David A. Demer

Southwest Fisheries Science Center · Advanced Survey Technologies Group

Three years after the 2015 collapse of the northern stock of Pacific Sardine that is predominantly located off the west coast of the United States, acoustic‐trawl (A‐T) surveys documented an increase in the presence and persistence of the southern stock off coastal Southern California. Then in 2020, the biomass of Sardine that was landed in Mexico...

Executive Summary This report provides: 1) a detailed description of the acoustic-trawl method (ATM) used by NOAA’s Southwest Fisheries Science Center (SWFSC) for direct assessments of the dominant species of coastal pelagic species (CPS; i.e.: Pacifc Sardine Sardinops sagax, Northern Anchovy Engraulis mordax, Pacifc Mackerel Scomber japonicus, Ja...

Fluctuations in prey abundance, composition, and distribution can impact predators, and when predators and fisheries target the same species, predators become essential to ecosystem‐based management. Because of the difficulty in collecting concomitant predator–prey data at appropriate scales in patchy environments, few studies have identified stron...

The Summer 2021 California Current Ecosystem Survey (CCES) (2107RL) was conducted by the Fisheries Resources Division (FRD) of the Southwest Fisheries Science Center (SWFSC) aboard NOAA ship Reuben Lasker (hereafter Lasker) (Fig. 1), 6 July to 15 October 2021, and augmented by data collected from the Mexican research vessel Dr. Jorge Carranza Frase...

Worldwide, marine protected areas (MPAs) are increasingly created to protect and restore selected parts of the ocean and to enhance recreation, fishing, and sustainable resources. However, this process has outpaced the development and implementation of methods for assessing and monitoring these habitats. Here, we combine data from an echosounder, a...

No PDF available ABSTRACT Offshore pelagic environments are dynamic and rich in biodiversity, yet our understanding of these habitats is lacking as they are surveyed less frequently than coastal areas. We present results from one-year (October 2016i–October 2017) of comprehensive oceanographic mooring measures from a site located 150 nmi offshore S...

The field study was conducted 1-2 November 2018 on the west coast of Isla de San Andrés, Colombia. Each day, two vessels transited to the study site and cooperatively deployed a 3-point optic-acoustic mooring in ~20 m depth, near the shelf break. There, using a novel setup, echosounder and stereo-camera data were collected simultaneously from the s...

The Gulf and Caribbean Fisheries Institute (GCFI) was founded in 1947 to promote the exchange of information on the use and management of marine resources in the Gulf of Mexico and Caribbean region. Fisheries management in this region is challenged by data-limited assessments and the need to improve survey monitoring programs. This is in part becau...

Echosounders may be calibrated by suspending a solid metal sphere in the acoustic beam and adjusting the system gain so that the measured target strength (TS) equals the theoretical value for the sphere. The suspension apparatus (i.e. net bag or loop of line glued into a hole, and control lines) may appreciably scatter sound, potentially affecting...

The environment influences the recruitment of small pelagic fishes, so environmental indices are used to match fishing mortalities to the stocks’ productivities. For example, the exploitation fraction for the northern stock of Pacific sardine in the Northeastern Pacific is a function of sea-surface temperature (SST). The functional relationship cha...

Length‐measurement conversions and seasonal mass–length relationships (MLR) for Pacific herring Clupea pallasii, northern anchovy Engraulis mordax, Pacific sardine Sardinops sagax, Pacific mackerel Scomber japonicus and jack mackerel Trachurus symmetricus in the California Current are presented. The conversions between total (LT), fork (LF,) and st...

Approximately 330 participants from 30 countries attended the 70th Gulf and Caribbean Fisheries Institute (GCFI) conference held in Merida, Mexico during 4-10 November 2017. The conference featured a symposium on Acoustic Technologies to Improve Reef Fish Ecosystem Surveys, which provided presentations on the various acoustic technologies used to m...

Large mobile predators take advantage of offshore pelagic environments that tend to be surveyed at a low temporal scale. We present results from a one-year observation of passive and active acoustics, as well as oceanographic measures, taken at a moored location 150 nmi offshore southern California in 4000 m deep pelagic waters. The passive acousti...

