The use of quantitative broadband echosounders for biological studies and surveys offers considerable advantages over narrowband echosounders. These include improved spectral-based target identification and significantly increased ability to resolve individual targets. Biological studies and surveys typically require accurate measures of backscatter strength and we present here a systematic and comprehensive explanation of how to derive quantitative estimates of target strength and volume backscattering, as a function of frequency from broadband echosounder signals.

The relationship between low frequency military sonar use and some whale stranding events has attracted negative attention towards the use of active sonar in the marine environment.As a consequence, there has been only limited use of active acoustic techniques by marine mammal researchers.Instead more attention has been given to the development and use of passive acoustic methods for the detection of cetaceans.Nevertheless there is great potential for the use of active acoustic systems in ecological studies, and studies aimed at improving conservation of whales and dolphins in their natural environment: active acoustic techniques can be used for the good of cetaceans, and should not just be considered a source of disturbance.We evaluated the capability of various acoustic systems -systems that are used commonly in fisheries research -to detect and track cetaceans underwater.We collected data initially with standard scientific echosounders (SIMRAD EK500) from moving vessels and from fixed moorings (EK60) at 38 and 120 kHz.Scientific echosounders have narrow beams and therefore enable sampling from only fixed and limited volumes.To overcome these limitations we progressed to use high frequency omnidirectional sonar (SIMRAD SH80) operating at 110 kHz.During Norwegian Sea ecosystem surveys (2006)(2007)(2008) we recorded the backscatter energy from three species of large whales: Fin whale (Balaenoptera physalus), Minke whale (Balaenoptera acutorostrata) and Humpback whale (Megaptera novaeanglie).During the observations no avoidance reactions by cetaceans were observed, even when the vessel's entire suite of active acoustic instruments were running (frequency range from 18 kHz to 200 kHz, maximum source level 210 dB re 1 µPa (at 20 and 110 kHz).Here we present a small sample of data from two Fin whales that show the animals backscatter response at broadside.In addition, we give an overview of the present status and possible future development of the use of standard active acoustic methods for cetacean studies.Omnidirectional sonar shows great potential as a tool for marine mammal detection, and could be developed to produce an automatic cetacean detector.Such a detector would be a highly valuable aid for seismic mitigation and ship strike prevention, and in addition could contribute to behavioural studies with its high definition time scale.Much work is required to understand relationships between cetacean body shape, physiology, kinematics and acoustic reflectivity, before a detector can be realized.

Marine mammals are very seldom detected and tracked acoustically at different depths. The air contained in body cavities, such as lungs or swimbladders, has a significant effect on the acoustic energy backscattered from whale and fish species. Target strength data were obtained while a humpback whale (Megaptera novaeangliae) swam at the surface and dove underneath a research vessel, providing valuable multi-frequency echosounder recordings of its scattering characteristics from near surface to a depth of about 240 m. Increasing depth dramatically influenced the backscattered energy coming from the large cetacean. This study is tightly linked to the ultimate goal of developing an automated whale detection system for mitigation purposes.

Scientific split-beam echosounders are sensitive instruments for observing biomass densities and individual behaviour. Earlier studies have demonstrated that these instruments can be used to study diving behaviour of cetaceans. In this paper, we go into more detail about the recorded signal to see if and how acoustic split-beam data can be used to extract information about synchronous behaviour and other species related characteristics.Data of several cetacean individuals were collected by a moored echosounder pinging upwards from about 900 m in the Charlie Gibbs Fracture Zone. In this paper, we discuss methodological issues associated with using split-beam tracking of large marine animals. Further we demonstrate that target tracking of cetaceans can be used to study solo dives as well as behavioural synchrony. We also show that paired signals can easily be interpreted as false synchrony due to the size of the animals. In such cases a rough estimate of the diameter, and hence size, of the animals can be estimated. We emphasise on four examples that clarify methodological challenges including synchronous swimmers as well as large single cetaceans that might be interpreted as two synchronous swimmers.The applied technology requires that the animals remain in a narrow acoustic beam for long enough time to extract behavioural information. The technology can be improved by developing automatic tracking of cetaceans with a steerable transducer. This will substantially increase the search volume and enable tracking of cetaceans over longer periods and thus, produce more realistic information about the whale behaviour. (C) 2013 Elsevier Ltd. All rights reserved.

