Mean Volume Backscattering Research Articles

Verification of the acoustic techniques used to identify Antarctic krill

Acoustic surveys to estimate krill biomass require that the sound backscattered by krill can be identified and distinguished from all other types of backscatter. Sampling acoustic targets with nets to verify their identity have achieved this traditionally. More recently backscattered sound has been partitioned into krill or non-krill groups using differences in mean volume backscattering strength (MVBS) at two acoustic frequencies (ΔMVBS=MVBS120−MVBS38; where ΔMVBS between 2 and 12 dB indicated krill). Here we compare net and acoustic data from two cruises around South Georgia in 1996 to assess the reliability of acoustic-based, target-identification techniques. MVBS data at 120 and 38 kHz were collected with a Simrad EK500 echosounder and net samples were collected with an RMT8. Around 80% of the echo integration cells from targets believed to be krill on the basis of their appearance on echo-charts, were also identified as krill from their difference in backscatter at 38 and 120 kHz. Krill biomass estimated from acoustic targets identified using echo-chart appearance or ΔMVBS were broadly similar (regression: ΔMVBS=0.94 visual classification, r2=0.99). Krill size was calculated from scattering models using the two frequency data and compared with that obtained in net hauls. This comparison revealed that a simplified bent-cylinder model was a better predictor of krill length than a fluid-filled, sphere model. We conclude that use of ΔMVBS to identify Antarctic krill is advantageous because it is more objective than using echo-chart appearance.

1997 년 춘계의 동중국해 및 황해에 대한 어군의 분포특성

황해, 동중국 해역에서 서식하는 어업생물 자원의 평가 및 그 관리에 필요한 기초 자료를 수집하기 위한 시도로서, 1997 년도에 부경대학교와 일본 장기대학이 공동으로 실시한 어업자원조사중에서 주로 계량어군탐지기를 이용하여 수집한 자료를 분석, 고찰한 결과를 요약하면 다음과 같다. 1 황해, 동중국해에 설정한 9개의 조사정점에서 저층트롤에 의한 어획량은 7.7~182.48 kg/hour이었고, 그때 어획된 어종수는 7~34 종이었다. 2. 1997년 5월에 동중국해에 있어서의 어군의 분포는 황해 중부의 저층 냉수역과 동중국해 중앙해역의 저층 고온수역, 또 쿠로시오계의 난류와 황해 냉수 및 대륙 연안수의 사이에 형성된 제주도 남부의 전선역에서 그 어군도가 매우 높았다. 3. 황해, 동중국해의 조사대상해역의 전수층부와 저층부에 대한 어군의 평균 체적산란강도는 각각 -74.57 ㏈, -68.24 ㏈로서 저층부가 전수층부에 비해 5.67 ㏈ 더 높았다. 4. 본 연구의 조사대상해역에 대한 어군의 분포밀도를 추정한 결과, 전수층에 대해서는 6.65<TEX>$\times$</TEX><TEX>$10^{-5}$</TEX> kg/㎥이었고, 저층부에 있어서는 2.86<TEX>$\times$</TEX><TEX>$10^{-4}$</TEX> kg/㎥이었다. The combined hydroacoustic and demersal trawl surveys to investigate the distribution characteristics of fish schools in the Yellow Sea and the East China Sea were carried out during the spring of 1997 by the training ship "Nagasaki Maru" of Nagasaki University. Fish samples were collected by bottom trawling from 9 trawl stations randomly selected in the survey area, and the species and length compositions of trawl catches are examined. Hydroacoustic data were collected by using a Furuno FQ-50 scientific echo sounder operating at 50KHz and the data stored in field were later processed in the laboratory. The results obtained can be summarized as follows ：1. In the 9 trawl surveys conducted in the Yellow Sea and the East China Sea, 78 species including 80 species of fishes, 4 species of Cephalopoda and 6 species of Crustacea, were identified. The most abundant species in these stations were swimming crab(Portunus trituberculatus), Japanese horse mackere(Trachurus japonicus), redlip croaker(Larimichthys polyactis) and the catch per one hour in each station ranged 7.7 to 182.5 kg/hour. 2. The mean volume backscattering strength for the entire water column and the bottom layer of the 0-10 m from bottom friction were -74.6 ㏈ and -68.2 ㏈, respectively. That is, the mean volume backscattering strength for the bottom layer was 6.4 ㏈ higher than that for the entire water column 3. In the surveys during the spring of 1997, the geographical distribution characteristics of fish schools suggests a trend of decreasing fish abundance toward the coast area of Korea and the highest demersal concentrations appeared in waters between the Cheju Island and the Tsushima Island 4. The distribution density of fish school in the East China Sea and Yellow Sea during the 1997 acoustic survey were estimated to be 6.65<TEX>$\times$</TEX>10<TEX>$^{-5}$</TEX> kg/㎥ in the entire water column and 2.86<TEX>$\times$</TEX>l0<TEX>$^{-4}$</TEX> kg/㎥ in the bottom layer, respectively.pectively.