This paper presents measures of target strength (TS; dB re 1 m^2) and models of TS versus fork length (L; cm), i.e., TS=20log(L)+b_20, for skipjack tuna associated with Fish Aggregating Devices (FADs) in the Central Pacific Ocean. Measurements were made using 38-, 120-, and 200-kHz split-beam echosounders on a purse-seine workboat during fishing op...

Sound produced by fish spawning aggregations (FSAs) permits the use of passive acoustic methods to identify the timing and location of spawning. However, difficulties in relating sound levels to abundance have impeded the use of passive acoustics to conduct quantitative assessments of biomass. Here we show that models of measured fish sound product...

Supplementary Information T. J. Rowell et al. 2017. Estimating fish abundance at spawning aggregations from courtship sound levels. Scientific Reports 7: 3340 | DOI:10.1038/s41598-017-03383-8

The “northern” stock of Pacific Sardine Sardinops sagax is fished by Mexico, the USA, and Canada. Without an international management agreement, the U.S. Pacific Fishery Management Council prorates its target total fishing fraction (F) in its harvest control rule (HCR) by 0.87. This is the proportion of the stock that it deemed in 1998 to be presen...

Demersal marine organisms may be surveyed visually using cameras deployed on a submersible vehicle, e.g. a remotely operated vehicle (ROV). For estimating animal densities along visual transects, the sampled area may be calculated from the products of transect distance and width. In this study, distance was measured relative to submerged pipeline f...

Echosounder surveys of fish and zooplankton are typically conducted using fixed values of the echosounder transmit-pulse (“ping”) interval and data-logging range. The transmit-pulse interval must be long enough to sample to the range of the farthest (deepest) species under study, potentially causing undersampling of closer (shallower) species. We p...

The seabed can be classified using data from vertical, split-beam echosounders. This was demonstrated recently using a model parameterized with acoustic estimates of slope, roughness, normal-incidence backscattering strength, and variation of backscattering strength by frequency and incidence angle. These seabed classifications were interpreted and...

Analytical and numerical scattering models with accompanying digital representations are used increasingly to predict acoustic backscatter by fish and zooplankton in research and ecosystem monitoring applications. Ten such models were applied to targets with simple geometric shapes and parameterized (e.g., size and material properties) to represent...

With one small move across a street, NOAA Fisheries, our nation's steward of marine ecosystems, made a giant leap into the future of ocean science and technology development. The new Southwest Fisheries Science Center facility in La Jolla, California, houses dozens of interdisciplinary research and engineering groups and features a world-class Ocea...

The National Oceanic and Atmospheric Administration Marine Fisheries Service endeavors to manage fish stocks with an ecosystem perspective. This objective requires an understanding of the effects of the environment and fishing on all major ecosystem components. For example, in large upwelling systems like the California Current Ecosystem (CCE), nat...

Two stocks of Pacific Sardine Sardinops sagax migrate seasonally and synchronously along the west coasts of Mexico, the USA, and Canada. Landings from the two stocks are currently combined in U.S. assessments of the northern stock, but the stocks may be differentiated by their associated seawater habitats, which are predominantly characterized by d...

Cuttlefish Sepia esculenta are commercially important in Korea. Assessments of their biomass currently depend on fishery-landings data, which may be biased. Towards fishery-independent acoustic surveys of cuttlefish, target strength (TS) measurements at 70 and 120 kHz were made of 23 live cuttlefish, in early May 2010. The fish were caught by traps...

The multifrequency biplanar interferometric imaging technique (MBI) is applied to data from vertical, split-beam echosounders to produce sub-beam estimates of seabed surface-backscattering strength (Ss), incidence angle (θ), and roughness (${\rm {\cal R}}$). A simple model is used to quantify the variation of Ss versus θ = {2–20°} and acoustic freq...

To annually assess the northern stock of Pacific sardine (Sardinops sagax) in the California Current and set harvest quotas for the US fishery, managers have used an age-structured stock synthesis model fitted with results fromacoustic-trawl (ATM), daily-egg-production, and aerialphotogrammetric surveymethods, fishery landing and individual-length...

Zwolinski, J. P., and Demer, D. A. 2013. Measurements of natural mortality for Pacific sardine (Sardinops sagax). – ICES Journal of Marine Science, 70: . The northern stock of Pacific sardine (Sardinops sagax) in the California Current Ecosystem is periodically assessed to provide harvest guidelines for the United States and Canadian fisheries. The...

