are otherwise found in high densities) abundance can be underestimated (Sawada et al 1993;Appenzeller and Leggett 1992;Rudstam et al 2003).…”

Comparing acoustic model predictions to in situ backscatter measurements of fish with dual‐chambered swimbladders

“…Backscattering cross section or its logarithmic transformation-target strength (S T , dB), can be measured for any species using caged or tethered animals, measured in situ using dual or split beam echosounders, or modeled using numeric backscatter models (Table I; Horne & Clay, 1998). Experimental measures of acoustic backscatter and individual fish lengths can be used to derive empirical relationships between target strength and fish length at dorsal aspect (Love, 1971;Nakken & Olsen, 1977;Foote, 1987) or incorporating fish aspect angle (Foote, 1980;Olsen, unpubl.…”