Active acoustic techniques can be used to detect whales. The ability to detect whales from a moving vessel or stationary buoy could reduce conflicts between hazardous human activities and whales, enabling implementation of mitigation procedures. In order to identify acoustic targets correctly as whales, knowledge of whale target strength ( TS ) is required. Active acoustic detections of fin whales ( Balaenoptera physalus ) were made in the Norwegian Sea; acoustic data were collected using calibrated omnidirectional sonar, operating at a discrete frequency of 110 kH z. Three fin whales of similar size (estimated between 16 and 18 m total length) had an overall average TS for all insonified body aspects of −11.4 dB [95% CI −12.05, −10.8] at 110 kH z, with a total spread of nearly 14 dB . As expected, the received signals were stronger when the fin whales were insonified at broadside (−5.6 dB ). Individual fin whale TS varied by approximately 12 dB , probably due to variation in lung volume with breathing, and to dynamic swimming kinematics. Our TS values are consistent with values reported previously for other large whales. All data together pave the way for development of automated acoustic whale detection protocols that could aid whale conservation.

Many marine species have evolved mechanisms to use sound to communicate, interact, and hunt in aquatic environments (Fudge and Rose 2009; Tyack and Clark 2000). For these reasons, human activities that introduce noise to the world’s oceans may be of concern for marine life. It is the opinion among many scientists that whales and dolphins in particular may be sensitive to elevated sound levels and that noise may potentially result in negative physical and/or behavioral impacts (Hildebrand 2005). For example, mass strandings and serious injuries found in groups of marine mammals have been linked to the operation of low-frequency military sonar systems (Bernasconi et al. 2009). Concerns that the impact of sound may be widespread and detrimental has led to “bad press” about the application of active acoustic techniques to marine life. Nonetheless, ad hoc groups of experts have concluded that sonar used in fishery research is not a source of high potential risk for marine mammals (O’Brien 2004) and thus blanket “bad press” is unjustified. We have collected data on the acoustic target strength (TS) of cetaceans since 2007 and analyze here the observational data collected in conjunction with the acoustic data to see if any impact and disturbance of the vessel or its sonar systems on the animals under investigation can be detected.

Current visualization technology implemented in the software for 2D sonars used in marine research is limited to slicing whilst volume visualization is only possible as post processing. We designed and implemented a system which allows for instantaneous volume visualization of streamed scans from 2D sonars without prior resampling to a voxel grid. The volume is formed by a set of most recent scans which are being stored. We transform each scan using its associated transformations to the view-space and slice their bounding box by view-aligned planes. Each slicing plane is reconstructed from the underlying scans and directly used for slice-based volume rendering. We integrated a low frequency illumination model which enhances the depth perception of noisy acoustic measurements. While we visualize the 2D data and time as 3D volumes, the temporal dimension is not intuitively communicated. Therefore, we introduce a concept of temporal outlines. Our system is a result of an interdisciplinary collaboration between visualization and marine scientists. The application of our system was evaluated by independent domain experts who were not involved in the design process in order to determine real life applicability.

Mesoscale eddies stimulate biological production in the ocean, but knowledge of energy transfers to higher trophic levels within eddies remains fragmented and not quantified. Increasing the knowledge base is constrained by the inability of traditional sampling methods to adequately sample biological processes at the spatio-temporal scales at which they occur. By combining satellite and acoustic observations over spatial scales of 10 s of km horizontally and 100 s of m vertically, supported by hydrographical and biological sampling we show that anticyclonic eddies shape distribution and density of marine life from the surface to bathyal depths. Fish feed along density structures of eddies, demonstrating that eddies catalyze energy transfer across trophic levels. Eddies create attractive pelagic habitats, analogous to oases in the desert, for higher trophic level aquatic organisms through enhanced 3-D motion that accumulates and redistributes biomass, contributing to overall bioproduction in the ocean. Integrating multidisciplinary observation methodologies promoted a new understanding of biophysical interaction in mesoscale eddies. Our findings emphasize the impact of eddies on the patchiness of biomass in the sea and demonstrate that they provide rich feeding habitat for higher trophic marine life.