We confirm that sardine recruitment in the California Current, during the last three decades, mimics aspects of the environment in the North Pacific indicated by the Pacific Decadal Oscillation (PDO) index. The periods of stock increase and decrease followed consecutive years with positive and negative PDO values, respectively. During the "warm" pe...

Demer, D. A., and Zwolinski, J. P. 2014. Corroboration and refinement of a method for differentiating landings from two stocks of Pacific sardine (Sardinops sagax) in the California Current. – ICES Journal of Marine Science, 71: 328–335. Efforts to survey, assess and manage Pacific sardine (Sardinops sagax) in the California Current may depend on a...

Multifrequency biplanar interferometric imaging (MBI) is a contemporary acoustic-data processing technology designed to remotely image the seabed or objects in the ocean. MBI requires a computationally intense data conditioning algorithm. For example, it takes a serial central processing unit (CPU) 10 seconds to process each transmission for typica...

1] We sampled a front detected by SST gradient, ocean color imagery, and a Spray glider south of San Nicolas Island in the Southern California Bight between 14 and 18 October 2010. We sampled the front with an unusually extensive array of instrumentation, including the Continuous Underway Fish Egg Sampler (CUFES), the undulating In Situ Ichthyoplan...

The oceanographic conditions in the north Pacific have shifted to a colder period, Pacific sardine (Sardinops sagax) biomass has declined precipitously in the California Current, the international sardine fishery is collapsing, and mackerel (Trachurus symmetricus and Scomber japonicus) are thriving. This situation occurred in the mid-1900s, but ind...

The abundances and distributions of coastal pelagic fish species in the California Current Ecosystem from San Diego to southern Vancouver Island, were estimated from combined acoustic and trawl surveys conducted in the spring of 2006, 2008, and 2010. Pacific sardine (Sardinops sagax), jack mackerel (Trachurus symmetricus), and Pacific mackerel (Sco...

During the last century, the population of Pacif ic sardine (Sardinops sagax) in the California Current Ecosystem has exhibited large f luctuations in abundance and migration behavior. From approximately 1900 to 1940, the abundance of sardine reached 3.6 million metric tons and the "northern stock" migrated from offshore of California in the spring...

While over 100 fish families produce sounds during behaviors like spawning, aggression, and feeding, passive-acoustic sampling is not used commonly for long-term fish population monitoring. Fish sounds consist mostly of low-frequency pulses of variable duration, number, and repetition rate, but it is often difficult to identify their sources to spe...

We developed a point-and-click acoustic data viewer (FishViewer) for exploratory comparison of up to three acoustic survey transects (or three frequencies) at a time and other environmental and biological data (e.g., surface temperature and seabird abundance). FishViewer also contains image-processing tools (e.g., morphological and threshold filter...

Zwolinski, J. P., Emmett, R. L., and Demer, D. A. 2011. Predicting habitat to optimize sampling of Pacific sardine (Sardinops sagax). – ICES Journal of Marine Science, 68: 867–879. More than 40 years after the collapse of the fishery for Pacific sardine, a renewed fishery has emerged off the west coasts of the United States and Canada. The daily eg...

The bocaccio (Sebastes paucispinis) was a part of an important rockfish fishery to California anglers and commercial fishers until they were declared overfished by the Pacific Fisheries Management Council in 1999. Historically, rockfish stocks were estimated using ichthyoplankton sampling and catch per unit effort of the rockfish recreational fishe...

  Antarctic krill is a key species in the Antarctic food web, an important prey item for marine predators and a commercial fishery resource. Although single-beam echo-sounders are commonly used to survey the species, multi-beam echo-sounders may be more efficient because they sample a larger volume of water. However, multi-beam echo-sounders may mi...

Antarctic krill is a key species in the Antarctic food web, an important prey item for marine predators and a commercial fishery resource. Although single-beam echo-sounders are commonly used to survey the species, multi-beam echo-sounders may be more efficient because they sample a larger volume of water. However, multi-beam echo-sounders may miss...

Graham, T. R., Harvey, J. T., Benson, S. R., Renfree, J. S., and Demer, D. A. 2010. The acoustic identification and enumeration of scyphozoan jellyfish, prey for leatherback sea turtles (Dermochelys coriacea), off central California. – ICES Journal of Marine Science, 67: 1739–1748. Acoustic-sampling techniques were developed to estimate the abundan...