Effective and accurate use of difference in mean volume backscattering strength to identify fish and plankton

Acoustic species identification is very important for fisheries' operations and surveys. One of the most promising methods for identification is to utilize the difference of mean volume backscattering strengths (ΔMVBS) among frequencies. Improvement of this technique is the aim of this study. The ΔMVBS must be obtained for a common observation range among frequencies so that the difference can be attributed solely to frequency characteristics of the sound scattering of targets organisms. We derived the common observation range of at least up to 150 m for our quantitative echosounder operating at 38 and 120 kHz with the same beam widths of 8.5°. We related ΔMVBS data obtained off northeastern Japan to specific marine organisms combined with the swimming depth and water temperature information. The echoes with small ΔMVBS (−1 dB<ΔMVBS<4 dB) were attributed to the walleye pollock ( Theragra chalcogramma ) and the echoes with large ΔMVBS (>10 dB) to krill ( Euphausia pacifica ). The changing pattern of ΔMVBS suggested complicated behaviour between species such as predator and prey interaction. In order to obtain reliable and detailed information the integration cell should be small and the ΔMVBS should be displayed as an echogram in an absolute colour scale.

Distribution of zooplankton inferred from hydroacoustic backscatter data in coastal waters off Ningaloo Reef, Western Australia

Hydroacoustic surveys were used to examine zooplankton distributions in coastal waters off Ningaloo Reef, Western Australia. Surveys were timed to coincide with the seasonal aggregation of whale sharks, Rhincodon typus, and other large zooplanktivores in these waters. The surveys examined scattering features of lagoon/shelf fronts, a series of cross-shelf transects and waters surrounding whale sharks swimming at the surface. These suggested that lagoon waters flow intrusively into shelf waters at reef passages in a layered exchange. Cross-shelf transects identified three vertical scattering layers: a surface bubble layer; a near-surface minimum layer; and a bottom maximum layer. Regions of intense mixing of lagoon and shelf waters were detected seaward and to the north of reef passages. Integrated acoustic mean volume backscatter of the bottom maximum layer increased with depth and distance offshore. Large subsurface aggregations of unidentified fauna were detected beneath whale sharks in the same area that manta rays and surface schools of euphausiids were also observed.

Time variability of acoustic backscatter from zooplankton in the Ibiza Channel (western Mediterranean)

One year of mean volume backscattering strength (MVBS) data obtained from an acoustic Doppler current profiler (ADCP) moored at 240 m in the Ibiza Channel is analysed. It shows the prominent effect of light on the zooplankton distribution in the surface layer. Two communities of zooplankton can be differentiated, both of which perform diel vertical migrations (DVM). They initiate descent at dawn, reach a maximum depth at noon, and finish ascent at dusk. Fast “primary” migrators, which quite homogeneously filled the surface layer at night, generally descended below the ADCP during day-time, whereas slower “secondary” migrators always remained above the instrument. The vertical motion of primary zooplankton at dawn and dusk was strong enough to be directly measured by the ADCP (±3– 4 cm s −1 ). The vertical velocity of secondary zooplankton inferred from the slope of MVBS contours did not exceed a few tenths of cm s −1 . Variations in DVM show a great dependence on light intensity: (1) The depth attained by zooplankton at noon was modulated by the annual cycle of sunlight and was maximum/minimum around the summer/winter solstice. (2) Some primary zooplankton remained above the ADCP during day-time on days of heavy cloud cover in winter. (3) The preferential depth of zooplankton at night followed the cycle of moonlight (29.5 days). Primary zooplankton were present all year at the mooring, whereas seasonal variations are seen in the abundance of secondary zooplankton. Maximum abundance is found in summer with enhanced concentration in the thermocline (upper 60 m), and minimum abundance in early winter after the erosion of the thermocline. A bloom is also observed in late winter. These observations are discussed in the context of past zooplankton surveys in the area. Fast primary migrators could consist of euphausiids (alternatively, micronekton, e.g. myctophids). A fraction of slow secondary migrators very likely are copepods.

