We measured the density and sound speed contrasts relative to seawater of Northeast Pacific zooplankton. The density contrast (g) was measured for euphausiids, decapods (Sergestes similis), amphipods (Primno macropa, Phronima sp., and Hyperiid spp.), siphonophore bracts, chaetognaths, larval fish, crab megalopae, larval squid, and medusae. Morphometric data (length, width, and height) were collected for these taxa. Density contrasts varied within and between zooplankton taxa. The mean and standard deviation (s.d.) for euphausiid density contrast were 1.059 ± 0.009. Relationships between zooplankton density contrast and morphometric measurements, geographic location, and environmental conditions were investigated. Site had a significant effect on euphausiid density contrast. Density contrasts of euphausiids collected in the same geographic area ∼4–10 d apart were significantly higher (p< 0.001). Sound speed contrast (h) was measured for euphausiids and pelagic decapods (S. similis) and it varied between taxa. The mean and s.d. for euphausiid sound speed were 1.019 ± 0.009. Euphausiid mass was calculated from measured density and volume, and a relationship between euphausiid mass and length was produced. We determined that euphausiid volume could be accurately estimated from two-dimensional measurements of animal body shape, and that biomass (or biovolume) could be accurately calculated from digital photographs of animals. Data from this study can improve the accuracy of theoretical acoustic scattering models for these taxa, resulting in more accurate estimates of zooplankton biomass in this region.
Material properties of the flesh from three fish species (Merluccius productus, Symbolophorus californiensis, and Diaphus theta), and several body parts of the Humboldt squid (Dosidicus gigas) collected from the California Current ecosystem were measured. The density contrast relative to seawater varied within and among taxa for fish flesh (0.9919-1.036), squid soft body parts (mantle, arms, tentacle, braincase, eyes; 1.009-1.057), and squid hard body parts (beak and pen; 1.085-1.459). Effects of animal length and environmental conditions on nekton density contrast were investigated. The sound speed contrast relative to seawater varied within and among taxa for fish flesh (0.986-1.027) and Humboldt squid mantle and braincase (0.937-1.028). Material properties in this study are similar to values from previous studies on species with similar life histories. In general, the sound speed and density of soft body parts of fish and squid were 1%-3% and 1%-6%, respectively, greater than the surrounding seawater. Hard parts of the squid were significantly more dense (6%-46%) than seawater. The material properties reported here can be used to improve target strength estimates from acoustic scattering models, which could increase the accuracy of biomass estimates from acoustic surveys for these nekton.
Measurements of several acoustic scattering characteristics were made for a variety of different nekton. At-sea broadband high-frequency (100s to 1000s kHz) backscatter measurements were made on several species of myctophids and other types of pelagic nekton (e.g. fish, shrimp). Animals were caught in mid-water trawls off the coast of Oregon during the summer of 2012, and measurements were made on fresh specimens from multiple species. Both broadside (dorsal) and end-on (head/tail) measurements were recorded. There was strong variability among and within species as well as with animal orientation. We also report measurements of swim-bladder size, shape, and fullness for the pelagic myctophids. Additionally, high-resolution computerized tomography (CT) scans were made for several coastal nekton (including squid, silverside, and sea bass) species from New York. These scans provide information on the density contrast of the various organs and other structures in the animal. These data provide useful information for acoustic scattering models of these and other similar animals.
We measured the material properties of Pacific hake (Merluccius productus), Humboldt squid (Dosidicus gigas), and two species of myctophids (Symbolophorus californiensis and Diaphus theta) collected from the California Current. Density contrast (g) was measured for pieces of hake and myctophid flesh, and the following Humboldt squid body parts: mantle, arms, tentacle, braincase, eyes, pen, and beak. Density contrast varied with fish species, as well as among squid body parts. Effects of animal length and environmental conditions on nekton density contrast were investigated. Sound speed contrast (h) was measured for hake and myctophid flesh, Humboldt squid mantle and braincase, and varied within and between nekton taxa. These material property measurements can be used to more accurately parameterize target strength models and increase the accuracy of nekton biomass from acoustic surveys.
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