Cutter, G. R. Jr, Berger, L., and Demer, D. A. 2010. A comparison of bathymetry mapped with the Simrad ME70 multibeam echosounder operated in bathymetric and fisheries modes. – ICES Journal of Marine Science, 67: 1301–1309. The Simrad ME70 multibeam echosounder was designed for quantitative fisheries research and is currently installed on Ifremer's...

Abundance and distribution of Antarctic krill (Euphausia superba) in the nearshore waters north of Livingston Island, Antarctica, were characterized from six small-boat surveys conducted in late January or early February from 2000 to 2007. The first three surveys (2000, 2002, 2004) were conducted using a 120 kHz split-beam echosounder to measure wa...

Antarctic krill (Euphausia superba) aggregate in dense swarms. Previous investigations of krill swarms have used conventional single- or split-beam echosounders that, with post-processing, provide a two-dimensional (2-D) view of the water column, leaving the third dimension to be inferred. We used a multi-beam echosounder system (SM20, 200 kHz, Kon...

Polar regions contain some of the most productive (and in some cases unspoiled) ecosystems on the planet. Zooplankton, particularly krill, are the preferred prey for numerous fish, bird, and mammal species including animals that are endangered, commercially fished, or ecologically important. Acoustic echosounders provide an excellent method for ass...

The resolution of 3-D imaging is greatly improved using a new multifrequency biplanar interferometry (MBI) technique. Using data from a multifrequency-acoustic pulse-echo system, ranges to coherent targets, estimated from propagation delays, and the phase differences between echoes received with four quadrants of a split-aperture array are converte...

We compared measurements of integrated optical volume backscattering coefficients β(π) with integrated acoustic volume backscattering coefficients (s v ) and surface-trawl catches over a large-scale (roughly 300 km by 450 km) region, and a small-scale (roughly 50 km by 50 km) region off the coasts of Oregon and Washington. In both cases, the data w...

A recently‐developed statistical‐spectral approach to acoustic‐target identification (SSID) incorporates information contained in the frequency‐dependent signal amplitudes and their variances. In addition to identifying biological targets, the SSID has demonstrated utility for estimating fish aggregation densities, abundances, and behaviors, and de...

Acoustic backscattermeasurements from aggregations of Antarctic krill were collected over two field seasons (2006 and 2007) in the waters north of Livingston Island using two 5.5‐m inflatable vessels. One was equipped with a dual‐frequency (38‐ and 200‐kHz) single‐beam echosounder, while the other used a 200‐kHz multi‐beam system. In addition to in...

Acoustic classifications of fish and estimations of their orientation distributions are possible using multi‐frequency or broad bandwidth measurements of their acoustic target strengths (TSs) with knowledge of their scattering directivity pattern and size distributions. To measure TS of in situ fish, single‐frequency interferometric methods provide...

Demer, D. A., Cutter, G. R., Renfree, J. S., and Butler, J. L. 2009. A statistical-spectral method for echo classification. – ICES Journal of Marine Science, 66: 1081–1090. The frequency dependence of sound-scatter intensity is commonly exploited to classify fish, zooplankton, and the seabed observed in acoustic surveys. Although less utilized, tec...

Cutter, G. R., Renfree, J. S., Cox, M. J., Brierley, A. S., and Demer, D. A. 2009. Modelling three-dimensional directivity of sound scattering by Antarctic krill: progress towards biomass estimation using multibeam sonar. – ICES Journal of Marine Science, 66: 1245–1251. Target strength (TS) estimation is a principal source of uncertainty in acousti...

Renfree, J. S., Hayes, S. A., and Demer, D. A. 2009. Sound-scattering spectra of steelhead (Oncorhynchus mykiss), coho (O. kisutch), and Chinook (O. tshawytscha) salmonids. – ICES Journal of Marine Science, 66: 1091–1099. A recently developed method for measuring total target strength (TTS) allows calculation of the absolute scattered energy from f...

Demer, D. A., Kloser, R. J., MacLennan, D. N., and Ona, E. 2009. An introduction to the proceedings and a synthesis of the 2008 ICES Symposium on the Ecosystem Approach with Fisheries Acoustics and Complementary Technologies (SEAFACTS). – ICES Journal of Marine Science, 66: 961–965.