Acoustic backscatter observations of zooplankton abundance and behaviour and the influence of oceanic fronts in the northeast Atlantic

Acoustic Doppler Current Profiler-derived acoustic backscatter data were compared with physical and biological data collected during leg 2 of the PRIME cruise in the northeastern Atlantic during July 1996. The leg 2 transect from 59 to 37°N ran approximately along the 20°W meridian and crossed a range of hydrographic conditions from near subarctic in the north to subtropical in the south. A front at 52.5°N was an extension of a southern branch of the North Atlantic Current system. A stronger frontal feature was observed at 48°N and was considered to be the northern limit of Eastern North Atlantic Water that had been ventilated at the eastern boundary. At 37°N a Lagrangian time-series study was performed. High acoustic backscatter was observed within the PRIME eddy at 59°N and at the southern site. Other changes in backscatter intensity, or more correctly, in ‘mean volume backscattering strength’ (MVBS), coincided with two main fronts crossed during the transect and upon arrival at the southern site. An increased MVBS in the surface 100 m in the north was not present south of 52.5°N. Between 48 and 37°N the MVBS approached its lowest of the transect. At 37°N marked changes in the backscatter were observed, with a strong signal seen in the top 75 m. As well as changes in the MVBS there were differences in the diel migratory patterns observed in the backscatter. Scattering at 37°N was concentrated above a strong pycnocline at 75 m. In the north the scattering signal was generally noisier and less well defined than in the south. Biomass has been estimated from the MVBS by comparing it with biomass estimates derived from Optical Plankton Counter data and carbon/nitrogen measurements. Although such calibrations have their limitations, the results give important information concerning net sampling strategies and are a good test of the representativeness of such sampling.

Digital echo visualization and information system (DEVIS) for processing spatially-explicit fisheries acoustic data

Spatially explicit analysis of fisheries acoustic data preserves heterogeneity observed in spatial distributions of fish. A software system — Digital Echo Visualization and Information System (DEVIS) — has been developed to process digital underwater acoustic data for spatially-explicit fisheries acoustic research. This system can be used to obtain spatial and temporal distributions of fish density, fish abundance, and fish lengths for management applications and for ecological modeling. DEVIS first reads digital data, corrects the data according to the sonar equation, discriminates individual targets, and vertically and horizontally integrates the data into a two-dimensional array of mean volume backscattering strength. Individual target information (TS, spatial location) is meshed with the volume backscattering array, and representative acoustic sizes are estimated in array cells with missing target information. Estimation methods for acoustic sizes and potential biases in abundance estimates are introduced and discussed. The final output is the spatial distribution of numeric density and fish length by length classes and for all fish. Data obtained on Lake Erie in September 1994 and on Chesapeake Bay in July 1995 were processed using DEVIS and are shown graphically. Steps required to process digital data are described and how these data can be applied to fish ecology is shown.

Acoustic discrimination of Southern Ocean zooplankton

Acoustic surveys in the vicinity of the sub-Antarctic island of South Georgia during a period of exceptionally calm weather revealed the existence of a number of horizontally extensive yet vertically discrete scattering layers in the upper 250 m of the water column. These layers were fished with a Longhurst-Hardy plankton recorder (LHPR) and a multiple-opening 8 m 2 rectangular mid-water trawl (RMT8). Analysis of catches suggested that each scattering layer was composed predominantly of a single species (biovolume>95%) of either the euphausiids Euphausia frigida or Thysanöessa macrura, the hyperiid amphipod Themisto gaudichaudii, or the eucalaniid copepod Rhincalanus gigas. Instrumentation on the nets allowed their trajectories to be reconstructed precisely, and thus catch data to be related directly to the corresponding acoustic signals. Discriminant function analysis of differences between mean volume backscattering strength at 38, 120 and 200 kHz separated echoes originating from each of the dominant scattering layers, and other signals identified as originating from Antarctic krill ( Euphausia superba), with an overall correct classification rate of 77%. Using echo intensity data alone, gathered using hardware commonly employed for fishery acoustics, it is therefore possible to discriminate in situ between several zooplanktonic taxa, taxa which in some instances exhibit similar gross morphological characteristics and have overlapping length– frequency distributions. Acoustic signals from the mysid Antarctomysis maxima could also be discriminated once information on target distribution was considered, highlighting the value of incorporating multiple descriptors of echo characteristics into signal identification procedures. The ability to discriminate acoustically between zooplankton taxa could be applied to provide improved acoustic estimates of species abundance, and to enhance field studies of zooplankton ecology, distribution and species interactions.

