Acoustic Volume Backscattering Strength Research Articles

Behaviours of Atlantic herring and mackerel in a purse-seine net, observed using multibeam sonar

To ensure efficient and sustainable purse-seine fisheries, the catch process must be monitored to better understand the reactions of fish to the gear. In this study, we monitored the behaviours of herring (Clupea harengus) and mackerel (Scomber scombrus) schools during purse-seine capture using a multibeam imaging sonar (Simrad MS70, 75–112 kHz) mounted on a research vessel. The fish behaviours differed between species and purse-seine sets. For both species, the acoustic volume backscattering coefficient increased as 0–80% of the seine was hauled aboard, indicating a corresponding increase in fish spatial density. This increase was significantly greater for herring than mackerel. As 0–40% of the seine was hauled aboard the fishing vessel, schools changed their spatial distribution and volume independent of seine hauling, while for some schools, depth and height decreased. The acoustic volume backscattering strength was up to 25 dB higher in the centre of the school than in the edges. The average lateral target strength was estimated for individual fish in the captured herring schools, and the effect of incident angle on the backscattering strength is considered.

DIEL CHANGE IN ACOUSTIC CHARACTERISTICS AND ZOOPLANKTON COMPOSITION OF THE SOUND SCATTERING LAYER IN I-LAN BAY IN NORTHEASTERN TAIWAN

A 36-h acoustic observation of the I-Lan Bay was conducted on board the Ocean Research II from June 2 to 4, 2013. The acoustic volume backscattering strength (SV) was measured using a scientific echo sounder (EK500, 38 and 120 kHz) to determine the acoustic characteristics of the deep scattering layers (DSLs) before and after the vertical movement. Two DSLs were found during the daytime at a depth of 80-130 m (upper DSL, UD) and 130-230 m (lower DSL, LD), with mean SVs of approximately -78 dB (UD) and -77 dB (LD) at 38 kHz and -65 dB (UD) and −60 dB (LD, 130-200 m) at 120 kHz. Two clear diel vertical movements (DVMs) were detected for both DSLs, which started to ascend at dusk at a speed of 1.04 cm/s (UD) and 1.92 cm/s (LD) and remained at 10-80 m at nighttime. These 2 DSLs started to descend at dawn with a speed of 0.59 cm/s (UD) and 1.38 cm/s (LD) and then remained at depths of 80-130 m (UD) and 130-230 m (LD) during the daytime. The habited depth of both DSLs may influenced by the change of water temperature. The dominant species of DSLs in abundance and biomass were copepods and decapods, respectively. The vertical nighttime distribution of the dominant scatterer of smaller and larger decapods was discussed in this paper.

Abundance and spatial distribution of Mysis diluviana in Lake Ontario in 2008 estimated with 120 kHz hydroacoustic surveys and net tows

Mysids are an important component of Great Lakes foodwebs, both as a prey for fish and as a predator on zooplankton. We monitored mysid abundance in Lake Ontario using lake-wide hydroacoustics data and vertical net hauls collected 1–5 August 2008 during the Ontario Ministry of Natural Resources and New York State Department of Conservation forage fish survey. Acoustic volume backscattering strength was highly correlated with both density and biomass of mysids although the correlation with biomass was stronger. The slopes of these relationships were not significantly different from theory (0.10) indicating a linear relationship between abundance and backscattering strength. Size structure significantly affected the relationship between backscattering strength and density but not between backscattering strength and biomass. Average target strength for areas with small mysids was −91.9 dB per mysid, and in areas with both small and large mysids it was −88.2 dB per animal. Acoustic estimates for Lake Ontario calculated with these regressions provided a lake-wide estimate weighted by lake area within depth intervals, of 228 mysids m−2 (CV 17.6%) and 0.93 g dw m−2 (CV 12.9%). Density and biomass in 14 net hauls averaged 316 mysids m−2 (range 0 to 1113) and 1.35 g dw m−2 (range 0 to 3.89). Mysid biomass was 33% of the biomass of other crustacean zooplankton in the summer of 2008. As in 2005, areas of low mysid abundance were detected in the middle of the transects and along the north shore. Lakewide mysid abundance in the summer of 2008 was higher than in a similar acoustic survey in the summer of 2005 and in whole-lake autumn 2000–2007 net surveys, suggesting that mysids are persisting in Lake Ontario and abundant relative to other Great Lakes.