Acoustic measurements show that the biomass of zooplankton and mesopelagic fish is redistributed by mesoscale variability and that the signal extends over several hundred meters depth. The mechanisms governing this distribution are not well understood, but influences from both physical (i.e. redistribution) and biological processes (i.e. nutrient transport, primary production, active swimming, etc.) are likely. This study examines how hydrodynamic conditions and basic vertical swimming behavior act to distribute biomass in an anticyclonic eddy. Using an eddy-resolving 2.3 km-resolution physical ocean model as forcing for a particle-tracking module, particles representing passively floating organisms and organisms with vertical swimming behavior are released within an eddy and monitored for 20 to 30 days. The role of hydrodynamic conditions on the distribution of biomass is discussed in relation to the acoustic measurements. Particles released close to the surface tend, in agreement with the observations, to accumulate around the edge of the eddy, whereas particles released at depth gradually become distributed along the isopycnals. After a month they are displaced several hundreds meters in the vertical with the deepest particles found close to the eddy center and the shallowest close to the edge. There is no evidence of aggregation of particles along the eddy rim in the last simulation. The model results points towards a physical mechanism for aggregation at the surface, however biological processes cannot be ruled out using the current modeling tool.

During a calibration trial with the Kongsberg TOPAS PS18 sub-bottom profiling parametric sonar, the Simrad EK60 split-beam scientific echo sounder was used to track the position of a standard target in the TOPAS beam. The target was a 280-mm-diameter solid sphere composed of the aluminum alloy 6082 T6. Simultaneous measurements were performed with proximate transducers operating at 18, 38, 70, 120, 200, and 333 kHz. The beamwidths were 7 deg at all frequencies except at 18 kHz, where the beamwidth was 11 deg. The target range was about 90 m. Position data are compared ping-by-ping relative to the individual transducers and in a common reference frame defined by the vessel motion reference unit and sea surface.

A prototype broadband echo sounder has been used for measuring zooplankton and fish in a new collaborative project between the Institute of Marine Research and Kongsberg Maritime-Simrad. The prototype system used is a Simrad broadband system which includes a transceiver operating from 10–500 kHz. We have mainly used the transceiver together with four separate Simrad ESXX-7CD standard pressure resistant transducers, the ES70-7CD, ES120-7CD, ES200-7CD, and ES333-7CD, covering the band from 50 to 450 kHz. The research has so far concentrated on data output formats, calibration methods, and measurements from single targets in ex situ and in situ situations. Examples of recorded reflected spectra for selected calibration and biological targets will be shown and the potential for improved aquatic ecosystem assessment discussed.

The low-frequency target strength of shoaling Atlantic herring (Clupea harengus) in the Gulf of Maine during Autumn 2006 spawning season is estimated from experimental data acquired simultaneously at multiple frequencies in the 300-1200 Hz range using (1) a low-frequency ocean acoustic waveguide remote sensing (OAWRS) system, (2) areal population density calibration with several conventional fish finding sonar (CFFS) systems, and (3) low-frequency transmission loss measurements. The OAWRS system's instantaneous imaging diameter of 100 km and regular updating enabled unaliased monitoring of fish populations over ecosystem scales including shoals of Atlantic herring containing hundreds of millions of individuals, as confirmed by concurrent trawl and CFFS sampling. High spatial-temporal coregistration was found between herring shoals imaged by OAWRS and concurrent CFFS line-transects, which also provided fish depth distributions. The mean scattering cross-section of an individual shoaling herring is found to consistently exhibit a strong, roughly 20 dB/octave roll-off with decreasing frequency in the range of the OAWRS survey over all days of the roughly 2-week experiment, consistent with the steep roll-offs expected for sub-resonance scattering from fish with air-filled swimbladders.

The difference-frequency band of the Kongsberg TOPAS PS18 sub-bottom profiling parametric sonar is very directional. Calibration of this by the standard-target method requires precise positioning and knowledge of target location. This was apparent during the first calibration trial performed on 10 December 2008. The proximity of split-beam transducers of the Simrad EK60/18- and 38-kHz scientific echo sounder to the TOPAS transducer on the hull of R/V “G. O. Sars” has been exploited, with synchronous operation of the several sonars. While results with a single split-beam transducer would be unambiguous, the availability of data from two split-beam transducers allows quantitative comparison of performance. Results from this comparison are presented here. Significantly, agreement of the respective measurements is strong at an approximate target range of 90 m, fair at 220 m, but rather weak at 325 m. The context of a moving target is noted. The challenge of target-positioning and -tracking in a dynamic environment, more so in the presence of wind and underwater currents, may be met by use of a split-beam scientific echo sounder, if with a range limitation still to be defined.