The use of two freqencies to interpret acoustic scattering layers

The use of multiple frequencies during acoustic stock assessment surveys allow methods to be used, which can discriminate between different species groups based on their scattering properties and assist in target species identification, thus improving estimates of biomass. Research, using multiple frequencies to study zooplankton, have used the difference in backscatter between several frequencies to identify the species or species groups comprising the scattering layers. This paper presents the results of using a dual-frequency differencing algorithm on 120- and 38-kHz mean volume backscatter strength (MVBS) data collected during a 1995 United States west coast acoustic survey of Pacific hake (Merluccius productus) as a means to acoustically separate hake from zooplankton. Scatterers were sampled using various nets with mouth areas ranging from 5 to 1000 m2. Backscatter data from both frequencies were corrected for background noise. The effect of species composition, time of day, depth, and density of scatterers on the (delta)MVBS data were evaluated. Results indicate that the dual-frequency differencing algorithm shows a marked difference between fish and zooplankton (delta)MVBS and can be used to distinguish between fish and zooplankton scattering layers.

Variability in biological distributions and hydrography from concurrent Acoustic Doppler Current Profiler and SeaSoar surveys

Concurrent Acoustic Doppler Current Profiler (ADCP)/SeaSoar surveys can provide a range of quantitative and qualitative biological data at the same time and space scales as hydrographic data. The data resolve mesoscale variability for both biology and physics and can be displayed in real time – thereby allowing targeting of conventional sampling programmes. Examples from such surveys in a range of oceanic conditions and from different oceans demonstrate the complex patterns of biological distributions, and thereby acoustic variability, which change in space and time in response to both biological and physical forcing. Direct comparison of data from different areas requires absolute acoustic data. A method for converting relative ADCP backscatter to mean volume backscattering strength that includes using concurrent temperature and salinity data from SeaSoar is described; the resultant data sets can be directly compared in time and space.

Comparison of acoustic backscatter measurements from a ship-mounted Acoustic Doppler Current Profiler and an EK500 scientific echo-sounder

We compare the calibration of a 150 kHz RD Instruments Acoustic Doppler Current Profiler against the 200 kHz channel of a SIMRAD EK500 scientific echo-sounder. Over a range of mean volume backscattering strength of −88 to −68 dB (relative to a scattering cross-section of 1 m−1), from 10 to 90 m depth, the data from the two instruments were well correlated but with significant slope and offset errors (≈10% and ±3 dB, respectively). After correction for these systematic errors the residual differences between the ADCP and the EK500 were less than 1 dB. This accuracy is sufficient for many qualitative studies of the coupling of ocean physics and biology.

Acoustic targets at South Georgia and the South Orkney Islands during a season of krill scarcity

