Density and sound-speed contrasts, and target strength of Japanese sandeel Ammodytes personatus

Abstract

Sound-speed and density contrasts (h and g, respectively), important acoustic material properties, of Japanese sandeel Ammodytes personatus were measured to estimate theoretical target strength (TS). The measured sound-speed contrast of adult fish varied between 1.016 and 1.023 (mean, 1.020), which showed temperature dependence. The measured density contrast differed significantly between juvenile and adult. The density contrast of juvenile varied between 1.017 and 1.024 (1.021), and that of adult varied between 1.026 and 1.038 (1.032). Using these results, TS at 38 and 120 kHz in the fishing season were estimated by an empirical sound scattering model. TS of an individual fish varied significantly with change of tilt angle. TS of near dorsal aspect (TS max ) and tilt-averaged TS (TS ave ) differed up to 7 dB. At both frequencies, two different TS ave -length relationships (TS ave = a log L + b) were obtained for adult and juvenile. The coefficients of log L of adult were close to 20, suggested that backscattering strength was proportional to the square of body length. These values were larger in juvenile (34.0 at 120 kHz, 56.5 at 38 kHz), suggested that backscattering strength varied drastically with the cube or fifth power of body length.

Material properties of Pacific hake, Humboldt squid, and two species of myctophids in the California Current

“…7). Pacific hake density contrast overlaps with the density contrast reported by Shibata (1970) and Brawn (1969) for Pacific herring, and the value reported for Japanese sand eel by Yasuma et al (2009). These species are all pelagic, and the similarities in their density contrasts may be due to these species having a similar life history.…”

“…Echosounders transmit acoustic energy into the water column which produces backscatter when the acoustic wave encounters a region or object with different acoustic impedance than the surrounding seawater (Shibata, 1970;Simmonds and MacLennan, 2005). This backscattered energy can be used to estimate animal biomass; however, to have an accurate estimate of biomass, a constrained estimate of target strength is needed for the scatterer (Simmonds and MacLennan, 2005;Yasuma et al, 2009). Target strength is a logarithmic measure of the energy scattered by an object back toward the source and is a function of the size, shape, orientation, and material properties of the target (Shibata, 1970;Simmonds and MacLennan, 2005).…”

“…Target strength is a logarithmic measure of the energy scattered by an object back toward the source and is a function of the size, shape, orientation, and material properties of the target (Shibata, 1970;Simmonds and MacLennan, 2005). It can be measured empirically; however, few studies have been conducted where TS and all the various parameters which may affect it are measured concurrently because it is logistically difficult (Foote et al, 1987;Simmonds and MacLennan, 2005;Yasuma et al, 2009). Physics-based mathematical target strength models can estimate target strength by using parameters such as length and material properties to calculate the target strength of an organism (Shibata, 1970;Yasuma et al, 2009).…”

“…It can be measured empirically; however, few studies have been conducted where TS and all the various parameters which may affect it are measured concurrently because it is logistically difficult (Foote et al, 1987;Simmonds and MacLennan, 2005;Yasuma et al, 2009). Physics-based mathematical target strength models can estimate target strength by using parameters such as length and material properties to calculate the target strength of an organism (Shibata, 1970;Yasuma et al, 2009). Two material properties needed for the target strength models are the animal's density (g) and sound speed (h) contrasts relative to the density of the surrounding seawater (Shibata, 1970).…”

“…The material properties for fish flesh used in the scattering model in Gorska et al (2007) were from calculations and were not measured directly, so empirical material property values may improve the accuracy of these models. Several studies have measured various acoustic properties of fish: Brawn (1969) measured the density of different body parts of Pacific herring (Clupea pallasii); Shibata (1970) measured the material properties of fish flesh from the several species including Pacific herring and common mackerel (Pneumatophorus j. japonicas); Butler and Pearcy (1972) report the specific gravity of previously frozen Oregon myctophids including California headlight fish (Diaphus theta); Iida et al (2006) reports estimates of material properties for fish flesh from an acoustic camera (Sand lance and Stichaeidae); Yasuma et al (2009) measured the material properties of whole Japanese sand eel (Ammodytes personatus); Forman and Warren (2010) reported material property values for whole striped mummichug (Fundulus majalis) and common mummichug (Fundulus heteroclitus); and Davison (2011) measured the specific gravity (relative to freshwater) of 71 species of mesopelagic fish species including California headlight fish and California lantern fish (Symbolophorus californiensis) after air was removed from the swimbladder. In addition Neighbors and Nafpaktitis (1982) report the buoyancy of many species of mesopelagic fish including the California headlight fish and California lantern fish.…”