Moonlit swimming: vertical distributions of macrozooplankton and nekton during the polar night

Macrozooplankton (e.g. krill, amphipods and jellyfish) and nekton (e.g. decapod shrimp, squid and fish) are integral parts of pelagic ecosystems, but knowledge of their vertical distributions and migrations during winter at high latitudes is lacking. This study provides the quantification of macrozooplankton and nekton distributions during the polar night in a partially ice-covered high Arctic fjord. In January 2012, mid-water trawls and MIK nets were deployed in Rijpfjorden, Svalbard (80° 18′ N, 22° 15′ E) at three depths (20, 75 and 200 m) day and night. Simultaneously, acoustic volume backscattering strength (a measure of biomass) was recorded using 18-, 38- and 120-kHz echosounders. We observed that the majority of nekton were below the thermocline (>100 m) day and night. A diverse fish community (10 species present) dominated the nekton biomass closely followed by shrimp and squid. Macrozooplankton, mostly large Calanus spp. copepods and gelatinous zooplankton, were found throughout the water column, but the majority were above the thermocline day and night. A general additive model with depth, time and moonlight predicted biomass to increase with depth for both macrozooplankton (over the top 100 m) and nekton, but revealed no patterns in biomass over time. The model also indicated that full moon presence increased depth of macrozooplankton backscatter. Our findings suggest a diverse and to some degree active pelagic community during the polar night, and provide some support for the hypothesis that moonlight induces downward vertical migrations of macrozooplankton.

Vertical Migration of Sound Scatterers in the Southern Yellow Sea in Summer

Acoustic volume backscattering strength data were collected and Conductivity Temperature Depth (CTD) measur e m e n t s were conducted in the southern Yellow Sea in summer 2005 and 2006. The high temporal and vertical resolution acoustic data measured with a 307 kHz Acoustic Doppler Current Profiler (ADCP) and a 250 kHz acoustic Doppler profile (ADP) had dominant diel variation, which resulted from vertical migration of sound scatterers. Some scatterers congregating in the bottom layer in the daytime migrated upward at dusk, and migrated downward into the bottom layer at dawn. The migration speeds were estimated. More than 33 days data show that the diel migration varies with time. The feature of migration measured with ADCP and ADP is consistent to some extent with what is described in the study on vertical migration of zooplankton in the southern Yellow Sea with conventional net samples.

The spatial distribution of euphausiid aggregations in the Northern California Current during August 2000

Abstract The location and size of euphausiid aggregations (patches) were identified using acoustic volume backscattering strength at 38 and 120 kHz and vertically stratified MOCNESS sampling during a US GLOBEC cruise off southern Oregon and northern California in August 2000. Euphausiid patches were clustered inshore of Heceta Bank (44.0°N) and off Cape Blanco (42.8°N), but were not common elsewhere. The distribution of euphausiid patches throughout the study area was correlated with that of near-surface chlorophyll. We suggest that Heceta Bank and Cape Blanco were zones of plankton retention and concentration along the shelf and slope, and also that large offshore meanders in the equatorward upwelling jet carried upwelled water, chlorophyll, and euphausiids offshore. In particular, the complex interaction of mesoscale physical features, shoaling bottom topography, and diel vertical migration created large patches of euphausiids. These euphausiid patches, probably persisting on timescales of days to weeks, could function as relatively large and persistent resources for euphausiid predators.