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 138:51-61 (1996) - doi:10.3354/meps138051 Acoustic targets at South Georgia and the South Orkney Islands during a season of krill scarcity Brierley AS, Watkins JL Dual frequency (38 and 120 kHz) acoustic surveys of shelf-break regions in the vicinity of the Willis Islands, South Georgia, and Coronation Island, South Orkneys, both in the Atlantic sector of the Southern Ocean, were carried out from RRS 'James Clark Ross' during January 1994. The difference in echo signal strength between the 2 frequencies [deltaMVBS (mean volume backscattering strength) = MVBS 120 kHz - MVBS 38 kHz] was used to partition acoustic targets into 3 biological categories: Antarctic krill Euphausia superba, nekton (larger than krill) and zooplankton (smaller than krill). Krill density estimates were derived for both survey areas, and the relative contribution of each class of target to overall regional pelagic biomass determined. Krill distribution in both regions was extremely patchy, resulting in a highly skewed frequency distribution of density; for example one echo-integration interval contained 104 times the mean krill density. An approximate 6-fold difference in mean krill abundance was detected between the 2 areas but, in comparison with previously published density estimates, krill densities in both regions were extremely low (1.7 and 10.7 gm-2 for Willis and Coronation Islands respectively). Gross regional differences in pelagic faunal composition were apparent from inspection of echo charts, and were quantified by detailed analysis of acoustic signals. The water column in the vicinity of the Willis Islands was characterised by a series of diffuse but continuous scattering layers visible only at 120 kHz, and 73% by mass of acoustic targets in this region were classified as zooplankton. In marked contrast, the Coronation Island region was dominated by dense, discrete acoustic target patches which were classified as larger, nektonic, sound scatterers (fish or squid), and only 12% of targets there were classified as zooplankton. Elsewhere, data from predator diet analyses carried out at Bird Island, South Georgia, during the same season that our acoustic measurements were made, confirm the profoundly low levels of krill availability there and support the acoustic observation that the Willis Islands region contained large numbers of small zooplankton such as the amphipod Themisto gaudichaudii. Similarly, available data on analyses of stomach contents of Chinstrap penguins nesting in a neighbouring region within an oceanographic setting similar to that of Coronation Island reveal the presence of more fish in the diet than usual. Conclusions drawn from acoustic observations as to the relative regional composition of pelagic biomass are therefore clearly supported by independent evidence gathered from predators, and the value of integrating predator/prey and acoustic studies to characterise the content of localised marine ecosystems is demonstrated. Antarctic krill . Euphausia superba . Dual frequency acoustic survey . Predators . Penguin diet . Themisto gaudichaudii . Target discrimination . Southern Ocean Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 138. Publication date: July 25, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

A biological acoustic survey in the marginal ice-edge zone of the Bellingshausen Sea

An acoustic survey at 38 kHz was carried out from R.R.S. Discovery in the Bellingshausen Sea from 23 November to 7 December 1992 as part of the U.K. Joint Global Ocean Flux Study (JGOFS) Southern Ocean investigations [Turner D. and N. J. P. Owens (1995) Deep-Sea Research II, 42, 907–932]. A total of 285 targets were identified and described from the chart record of the echo sounder. Mean volume backscattering strength data were collected using an echo integration system. These data are used to describe the spatial and temporal variability of krill ( Euphausia superba) distribution and biomass in the marginal ice-edge zone. Krill biomass density varied from a mean of 42 g m −2 during the first survey phase to 20 g m −2 on the second survey phase. The number of small swarms detected during the second phase was greater than during the first phase. On this first survey a large swarm (2.8 km in extent) had a potential biomass of 3.7 × 10 4 tonnes. Some calculations are presented to show the potential impact of krill on the flux of carbon in the area of the survey.

ADCP measurements of the distribution and abundance of euphausiids near the Antarctic Peninsula in winter

Euphausiid populations were observed in Gerlache Strait, Antarctic Peninsula region, during austral winter 1992 (12 July–14 Aug) using a hull-mounted 153-kHz acoustic Doppler current profiler and a multiple opening and closing net and environmental sensing system (MOCNESS). Theoretical estimates of target strength based on a straight cylinder model and applied to the numbers and sizes of euphausiids in net samples yielded estimates of mean volume backscattering strength (MVBS) that compared favorably with observed values at low abundances. However, net samples underestimated biomass by almost two orders of magnitude at high abundances, a feature we attribute to net avoidance. Other taxonomic groups of macrozooplankton did not appear to contribute significantly to acoustic backscatter. Distributions of euphausiids were similar to those observed in summer; we found no evidence for abundant aggregations immediately below surface sea-ice. Euphausia superba dominated the biomass at all stations; individuals of approximately 22 mm were most abundant at 30–70 m depth, whereas 40-mm individuals were most abundant at 90–130 m. Thysanoessa macrura was positioned deeper in the water column, from 150 to 250 m, and Euphausia crystallorophias was centered on 100 m. The first two species were distributed throughout the study area, but E. crystallorophias was concentrated in coastal embayments. Total wet weight biomass ranged from 0.0009 to 9 g m −3, dry weight ranged from 0.002 to 18 g m −3, and protein ranged from 0.0009 to 9 g m −3, mostly concentrated in the 30–70 m depth stratum. Over the upper 200 m of the water column integrated wet weight biomass estimated from ADCP data was in the range from 29 g m −2 to 92 kg m −2.