The difference-frequency band of the Kongsberg TOPAS PS18 parametric sub-bottom profiling sonar, nominally 1-6 kHz, is being used to observe Atlantic herring. Representative TOPAS echograms of herring layers and schools observed in situ in December 2008 and November 2009 are presented. These agree well with echograms of volume backscattering strength derived simultaneously with the narrowband Simrad EK60/18- and 38-kHz scientific echo sounder, also giving insight into herring avoidance behavior in relation to survey vessel passage. Progress in rendering the TOPAS echograms quantitative is described.

Many fish with swimbladders exhibit diel vertical migrations (DVM). Ascents and descents of hundreds of metres occur, and altered swimbladder volume and buoyancy can result from incomplete secretion and resorption of gas. When acoustic observations are made near the resonance frequency of the swimbladder, the estimated fish biomass can be positively biased. When multiple-frequency echosounders are used, the frequency response of the backscatter might vary temporally and spatially and compromise the effectiveness of conventional target-identification methods. In this paper, variations in backscatter from mesopelagic fish are studied using data collected west of the British Isles with a five-frequency echosounder (Simrad EK60). Two acoustic layers, one dominated by pearlsides (Maurolicus muelleri) and the other by myctophids (Myctophidae), were monitored during their DVM. The frequency responses of the layers changed systematically, mainly characterized by increases in the nautical-area-backscattering coefficient (s(A)) values at 18 kHz relative to those at 38 kHz. This could have been caused by changes in the resonance frequencies of fish swimbladders, as they expanded and contracted during ascent and descent. Two s(A) maxima in the myctophids layer suggest the presence of two types of target with different scattering characteristics. Models of sound scatter from myctophid swimbladders suggest that these peaks have resulted from resonance scattering. The s(A) at 18 kHz attributed to M. muelleri also peaked, but at the maximum depth of their distribution. Spatial and temporal changes in the frequency responses of fish should be taken into account when pelagic fish communities are surveyed with multiple-frequency echosounders.

A seabed-mounted, upward-looking, 38 kHz echosounder (Simrad EK60) was used to monitor cetaceans for 128 d between July 2004 and June 2005. The echosounder was placed at 52 degrees N 30 degrees W at a depth of 910 m, in a frontal area with continuous upwelling creating a hotspot of marine life at all trophic levels. Echo tracks were observed, apparently resulting from one or more animals in close temporal and spatial proximity. The 69 "acoustic detections" were examined with a principal component analysis (PCA) considering mean target strength (TS), group size, and maximum dive depth and duration. The PCA conservatively classified 45 of the acoustic detections as "cetaceans". Their mean TS values varied between -5 and -36 dB, in agreement with earlier TS measurements of whales and dolphins. Several of the supposed cetaceans were apparently feeding in the "deep-scattering layer" when this ascended at night. This study demonstrates that cetacean behaviour, including swimming, feeding, and vocalizing, can be observed and monitored with stationary active-acoustic instrumentation.

Vessel-induced avoidance can potentially cause a large bias in acoustic estimates of schooling, pelagic-fish biomass. This paper presents a method for quantifying this uncertainty. Volume-backscattering strength (S-v) from a horizontally projecting, multibeam sonar (Simrad MS70) is resampled to form synthetic, vertical, echosounder beams to the side of the survey vessel. These data are analysed as if they were collected from phantom vessels surveying parallel transects at fixed ranges from the real vessel. The nautical-area-backscattering coefficients (s(A)) from the synthetic echograms are compared with those measured by conventional 70 and 120 kHz echosounders (Simrad EK60) on the real vessel. Data collected in 2006 from schools of Norwegian spring-spawning herring are used to illustrate the method and explore its limitations. Potential effects of vessel-induced avoidance are evaluated by comparing the mean sA values observed from the phantom vessels with those observed from the real vessel. The technique also allows direct estimates of the mean lateral-aspect target strength of in situ herring.