Širović, A., Cutter, G. R., Butler, J. L., and Demer, D. A. 2009. Rockfish sounds and their potential use for population monitoring in the Southern California Bight. – ICES Journal of Marine Science, 66: 981–990. Non-lethal methods are being developed to assess and monitor the depleted rockfish stocks off southern California. For example, data from...

Sound production by many fish species has been studied extensively, but little is known about sound production by rockfishes (genus Sebastes), and only a few species have been reported to be soniferous. Passive acoustic recordings were made during 2007 and 2008 at Hubbs‐SeaWorld Research Institute and Southwest Fisheries Science Center tanks contai...

A multibeam echosounder (MBE) was deployed on an inflatable boat (length = 5.5 m) to observe swarms of Antarctic krill Euphausia superba in the nearshore environment off Livingston Island, South Shetland Islands, Antarctica. Visual observations of air-breathing predators, including penguins and fur seals, were made from the boat at the same time. M...

We conducted two ship-based surveys of the nearshore ecosystem north of Livingston Island, Antarctica during 2–10 February 2005. Between the two surveys, a low-pressure system (963mbar) passed through the area providing the opportunity to measure ecosystem parameters before and after a near gale. A ship-based multiple-frequency acoustic-backscatter...

Demer, D. A., and Renfree, J. S. 2008. Variations in echosounder–transducer performance with water temperature. – ICES Journal of Marine Science, 65: 1021–1035. Electro-acoustic transducers are central components of multifrequency echosounders used in remote-target identification and acoustic surveys for fish and zooplankton. Appreciable changes in...

The time-series of acoustically surveyed Antarctic krill (Euphausia superba) biomass near the South Shetland Islands (SSI) between 1996 and 2006 is re-estimated using a validated physics-based model of target strength (TS), and a species-discrimination algorithm based on the length-range of krill in plankton samples to identify krill acoustically,...

Cutter, G. R. Jr and Demer, D. A. 2007. Accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys. – ICES Journal of Marine Science, 64. Multibeam echosounders can improve the efficiency and the precision of acoustic-survey estimates by providing greater sampling volumes than single-beam echosounders. For a multibeam...

Electroacoustic transducers are central components of multi‐frequency echosounders used in remote target identification and acoustical surveys for fish and zooplankton. While these transducers, constructed from multiple materials such as piezoelectricceramics, polyurethanes, and metals, are designed to be insensitive to performance changes versus t...

Fish dispersion and abundance are commonly estimated from volume backscattering coefficients (sv) of animals located directly beneath the survey vessel, inside a narrow transducer beamwidth. Fish density (♯/m2) is estimated from a ratio of integrated sv and a representative backscattering cross‐sectional area. Fish biomass is estimated from the pro...

Life-history theory predicts that adults of long-lived species such as seabirds should optimally balance investment in current and future offspring. However, when trying to optimize investment in offspring provisioning, the most energetically costly component of seabird parental care, adults need to contend with large interannual fluctuations in pr...

ffiffiffi 2 p =2 radians, and predictions were validated with empirical TS and total TS data at 120 kHz, and over a broad bandwidth, respectively. For general application, parameteriza- tion of the SDWBA model is improved to account explicitly for dependence among four of the model parameters: standard length of krill, number of cylinders used to d...

A challenge for the aquaculture community has long been the development of harmless techniques for monitoring fish in a tank. Acoustic telemetry has been used to monitor fish swimming behavior, and passive acoustics have been used to monitor fish feeding, but new techniques are needed to monitor non-invasively their numbers and growth rates. Recent...

Upward-looking acoustic Doppler current profilers (ADCPs) (300 kHz) and echosounders (125 kHz) were deployed on moorings at South Georgia to measure abundance of Antarctic krill continuously over several months. Echoes from krill were identified using the theoretical difference in echo intensity at 300 and 125 kHz and scaled to krill density using...

Recently, an acoustic technique has been proposed to measure the scattering strength and the dynamics of weak moving scatterers in a reverberant cavity: diffusing reverberant acoustic wave spectroscopy (DRAWS). Both parameters are obtained from the correlations of the reverberated-scattered transient pressure fields for different scatterers positio...