Exploration of geostatistical methods for mapping and estimating acoustic biomass of pelagic fish in the Gulf of St. Lawrence: size of echo-integration unit and auxiliary environmental variables

Daytime 38 kHz mean volume backscattering strength (MVBS) of small pelagic fish in the Gulf of St. Lawrence was recorded at 60-m intervals along a regular network of 11 parallel transects in August 1991. Profiles of temperature, salinity, density and transmittance were collected on a grid of 44 punctual stations along the transects. The MVBS was contagiously distributed and autocorrelated for al1 transects. No significant monotone relationship with any of the environmental variables was evidenced, except that the highest concentrations of fish were located over shallow bottoms. The anisotropic variogram of MVBS indicated three different scales of spatial structures: an unresolved small-scale variability, which accounted for only 7.7% of variance, and two structure components of along-transect scales of ~500 m (~40%) and 15 km (~50%). The theoretical variograms deduced for larger echo-integration units (EIUs) than the recorded 60-m unit, provided a good fit to the corresponding experimental variograms up to EIU of 1920-m, when the disturbing effect of outliers on the variograms was removed. Global estimates and maps produced with 240-m and 1920-m EIU data were almost identical, showing that no significant loss of information was caused by increasing the EIU. Asymmetric confidence intervals of the global estimates are obtained using two variograms, one controlled for the effect of the outliers, the other one not controlled. In an effort to take the relation of MVBS with the bottom depth into account, four different kriging methods were compared: ordinary kriging, kriging with the bottom depth or its indicator-transform as external drift, and 3-D ordinary kriging with depth as the third dimension. Because of the weak relation of MVBS with the bottom depth, ordinary kriging had the best performance. Results are discussed in the context of applications of geostatistics to estimation and mapping of acoustic survey data.

Interpretation of acoustic data at two frequencies to discriminate between Antarctic krill (Euphausia superba Dana) and other scatterers

A visual classification was carried out of echo trace characteristics derived from 113 integration intervals of 1 n mi (nautical mile) each taken from the Southern Ocean in the area of South Georgia. This separated the echo traces into five categories: three biotic and two abiotic in origin. The acoustic data were subsequently analysed in detail by comparing the acoustic parameter Mean Volume Backscattering Strength (MVBS) at 120 kHz with that at 38 kHz. The three biotic categories separated visually had significantly different mean AMVBS values (δMVBS = MVBS 120 kHz - MVBS 38 kHz). To investigate if the biotic categories could be identified solely on the basis of δMVBS values, maximum-likelihood techniques were used to analyse the multimodal distribution of a further 1286 δMVBS records. These analyses allowed ranges to be defined for AMVBS which separate three biotic categories. On the basis of the target strength properties of the most likely scatterers in the region, the three categories were identified as krill, organisms larger than krill (fish or squid) and organisms smaller than krill (small zooplankton). The acoustic techniques described in this paper indicate a useful methodology in the study of Southern Ocean pelagic organisms.

Differences in backscattering strength determined at 120 and 38 kHz for three species of Antarctic macroplankton

The ability to acoustically separate zooplankton species is an important requirement for ecological studies and to improve biomass estimates. In order to distinguish between Euphausia superba and other swarm-forming macroplankters we used a dual frequency echo-sounder (120 and 38 kHz) and echo-integrator during a series of Longhurst Hardy Plankton Recorder (LHPR) hauls near South Georgia. We compared the acoustic parameter Mean Volume Backscattering Strength (MVBS) according to the equation: DELTAMVBS (dB) = MVBS 120 kHz - MVBS 38 kHz. Mean values of DELTAMVBS for E. superba, Themisto gaudichaudii and E. frigida were 4.6, 9.7 and 15.6 dB, respectively, and were significantly different, allowing the 3 species to be separated acoustically.

Contribution to Antarctic krill acoustic studies ? Results of the NO Marion-Dufresne MD25/FIBEX cruise

Antarctic biomass acoustic surveys generally yield biomass estimations expressed in terms of krill biomass, on basis of its being the major component of the Antarctic ecosystem. The influence of other scatterers is often left aside, thus introducing a bias in the estimations. The correlation between krill density distribution, as assessed by trawls, and Mean Volume Backscattering Strength frequency distribution in the range (-80, -60) dB(m−1·sr−1), allows to propose a method to reduce the bias. This method could be used to study the influence of each kind of scatterers on the recorded acoustic values, thus leading to better estimations.