Acoustically-inferred zooplankton distribution in relation to hydrography west of the Antarctic Peninsula

Abstract The relationship between the distribution of zooplankton, especially euphausiids (Euphausia and Thysanoessa spp.), and hydrographic regimes of the Western Antarctic Peninsula continental shelf in and around Marguerite Bay was studied as part of the Southern Ocean GLOBEC program. Surveys were conducted from the RVIB N.B. Palmer in austral fall (April–June) and winter (July–August) of 2001. Acoustic, video, and environmental data were collected along 13 transect lines running across the shelf and perpendicular to the Western Antarctic Peninsula coastline, between 65°S and 70°S. Depth-stratified net tows conducted at selected locations provided ground-truthing for acoustic observations. In fall, acoustic volume backscattering strength at 120 kHz was greatest in the southern reaches of the survey area and inside Marguerite Bay, suggestive of high zooplankton and micronekton biomass in these regions. Vertically, highest backscattering was in the 150–450 m depth range, associated with modified Circumpolar Deep Water (CDW). The two deep troughs that intersect the shelf break were characterized by reduced backscattering, similar to levels observed off-shelf and indicative of lower zooplankton biomass in recent intrusions of CDW onto the continental shelf. Estimates of dynamic height suggested that geostrophic circulation likely caused both along- and across-shelf transport of zooplankton. By winter, scattering had decreased by an order of magnitude (10 dB) in the upper 300 m of the water column in most areas, and high backscattering levels were found primarily in a deep (>300 m) scattering layer present close to the bottom. The seasonal decrease is potentially explained by advection of zooplankton, vertical and horizontal movements, and mortality. Predictions of expected backscattering levels based on net samples suggested that large euphausiids were the dominant source of backscattering only at very particular locations and depths, and that copepods, siphonophores, and pteropods were more important in many locations.

Quantification of a multibeam sonar for fisheries assessment applications.

The acoustic theory is developed for a multibeam fisheries-type sonar employing a circular arc of transducer elements. Specifically, numerical relations for transmit and receive beam patterns are derived and methodologies set forth for the derivation of appropriately scaled acoustic target strength and acoustic volume backscattering strength from an ideally performing multibeam device. Predicted and measured beam characteristics of a realizable multibeam sonar, a Kongsberg Simrad-Mesotech SM 2000, are compared. Practical techniques for the extraction of calibrated acoustic volume backscattering strength from real systems are advanced.

Hydrographic and acoustic evidence for enhanced plankton stocks in a small cyclone in the northeastern Gulf of Mexico

Mesoscale eddies (diameters of hundreds of km) have been shown to influence plankton production as well as the distribution of seabirds and marine mammals in the Gulf of Mexico. Smaller circulation features (eddies with diameters of tens of km) may have similar effects. We show that a small, sub-mesoscale cyclone located on the continental shelf and slope in the northeastern Gulf of Mexico during November 1997 was an area of enhanced nutrients, chlorophyll, and acoustic volume backscattering strength ( S v ). Nitrate concentrations at mid-depth in the euphotic zone were as high as 2 μM, surface chlorophyll exceeded 1 μg l −1, and S v was as much as 15 dB referenced to 1 m −1 4 π −1 greater within this feature than in surrounding waters. Since S v at 153 kHz is a proxy for the abundance of sound-scattering mesozooplankton and micronekton, we infer that this cyclone was locally enriched in biomass of these organisms. As with mesoscale eddies, smaller cyclones could also function as patches of favorable habitat for higher-trophic-level organisms and their prey.

Biomass distribution and zooplankton composition of the sound-scattering layer in the waters of southern East China Sea

Fuzzy classification was used to analyse hydroacoustic survey data (Sv, an acoustic volume backscattering strength index of biomass) and oceanographic data (temperature, salinity, water density, and chlorophyll a fluorescence) collected from the waters of the southern East China Sea off northeastern Taiwan. Four types of water masses were identified. Type A water mass corresponded to warm continental-shelf mixed water and had a high acoustic volume backscatter (Sv>59.37 dB). Type D corresponded to cold upwelling Kuroshio subsurface water with Sv<65.24 dB, i.e. about 6 dB lower than that of Type A. Type B and C waters were intermediate between Type A and D waters. Twenty-two IKMT (net) samples were taken, and their estimated biomasses showed a significant relationship to corresponding average Sv values. Copepoda were most dominant in the zooplankton community. Decapoda, appendicularia, and chaetognatha were abundant in Type A waters, and proportionally rarer in the other three types of water masses. Conversely, the percentage contribution made by ostracoda and gastropoda was greater in Type D waters than in Type A waters. The combination of IKMT net sampling and hydroacoustic measurements was found to be useful for assessing the biomass distribution and species composition of zooplankton. We also conclude that the fuzzy classification method used here can usefully be applied to hydroacoustic data for the classification and identification of water masses and zooplankton composition in the ocean, particularly in an upwelling area. 1999 International Council for the Exploration of the Sea

