Physical Characteristics of Bering Sea Zooplankton and Their Use to Parameterize an Acoustic Scattering Model for Euphausiids

Abstract

Acoustic assessment of Bering Sea euphausiids and their predators can provide useful data for ecosystem studies if the acoustic scattering characteristics of these animals are known. The amount of acoustic energy that is scattered by different marine zooplankton taxa is strongly affected by the contrast of the animal's density (g) and sound speed (h) with the surrounding seawater. Density and sound speed contrast were measured in the Bering Sea during the summer of 2008 for several different zooplankton and nekton taxa including: euphausiids (Thysanoessa inermis, Thysanoessa raschii, and Thysanoessa spinifera), copepods, amphipods, chaetognaths, gastropods, fish larvae, jellyfish, and squid. Density contrast values varied between different taxa as well as between individual animals within the same species. Sound speed contrast was measured for monospecific groups of animals and differences were found among taxa. The range, mean, and standard deviation of g and h for all euphausiid species were: g = 1.001-1.041; 1.018 ñ 0.009 and h = 0.990-1.017; 1.006 ñ 0.008. Changes in the relationship between euphausiid material properties and animal length, seawater temperature, seawater density, and geographic location were also evaluated. Results suggest that environmental conditions at different sample locations led to significant differences in animal density and material properties.Acoustic surveys monitor euphausiid populations in the Bering Sea because of their importance as prey walleye pollock. Various scattering models exist to convert acoustic survey data to numerical density estimates of euphausiids, but a target strength (TS) model specific to Bering Sea euphausiids did not exist. This study parameterized a distorted wave Borne approximation model using measured lengths and material properties (density contrast, g, and sound speed contrast, h) from live euphausiids. All model parameters (length, g, h, shape, orientation) were evaluated for their effect on TS estimates.