Refinements have been made to the multiple-frequency method for rejecting overlapping echoes when making target-strength measurements with split-beam echosounders described in Demer et al. (1999). The technique requires that echoes, simultaneously detected with two or more adjacent split-beam transducers of different frequencies, pass multiple-targ...

Acoustic surveys of the distribution and abundance of Antarctic krill were conducted by two different vessels in February 2005 in the near-shore waters of Livingston Island, Antarctica. Multiple frequency acoustic backscatter, video observations, and net tow data were used to identify the acoustic scatterers and to estimate the abundance of krill....

Sound scattering and absorption by Northern krill (Meganyctiphanes norvegica) were measured over the acoustic bandwidth of 30–210 kHz and compared with similar scattering measurements for Antarctic krill (Euphausia superba). The measurements of total target strength (TTS; energy scattered in all directions, averaged over all angles of incidence) ma...

Annual surveys of the distribution and abundance of Antarctic krill are conducted by the United States Antarctic Marine Living Resources Program in order to assist fishery managers in conserving this economic and ecologic resource. These surveys utilize a large vessel which does not sample close (within 10 km) to the South Shetland Islands. These i...

Multifrequency backscattering measurements are more and more frequently used to estimate the populations of scatterers from echosounders data, and to separate the echoes from different species. Such analysis is based on the projection of the backscattered spectral data on the theoretical scattering spectra of the scatterers. Beside the accuracy of...

Antarctic krill, Euphausia superba, comprises the foundation of the foodweb in the Southern Ocean and is the target of a large fishery. Recently, the total abundance of krill in the Scotia Sea was estimated from an international echosounder and net survey (CCAMLR 2000) to be 44.3 million metric tonnes (Mt; CV 11.4%) (Hewitt et al., 2002). The new b...

The distributions of temperature, salinity, chlorophyll and zooplankton were measured in the Ligurian Sea, north of Corsica, in August 1999. To characterize the physical environment, hydrographic and fluorometric profites were collected. A net and two acoustic systems were used to measure the distribution of small (<5 mm) and large (>5 mm) zooplank...

In recent publications, it has been demonstrated that the total scattering cross section of fish moving in a tank can be estimated from ensembles of reverberation time series. However, the reproducibility of these measurements is influenced by parameters such as the motion or the behavior of the fish. In this work, we propose to observe acousticall...

It has been demonstrated that the total scattering cross section of a moving scatterer in a reverberant media can be estimated from ensembles of reverberation time series. In order to obtain an accurate estimate of the total scattering cross section, the positions of the scatterer must be uncorrelated between consecutive time series. Here we show t...

Vessels from Japan, Peru, and the USA conducted four sequential surveys designed to estimate the biomass density and demography ofAntarctic krill in the vicinity ofthe South Shetland Islands between late December 1999 and early March 2000. The surveys were conducted during the same austral summer as the CCAMLR 2000 Survey in the Scotia Sea (Watkins...

In January and February 2000, a collaborative survey designed to assess the biomass of Antarctic krill across the Scotia Sea was conducted aboard research vessels from Japan, Russia, the UK and the USA using active acoustic and net sampling. Survey design, sampling protocols, and data analysis procedures are described. Mean krill density across the...

Combined sampling and measurement error is estimated for the CCAMLR 2000 acoustic estimate of krill abundance in the Scotia Sea. First, some potential sources of uncertainty in generic echo-integration surveys are reviewed. Then, specific to the CCAMLR 2000 survey, some of the primary sources of measurement error is explored. The error in system ca...

In January and February 2000, four ships conducted an extensive hydrographic survey of the Scotia Sea as part of the CCAMLR 2000 Survey. There were 169 CTD stations to at least 1000 m depth, making this the largest synoptic dataset since 1981. A hydrographic section at Drake Passage was used to define water masses and ocean fronts. In 2000, the Sub...

During January and February 2000 four research vessels,f rom Russia,t he UK, Japan, and the United States, conducted an oceanographic survey with 137 hydrographic stations within the Scotia Sea and adjoining waters as part of a survey sponsored by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to estimate the biom...

Presented here are absolute measurements of the acoustic intensity scattered and absorbed by humans. The total scattering and absorption cross sections, σT and σa, were obtained for individual humans walking randomly in a room, using long-duration acoustic reverberation. Within the audible range, the sound scattering spectra of the human body is si...