Relationship between acoustic backscattering strength and density of zooplankton in the sound-scattering layer

Hydroacoustic sampling in the ocean is a useful technique for studying the biomass and structure of the distribution of marine organisms, including zooplankton. Acoustical and biological sampling of zooplankton in the sound-scattering layer (SSL) oV the east coast of Oshima Peninsula, northern Japan has been conducted during the last 10 years to obtain the scale factor for converting the backscattering strength to biological density. The volume backscattering strength (SV) was measured at 25, 50, 100, and 200 kHz, while an IKMT (Isaacs-Kidd Midwater Trawl) and a Norpac (North Pacific standard net) were used to sample the biological organisms in the SSL. During drifting observation at dusk, the maximum speed of the upward migration of the SSL toward the surface was m min 1 , and the SV changed over a range from 80 dB to 50 dB. Vertical tows of the Norpac indicated that euphausiids were the major zooplankton species of the migrating SSL. For quantitative analysis, the IKMT was towed horizontally about one hundred times, and the calculated density of zooplankton showed a maximum of 10 g m 3 . We assumed that the acoustic reflectivity of the individual plankton was related to the size of the plankton squared, and that the acoustic backscattering strength of biomass was proportional to the distribution density. Results of regression analysis showed a linear relationship between the log of zooplankton density ae (mg m 3 ) and the acoustic volume backscattering strength SV (dB), with correlation coeYcients greater than 0.5 at all frequencies. ? 1996 International Council for the Exploration of the Sea

Minimising errors in estimating fish population and biomass densities using the ‘acoustic volume backscattering strength’ method

Abstract The basic acoustical methods for estimating the volume backscattering strength and the absolute biomass density are commonly known. However, the detailed techniques for minimising errors in fitting the calibrated parameters of an echo sounding system in these computations are still not adequately examined and discussed. This paper describes and discusses the following computing techniques involved in the absolute biomass estimation: (i) the fitting of the calibrated time varied gain (TVG) curve, the selection of its operating interval, and the correction of its response; (ii) the compensation for the limitation of the signal bandwidth; (iii) the computation of the equivalent beam width of a transducer using its calibrated beam patterns; and (iv) the accuracy in the conversion from the volume backscattering to the absolute fish density using individual fish target strength measurements.

Backscattering of Underwater Sound in Saanich Inlet, British Columbia, Including Observations of Scattering from a Fish School

The frequency dependence of acoustic volume backscattering strength has been measured at depths of 100, 200, 300, and 400 ft in Saanich Inlet, British Columbia, Canada, over a 24-h period. The measurements were made for comparison with open-ocean (Pacific) results, since the biology of the inlet has been extensively studied, and it serves to some extent as a reference water volume. In the course of carrying out the above measurements, the backscattering spectral characteristics and strength of a school of young hake, of apparent mean size of 11 in., was obtained. The time series measurements revealed unchanging scattering characteristics at the 300- and 400-ft depths, with broad peaks near 1.2 and 4.8 kHz superimposed on otherwise flat spectra. Greater variability was observed at the 200-ft depth, while the greatest variability was observed near the surface, i.e., at the 100-ft observation depth. The fish school, which was observed to be closely packed (less than a fish length spacing near the surface) and randomly oriented, was shown to have a flat backscattering spectrum between 1 and 9 kHz and an estimated backscattering strength of −47±3 dB.